Glossary of physics terms

A B C D E F G H I J K L M N O P Q R S T U V W X Y Z

Absorption: Transfer of energy from an electromagnetic field to tissue.
Acquisition: Electronic recording of an RF signal or data set of multiple signals.
Acquisition matrix: Set of signal data in a multidimensional array of pixels (e.g. two-dimensional set of data forming a single 256 × 256 voxel image). In 2D Fourier transform imaging, matrix size is in the number of data points acquired in the phase encoding direction × the number acquired in the frequency-encoding direction.
Artifact: A false signal on an image. A signal that does not correspond to an anatomical structure or one which has an inappropriate value.; chemical-shift artifact These are bright or dark bands at fat water interfaces in MR images. Chemical-shift artifacts are caused by spatial misregistration (in the frequency-encoding direction) or spins with different chemical shifts, such as protons in water and protons in lipid.; data-clipping artifact Artifact caused by the signal being too large to be digitized properly for a given attenuation setting.; ghost artifact False images of a tissue or organ that propagate along the phase encoding direction. They are often due to motion.; gibb’s artifact (also truncation artifact) Artifact caused by the inability of a digital Fourier transform to reproduce faithfully an abrupt discontinuity at a boundary.; metallic artifact Geometric signal distortions caused by local magnetic field inhomogeneity from a metallic object. This artifact is usually worse on gradient echo (compared to spin echo) images and is worse for ferromagnetic (compared to non-ferromagnetic) metals.; motion artifact Spatial misregistration of signal usually along phase-encode direction caused by motion and typically resulting in ghosts.; susceptibility artifact Signal loss and geometric distortion due to different magnetic properties of tissues or materials. May appear at the boundary of two tissues with differing diamagnetic susceptibility (e.g. brain and air-containing paranasal sinuses).; truncation artifact Multiple bands of alternating high and low signal intensity that appear parallel to a boundary between two tissues with different signal intensities.; wrap-around (aliasing) artifact Image located outside the field of view on the opposite side of the image. This is caused by an inadequate number of phase-encoding or frequency-encoding samples.
ATP: Adenosine triphosphate.
Averaging: Combining more than one data acquisition.
Bandwidth: Range of frequencies received (measured in Hz per pixel or ± kHz) by the receiver. The MR receiver bandwidth must be sufficient to process the range of frequencies in the data sampled or wrap-around artifact occurs.
Bird-cage coil: Whole-volume RF coil that has a cage-like design which is used at higher fields.
Blood oxygen level dependent (BOLD): MRI techniques sensitive to the presence of deoxyhemoglobin in a tissue. Oxyhemoglobin is diamagnetic and has a lower magnetic susceptibility than deoxyhemoglobin, which is paramagnetic.
Bulk water: Water molecules whose molecular motion is determined solely by the interactions between water molecules (i.e. no macromolecular interactions are occurring).
Carr–Purcell sequence: T2 characteristics of tissue can be measured by applying a 90° RF pulse followed by a train of 180° pulses. This results in multiple spin echoes. It is technically difficult to produce perfect successive 180° pulses and this results in ‘pulse error accumulation’. This accumulation can be reduced by altering the direction of 90° or 180° pulses with reference to one another in 3D space, i.e. altering the phase of these pulses in relation to the rotating frame of reference. Such modified sequences are known as Carr–Purcell–Meiboom–Gill (CPMG) sequences.
Chemical shift: Protons in different molecules may have slightly different Larmor frequencies due to the ‘shielding’ effect of adjacent electron orbits. This results in signals from the same voxel being mapped to different pixels causing false signal voids and hyperintensities around the edges of structures or lesions. These effects are exaggerated by higher field strengths and are seen in the frequency encoded direction. The amount of shift is expressed as parts per million (ppm) of the resonance frequency and is given the greek symbol Delta.
Chemical-shift cancellation effect: Pulse sequence that exploits the phase difference between two components (usually protons in lipid and protons in water) in the same voxel.
CHESS (chemical-shift selective): Imaging sequence in which an RF pulse is applied at a frequency that saturates certain chemical components. If the conventional pulse sequence is then applied at a different frequency, only those chemical components that have not already been saturated will produce a signal. This imaging method can be used to differentiate fat from water.
Cine-MRI: Fast image acquisition with sequential looping of the resultant images which can be used to display dynamic processes. Cine-MRI has been particularly useful in studying CSF flow.
Coherence: Consider the old executive’s toy, Newton’s Cradle. If all five balls are pulled to one side and then let go, they will all swing together and the motion will continue for some time. This is because they are all swinging in phase and their motion is coherent. But if three balls are pulled to one side and two to the other, the balls will collide and soon cancel their motion out. They have been given opposite phase and their motion lacks coherence. In a similar manner, MR signals from protons in tissue with similar frequency and identical phase are said to be coherent. But proton spin/spin interactions lead to loss of phase coherence and thus the MR signal will decrease with time. This is the basis of T2 relaxation.
Coil: An electric current flowing through a wire will produce a magnetic field around it. Likewise, a changing magnetic field will induce a current in a wire. Specially shaped coils or wire are used in MRI to both produce the RF interrogating pulse and to detect the resultant MR signal from the protons. The voltage strength of these induced currents is proportional to the MR signal strength.
Computer: If the magnet is the engine then the computer is the driver in MRI. Unlike CT where significant computer processing only occurs after data acquisition, MR requires powerful computation to provide appropriate RF pulses and gradients for signal generation. Overall command is provided by a central processing unit (CPU) which links into pulse and gradient generators, spectrometers, memory and data storage facilities and image generation/manipulation outputs.
Contrast: Difference in signal intensity between two discrete regions of an image (e.g. two tissues), scaled to a reference such as signal intensity of one of the two regions (or tissues), their mean, another region (or tissue), the background noise level, or an external standard.
Contrast agent: Any drug or material that alters the contrast of a region (e.g. a tissue) on an image. MR contrast agents usually shorten T1 and/or T2 relaxation times. In MRI the contrast agents work by altering the relaxation characteristics of the target tissue, e.g. enhancement is achieved by gadolinium decreasing the T1 relaxation time of abnormal tissue. MR contrast agents may be ferromagnetic, paramagnetic or superparamagnetic.
Contrast-to-noise ratio (CNR): Ratio of signal intensity differences between two regions, scaled to image noise. The ratio indicates the ability to detect low-contrast lesions.
Coupling: Magnetic dipole interactions with local electric and magnetic fields created by neighboring nuclei. Atoms induce a relaxation because the motions of one particle are coupled to the motion of another. In general, only magnetic dipole–dipole coupling is important in proton MRI.
Cross-talk: Efforts to reduce imaging times in MR resulted in multiple slice acquisition programs. Unfortunately, perfect accuracy is impossible and each slice may receive some of the RF pulse intended for adjacent slices which results in degradation of the signal from that slice or spurious echoes. Hence truly contiguous slices cannot be obtained without resorting to volume acquisition techniques.
Cryogen: Superconducting magnets need to be kept at extremely low temperatures which are maintained by liquified gases or cryogens. The most commonly used are helium (boiling point = –269_C) and nitrogen (boiling point = –196_C). These gases need to boil off over time and can represent a major part of the running costs of a superconducting magnet unless a recycling system or cryosaver is employed.
CSE: Conventional spin echo.
1D: One-dimensional.
2D: Two-dimensional.
3D: Whole volume pulse sequence that uses phase encoding for two axes. 3D techniques are based on gradient echo sequences that employ short repetition times (TRs).
dB/dt: Rate of change of the gradient magnetic field with time. Time-varying magnetic fields can induce electric currents in materials that have free charges or dipoles. Such currents are a potential safety hazard.
Dead time: After sampling an echo, the central processing unit of an MR computer needs a finite time to process the next set of instructions before it can switch into signal generation mode. This is known as dead time.
Decay time: After excitation, the protons gradually lose their transverse magnetism (T2) as they process back to their resting state. The time taken can be referred to as the decay time.
Deconvolution: The frequency distribution of signals forming an image may not match the frequency distribution of signals from the object under study if, for example, spurious signals are incorporated. The frequency distribution can be ‘cleaned up’ by using a mathematical process called deconvolution. An example of its use would be in reducing wraparound by rejecting signals from part of the body not in the field of view of the image.
Decoupling: This is a strategy used in MR spectroscopy to suppress the dominant signals from hydrogen protons to allow study of other nuclei such as carbon or phosphorus. This is achieved by applying a stream of pulses at the Larmor frequency of hydrogen whilst acquiring data of the resonant frequency of the nuclei under study. A drawback to this technique is the high amount of RF energy deposited which may cause a rise in temperature of the patient or sample.
Dephasing: Loss of phase coherence within a voxel.
Depth resolved spectroscopy: Given the acronym DRESS, this is a technique using field gradients to obtain MR spectra from predetermined depth in a sample volume.
Detector: Also known as a demodulator, this device receives the MR signal and converts it into a lower frequency prior to image processing.
Dewar: This is the name given to an insulated container for the storage of cryogens. These are bulky, and consideration must be given to their storage and access for exchange when designing an MR facility.
Diamagnetic: If a substance placed in a magnetic field becomes magnetized in the opposite direction to that field, it will cause a localized decrease in the field and is said to have diamagnetic properties or negative susceptibility.
Diffusion: Translational molecular motion caused by random thermal or Brownian motion. The parameter that characterizes diffusion, the apparent diffusion coefficient, describes the amount of translational molecular motion in a period of time for a particular material.
Dipole: This is a term applied to any particle with a single pair of separated points of positive and negative charges of equal magnitude. When given spin this results in the particle behaving as a small magnet with a discrete field. Many dipoles together can have an effect on one another and this is an important contributing factor to the relaxation characteristics of tissues. These interactions can be modified by paramagnetic agents which is the basis of enhancement techniques in MR.
Dipole electric: Particle (nucleus, molecule, or larger) with a single pair of separated centers of positive and negative charges of equal magnitude.
Dipole–dipole interaction: Magnetic interaction between two spins with magnetic moments resulting from the magnetic field produced by one spin acting on the other. The proton–proton dipole–dipole interaction is the principal source of relaxation for nuclei.
Dispersion: The rate of relaxation of tissue will alter at different strengths of the external magnetic field (B_o). This variation is known as dispersion and it is an important quantity in the study of relaxation mechanics. Knowledge of dispersion profiles is important in the development of MR contrast agents.
Display matrix: This refers to the number of picture elements (pixels) which form each line of the image. A typical high quality image will have 512 pixels per line and 512 pixels per column – a ‘512 × 512’ image. However, in order to see a high-quality image, the acquisition matrix must equal or exceed that of the display matrix. If the display matrix is greater than the acquisition matrix then data are calculated in adjacent pixels by a process known as interpolation. The final image would therefore lack detail.
Dixon technique: This is a chemical shift imaging technique which requires the acquisition of two images in order to resolve the chemical shifts of fat and water. The pay-off is excellent spatial resolution.
Duty cycle: A finished image for study is the product of interrogation, data collection, data processing and image formation. The period of interrogation by the RF pulses is an example of duty cycle within the overall function of image production. There is also a more rigid application of the term which refers to the time taken for excitation and data sampling for a single slice.
Echo: During spin echo imaging the free induction decay of the 90° pulses used to create the information signal is ‘contaminated’ by the effects of inhomogeneities in the main magnetic field and diffusion within the sample volume. This overall time constant is known as T2*. In order to separate out the T2 component of this signal a 180° rephasing or refocusing pulse is used and the residual signal from this pulse is collected. The residual signal is known as the echo and represents the pure T2 component. This echo can be sampled more than once in the multiple echo imaging techniques to give varied T2 weighting to the image and so increase the diagnostic yield.
Echo planar imaging: This is a fast scan technique which can create images of single slices in fractions of a second by rapidly switching the phase encoded gradient during the free induction decay of a single excitation pulse. This produces a train of gradient echoes which can be Fourier transformed into a single image. The process is virtually real-time and offers the prospect of interactive MR at the cost of increased RF energy deposition in the patient. The rapid rate of change of the magnetic gradients can also cause peripheral nerve stimulation, although the switching rates now used in image formation do not cause observable effects.
Echo time: See TE.
Eddy currents: Any conductor placed in a changing magnetic field will experience the induction of eddy currents. These can cause problems with image degradation and are a potential hazard to patients in high field systems with rapidly varying gradients. Their effects can be minimized by using specially shielded gradients.
Effective transverse relaxation time (T₂*): When the observed transverse relaxation time is faster than the normal T2 transverse relaxation time because of spatial inhomogeneity of the magnetic field, it is termed T2*. Spins at different locations experience slightly different magnetic fields, causing a loss of phase coherence, consequently the transverse relaxation decreases more rapidly than it would on the basis of T2 alone.
Electromagnetic radiation: This is the transmission of energy by variations in electric and magnetic fields in the form of waves. The frequency of the waveform gives the radiation its physical properties. Radio, light and X-rays are merely electromagnetic radiation at different frequencies.
Electron: Negatively charged particle with a mass 1/1864 that of a proton and located in orbitals of discrete (quantized) energies surrounding the nucleus of atoms.
Energy level: In a magnetic field, nuclear spins can populate different energy states. The energy levels available to particular nuclei depend on the angular momentum of the spinning nuclear charge, which is normally designated by the spin quantum number, I.
Enhancement: In the context of nuclear relaxation, enhancement is an increase in the relaxation rate or a decrease in the relaxation time.
Excitation: Delivery of energy (in the form of a RF pulse or magnetic field change) into a spin system (e.g. a 90° transverse pulse).
F: See frequency.
Faraday’s law: A changing magnetic field will induce a changing current in a conducting loop (circuit). A changing current in one circuit can create a changing circuit in another circuit field to which it is not physically connected except through the magnetic flux between the circuits.
Fast Fourier transform (FFT): Fast computational algorithm for performing a Fourier transform that allows a periodic function to be expressed as an integral sum over a continuous range of frequencies.
Fast spin echo (FSE): Long TR, multispin echo technique where each echo is separately phase encoded. Interecho times are short, and typically 4, 8 or 16 echoes are obtained per excitation, proportionally reducing scan time.
Ferrite: Iron oxide with the formula Fe⁺²_xM⁺³_yO_z (e.g. magnetite Fe₃O₄). Ferrite magnetic properties are determined by crystal size and structure.
Ferromagnetic: Any substance that has a large, positive magnetic susceptibility and shows a magnetic memory or residual magnetization.
Field distortion: Local field inhomogeneity caused by presence of diamagnetic, paramagnetic or ferromagnetic substances and leading to artifacts in the image.
Field strength: Intensity of static magnetic field. In the context of MRI, low field strengths range from 0.02 to 0.3 T, medium field strengths range from 0.3 to 1.0 T, and high field strengths are greater than 1.0 T. These definitions are evolving.
Field of view (FOV): Size of anatomical region imaged, which may be square or rectangular (i.e. asymmetric). FOV is also the product of the acquisition matrix (e.g. 128 × 256) and pixel dimensions (e.g. 3.1 × 1.6mm) (FOV in this example would be 40 × 40cm).
Filling factor: Ratio of the volume of sample within an RF coil to the volume of the coil. Filling factor affects the efficiency of irradiating the object and detecting MR signals, thereby affecting the signal-to-noise ratio. Achieving a high factor requires fitting the coil closely to the object.
FLAIR: Fluid-attenuated inversion recovery.
FLASH (fast low angle shot): Gradient echo imaging technique that uses gradient reversal and a 180° rephasing pulse. Tip angles (excitation pulses) of less than 90° are generally used, leaving a substantial fraction of the longitudinal magnetization unperturbed.
Flip angle: Angle of rotation of the macroscopic magnetization vector produced by an RF pulse. Flip angles are measured relative to the longitudinal (z) axis of the main magnetic field (B₀). For example, a 90° flip angle rotates the magnetization vector into the transverse (xy) plane. The flip angle is proportional to amplitude and duration of the RF pulse.
Flow artifacts: Both artifactual signal voids (time of flight signal loss) and hyperintense areas (flow-related enhancement) can be caused by flow phenomena in body fluids. Flowing matter can take signal away from an area of data acquisition to cause a void and it can equally bring signal into an area from adjacent excitations in multislice acquisitions.
Flow-related enhancement: Increase in signal intensity of flowing blood or cerebrospinal fluid relative to stationary tissue caused by entry of unsaturated spins.
Flow void: Signal loss observed with rapid flow caused by a combination of high-velocity signal loss, turbulence and intravoxel dephasing. Rapidly flowing arterial blood usually appears dark, whereas slowly flowing venous blood appears bright.
Fourier transform: This is a mathematical tool which is used to convert the raw MR signal into its frequency and phase components. In multiple-slice imaging frequency and phase are the x and y coordinates for the final image. Hence 2 Fourier transformations will be sufficient (2-DFT). However, if data have been acquired from a volume rather than a slice then a third coordinate will be needed which requires a third Fourier transformation (3-DFT).
Free induction decay (FID): When a gyroscope is tipped away from its vertical spinning axis it will return to its upright position in a spiralling motion and the amount of this precession around its axis will decay with a characteristic time constant. In the same way a spinning proton tipped from the main magnetic field of an MR magnet by the interrogating RF pulse will return to alignment with the main magnetic field (in time constant T2*).
Frequency (f): Number of repetitions of a periodic process (cycles) per unit time. Electromagnetic radiation frequency (radiowaves, microwaves, X-rays, etc.) is measured in Hertz (Hz) with units of inverse seconds.
Frequency encoding: In a flat plane any point on that plane can be described by its x and y coordinates. In a 2-D MRI image slice the equivalent of the x and y coordinates are the frequency and phase characteristics of each point. If the x scale is to be the ‘frequency’ characteristics then a range of frequency values across the slice must be provided by ‘encoding’ the MR signals from protons along the slice.
Gadolinium: The first clinically available contrast medium for MRI was the chelate of this metal from the lanthanide series with diethylenetriamine penta-acetic acid (DTPA). The ion of gadolinium has the greatest paramagnetic effect of any ion but is highly toxic in the unbound state. Its chelation provides safe bioavailability without removing its paramagnetic properties.
Gating: In order to remover the effects of physiologic motion from tissues or fluids MR data can be acquired at the same point in each motion cycle. Successful gating requires regular or cyclical motion. Thus gating cannot overcome artifact due to bowel movement.
Gauss: A Gauss (G) is a unit of magnetic flux density. This has been replaced in the SI system by the Tesla (T), where 1T = 10000G. The Gauss is still used as a convenient method of describing very small fields such as the stray field around an MR system.
Gradient: Spatial variation of some quantity, such as magnetic field strength.
Gradients: In order to localize MR signals emanating from protons in three dimensions a series of three magnetic field gradients are superimposed on the external magnetic field (B₀). This results in each point in a volume of tissue experiencing a unique magnetic field strength. Thus the Larmor frequency of precession will be unique for each point which allows for selective excitation or detection. The three gradients are: slice selection gradient; phase encoding gradient; and readout gradient.
Gradient coils: Coils producing a magnetic field gradient. Proper design of the size and configuration of the coils is necessary to produce a controlled and uniform gradient.
Gradient echo: Echo produced by reversing the direction of the frequency-encoding (readout) magnetic field gradient so as to cancel out the position-dependent phase shifts that have accumulated because of the gradient.
Gradient echo imaging: Spin echo imaging achieves T2 weighting by nullifying the effects of the magnetic field in homogeneities with a 180° ‘rephasing’ pulse. Unfortunately, this tactic increases scan times and quadruples the amount of RF power deposited in tissues. However, T2 weighting can also be achieved by rapid reversals of the readout gradients and acquiring the resultant echoes. The contrast in these images are dependent on the T2* signal (the sum of T2 and the effects of magnetic field inhomogeneities) and so they are sensitive to unstable or impure static fields. Image acquisition time is reduced by both the deletion of the 180° rephasing pulse and by the ability to reduce flip angles owing to the rapidity with which gradient reversals can be applied.
Gradient-recalled echo (GRE): Pulse sequence used in fast-scanning techniques in which the free induction decay is dephased and subsequently rephased into an echo via a gradient.
Gradient reversal: Pulse sequence that has both a negative dephasing gradient and a positive rephasing gradient. In gradient echo sequences the gradient reversal is applied along the slice-selection and frequency-encoding directions.
GRASS (gradient-refocused or recalled acquisition in the steady state): Gradient echo imaging technique with rewinder gradient in which the transverse magnetization is not spoiled, allowing persistence of the steady state.
G_x, G_y, G_z: Symbols for magnetic field gradients. Used with subscripts to denote spatial direction of the gradient in cartesian coordinates.
Gyromagnetic ratio (g): Ratio for the magnetic moment to the associated angular moment of a nucleus. A constant for all nuclei of a given isotope. For example, g is 42.58 MHz/T for 1H, and 17.24 MHz/T for 31P.
Helmhotz coil: Paired RF coils having a uniform sensitivity along their central axis. The distance between two single wire loops is set equal to their radius.
Hertz (Hz): SI unit of frequency (equal to the cgs unit of cycles per second).
High-velocity signal loss: Signal loss of moving spins that were not in the selected slice to receive both 90° and 180° RF pulses required to generate a spin echo.
Homogeneity: Homogeneity of the static magnetic field is an important criterion of magnetic field quality. Inhomogeneity is measured in parts per million (ppm) over a specified diameter spherical volume (DSV).
Hybrid magnet: Type of magnet in which the magnetic field is composed of two or more different types of magnets (i.e. resistive, permanent, superconducting) to form a more homogeneous or stronger field.
Image acquisition time: Time required to obtain MR data necessary for image reconstruction. For a 2D Fourier transform acquisition, the total image acquisition time equals the product of TR, the number of signals averaged (NSA or NEX) and the number of phase-encoding steps.
Image noise: Non-anatomical fluctuations in voxel signal intensity. Random or statistical noise (white noise) has a broad bandwidth and no phase coherence and appears at all points in the images as ‘graininess’.
Image reconstruction: Mathematical process of converting the composite signals obtained during the data acquisition phase into an image.
Induction: The term induction is used in two senses in MR, not entirely unrelated, but worthy of distinct explanation.; (1) Magnetic field strength is denoted by the symbol H and has the units of amperes/meter. Any object inserted into this field will have a magnetic flux induced within it. Magnetic induction, denoted by the symbol B, has the units of Tesla. The extent of the magnetic induction is dependent upon the magnetic permeability (m) of the material which is usually compared to the value for free space (m₀). Strictly we should be concerned with the magnetic induction B since this will depend on the nature of the material or body within the field H.; (2) A constant current passed through a wire loop generates a magnetic field along the axis of that loop. This is the basis upon which the static field B₀ is generated. The converse, that a magnetic field applied to a coil generates a current, is not true. In order to generate any current in the coil the magnetic field must be varying in time and this is an expression of Faraday’s law of magnetic induction. Strictly it is required that the magnetic flux cutting through the conductive loop varies in time in order to generate a time varying current. Transverse magnetization and its associated magnetic flux lines precess at the Larmor frequency with respect to the magnet and receiver coil. A time-dependent current is induced, with the small variations in precessional frequency being reflected by commensurate changes in current in the receiver coil. This signal is subsequently amplified prior to digitization and further processing within a computer.
Inhomogeneity: The term inhomogeneity can be applied to the applied static field B₀ and the radiofrequency (RF) or B₁ field employed in MR. In each case the quality of the respective magnetic field over the extent of the imaged region is of relevance.; The local magnetic field can be varying due to spatial variations caused by imperfections or inhomogeneities in B₀. A magnet system would usually have some corrective mechanism, called shimming, that allows field imperfections to be reduced. This procedure would be done at the time of installation of the magnet and would not require further attention as long as the environment of the magnet was not changed significantly.
In-phase image: Spin echo image acquired with 180° pulse at TE/2, so that temporal rephasing from the gradient reversal occurs simultaneously.
Inversion pulse: A 180° RF pulse that causes precessing nuclei to shift to the opposite spin state. This pulse causes the net equilibrium longitudinal magnetization vector, if in the low-energy state parallel to the applied magnetic field (B₀), to invert to the higher-energy state (antiparallel).
Inversion recovery sequence (IR): The IR sequence is specifically employed to measure the longitudinal magnetization following the initial 180° pulse. If a series of inversion recovery sequences is employed this can allow the relaxation behavior of the longitudinal magnetization to be measured and its associated time constant T1 calculated. In MRI it is simply employed as a means of emphasizing T1 by selecting a single TI.; As its name implies the sequence commences with a 180° or inversion RF pulse. It is usually assumed that the nuclear spins are all aligned along B₀ prior to this pulse. As a result there is no signal directly after the perfect inversion pulse since no transverse components of magnetization are generated. The magnetization is rotated from +z to –z. Following the inversion the nuclei are free to re-establish themselves as they please which will be to restore their orientations with respect to the magnet. The longitudinal component of magnetization becomes less negative, passes through zero and heads towards +z. Since longitudinal magnetization can not be seen directly we must apply an observing 90° pulse to take a look at the extent of the relaxation along B₀. The 90° pulse occurs at time TI after the 180° or inversion pulse. This ‘look’ can involve the image gradients to allow the construction of an image if required. The resultant image will contain a heavy weighting toward the T1 process.
Inversion time (TI): The interval TI denotes the interval between 180° inversion pulse and subsequent 90° observation pulse in an inversion recovery pulse sequence.
ISIS (image-selected in vivo spectroscopy): Surface-coil B1 gradient method that uses frequency-selected inversion pulses in the presence of magnetic field gradients to provide 3D localization of an image volume.
K space: Mathematical (transform) space whose coordinates are frequency and phase as opposed to physical space.
Laminar flow: Non-turbulent, linear flow with a parabolic velocity profile. The fluid at the center of the vessel moves twice as fast as the average; the fluid in contact with the wall has zero velocity.
Larmor equation: Larmor frequency (F) of precision of a nuclear magnetic moment is proportional to the magnetic field (B) and the gyromagnetic ratio (γ):
γF = gB.
Lattice: As used in NMR, lattice refers to the chemical environment.
Line shape: Distribution of the relative amplitude of resonances as a function of frequency, which establishes a particular spectral line. Common line shapes are Lorentzian and Gaussian.
Line width: Spread in frequency of a resonance line in an MR spectrum. A common measure of line width is Hz at full width half maximum (FWHM) for a specific field strength.
Localization technique: Techniques for selecting a restricted region from which a spectrum is desired. These can include the use of surface coils, magnetic field gradients, or a combination of both.
Longitudinal magnetization (M_z): Component of the macroscopic magnetization vector along the static B₀ magnetic field. Following excitation by an RF pulse, M_z will approach its equilibrium value, M₀, with a characteristic time constant, TI, according to the equation M_z/M₀ = 1 – exp(–t/TI).
Longitudinal relaxation: Return of the longitudinal magnetization to the equilibrium value after excitation. Energy is exchanged between the nuclear spins and the lattice.
M: Conventional symbol for macroscopic magnetization vector.
M₀: Equilibrium value of the magnetization, directed along the static magnetic field. At a given temperature the value of M_z is proportional to spin density (N), the gyromagnetic ratio (γ), and the static magnetic field (B₀).
Magnet: The magnet is the central piece of equipment of the MR scanner. It should provide a field (B₀) whose strength is stable over a period of time and spatially homogeneous over a region that must compare in size with the object to be imaged. The homogeneity is expressed as parts per million (ppm) figure for a defined volume of space, say a 30 or 50cm diameter sphere.; Magnets may be permanent, resistive or superconducting in construction. For higher fields, above 2.0 T, the superconducting magnets present the only choice. Permanent magnets are built from blocks of ferromagnetic alloys of metals such as nickel, iron and cobalt. The source of ferromagnetism is the unpaired electrons within the component elements of these alloys. Resistive magnets consist of many windings of copper wire and the field is maintained by the permanent supply of electrical current from a suitably stabilized power supply. Superconducting magnets are similar in the respect of having many windings. However, rather than copper, alloys of metal such as niobium and tin are employed whose characteristic is that they lose all electrical resistance when reduced in temperature. Once energized a superconducting magnet will maintain a field without additional power consumption. The catch is that incredibly low temperatures have to be maintained usually in the region of 20K (20_ above the zero [approximately –273_C]). The electricity bill is replaced by the expense of the cryogenic (cooling) gases nitrogen and helium, although many magnets now require helium only.
Magnetic dipole: Effective separation of magnetic north and south poles in a sample. An electric current loop, including the effective current of a spinning nucleon or nucleus, can create an equivalent magnetic dipole.
Magnetic dipole–dipole coupling: Small increment or decrement in the static magnetic field strength caused by a neighboring nucleus.
Magnetic dipole moment: Measure of magnitude and direction of the magnetic properties of a nucleus that has an I ≠ 0. From a classical viewpoint, a rotating nucleus (or charge) behaves analogously to an electric current flowing in a loop and thus possesses a magnetic dipole.
Magnetic field: The field is in the region surrounding a magnet in which magnetic properties can be detected. One property is that a small magnet in such a region experiences a torque that tends to align it with the magnetic field. The direction of the magnetic field is defined by a vector that points from south pole to north pole.
Magnetic field gradient: For example G_x, G_y and G_z consist of a magnetic field B, the strength of which varies linearly along one or more coordinate (spatial) directions. Magnetic gradients are used in MRI for selection of a region being imaged (slice selection) and to encode the in-plane location of MR signals received from the object being imaged.
Magnetic moment: Magnetic moment is a descriptive term employed interchangeably with, for example, nuclear spin, nuclear magnet or nucleus.; A conventional bar magnet will align itself within a magnetic field, a concept we are familiar with in the compass. The magnetic moment is strictly defined as the force required to keep the bar magnet or compass needle at right angles to this preferred alignment along the field. The greater the strength of the bar magnet or field, the greater the magnetic moment.; The nuclear magnetic moment when placed in a magnetic field can take only a small number of orientations and can never perfectly align itself along this field. The energy differences between these orientations are the basis of the magnetic resonance phenomenon. The size of the nuclear magnetic moment is directly proportional to the magnetogyric ratio of the element (or more specifically isotope) under investigation.
Magnetic interactions: These interactions or couplings are the basis for the MR phenomenon. Time independent or motionally averaged interactions determine the positions of nuclear resonances in the frequency spectrum. Fluctuating or time dependent magnetic couplings, particularly when they are varying at a rate close to the Larmor frequency, have a significant role in the relaxation of the nuclei under investigation.; The primary magnetic interaction is between the applied magnetic field (B₀) and the nuclear magnetic moment, the nuclear Zeeman effect. The size of this interaction is dependent upon the B₀ field strength and would be most commonly expressed as the Larmor frequency that lies in the range 5–64MHz for whole body MRI systems.
Magnetization: Magnetization is the bulk equivalent of magnetic moment and is defined as the density of magnetic moment per unit volume (of tissue in clinical MRS). There are various contributions to the overall magnetization induced when a body is placed in a magnetic field. These include the diamagnetism of the electrons and the paramagnetism of the nuclei.; Nuclear magnetism is generated by the alignment of atomic nuclei within the applied field. It represents one of the smaller components of the total magnetization.; The induced magnetization (M) generated is proportional to field (B) and this is expressed by the equation
M = χB
where c is the (nuclear) susceptibility and will vary between tissues.
Magnetic ratio, γ: The magnetogyric ratio (γ) relates the magnetic field strength (B) to the nuclear precessional or Larmor frequency (w). This is summarized by the Larmor equation
w = γB.
Note that w is the angular frequency which is simply the usual Larmor frequency expressed in Hertz multiplied by 2p.
Magnetic resonance (MR): Resonance phenomenon resulting in the absorption and/or emission of electromagnetic energy by nuclei or electrons in a static magnetic field after excitation by a resonance frequency pulse. The resonance frequency is proportional to the magnetic field and is given by the Larmor equation. Only unpaired electrons and nuclei with a non-zero spin exhibit MR.
Magnetic resonance imaging (MRI): Use of magnetic resonance to create images of hydrogen (protons), sodium, fluorine, phosphorus, etc. The image signal intensity depends on the density of the nucleus.
Magnetic resonance spectroscopy (MRS): Use of magnetic resonance to obtain spectral information in the form of spectral peaks. Peaks are analyzed according to their frequency or chemical shift (depending on substance being imaged), peak amplitude, and area under the peak (depending on number of nuclei).
Magnetic shielding: Reduction of magnetic field outside imaging area by passive ‘architectural’ measures, such as the Faraday cage for B₁ shielding and steel plates in the walls of the imaging suite for B₀ shielding.
Magnetic susceptibility: Ratio of the intensity of magnetization induced in a substance to the intensity of the magnetic field to which the substance is exposed; often expressed as a dimensionless quantity times 10–6.
Magnetite: Fe₃O₄; a cubic, close-packed crystal. It can be superparamagnetic or ferromagnetic.
Magnetization: Magnetic polarization of a material produced by a magnetic field. Magnetic moment per unit volume.
Magnetization transfer: Magnetic labeling of spectroscopic peaks and subsequent observation of chemical transfer of the magnetically labeled nuclei to other peaks.
Magnitude image: Form of image presentation, which is reconstructed from magnitude data. Magnitude data = ([Real data]2 + [Imaginary data]2)1/2. The magnitude image is generally used because of its insensitivity to phase errors.
Magnitude reconstruction: Image reconstruction technique that yields a modulus image.
MIP: Maximum-intensity projection.
Misregistration: Incorrect spatial mapping of an acquired MR signal. This artifact may be secondary to motion (flow, pulsation, respiration) chemical shift or aliasing.
M₀: Vector representation of the equilibrium value of the magnetization that is parallel to the static magnetic field. The value of M₀ is proportional to spin density. See also spin density.
MT: Magnetization transfer.
Multiple echo imaging: Imaging using a series of echoes acquired as a train following a single excitation pulse. In spin echo imaging, each echo is formed by a 180° pulse at TE_n–1 + (TE_n – TE_n–1) ÷ 2. Typically, a separate image is produced from each echo of the train.
Multislice imaging: MRI pulse sequences must be separated in order to build spatial encoding into the signals. The spatial encoding process is short, say 10.50 milliseconds (ms), in comparison to the relaxation delays (TR) that are conventionally employed to manipulate image contrast (2000–1500ms). It is apparent that there is plenty of time when there is nothing to do except wait for relaxation to take place. If all radiofrequency (RF) pulses are made slice-selective then this ‘dead time’ can be usefully employed to acquire data from different slices in an interleaved fashion.
M_xy: See transverse magnetization.
M_z: See longitudinal magnetization.
NAA: N-acetyl aspartate.
Negative enhancement: Signal intensity loss induced by a contrast agent (e.g. ferrite).
NMR: Nuclear magnetic resonance. See magnetic resonance.
Noise: This is comprised of electronic and acoustic forms. The random motions of electrons within conducting media constitute small, random electric currents. These currents give rise to spurious signals that can be detected by the sensitive MR receiver system. Electrolytes within the body, the copper within the receiver coils and its associated cabling and connectors and electronic components all contribute to the overall noise level. The aim would always be to be limited by noise generated in the patient rather than the equipment.
Non-selective pulses: RF pulses whose bandwidth contains all Larmor frequencies produced within the imaged object (i.e. the entire object is excited by the pulse).
NSA: Number of signals averaged together to determine each distinct position-encoded signal to be used in image reconstruction.
Nuclear magnetic resonance (NMR) signal: Electromagnetic signal in the RF range produced by the precession of the transverse magnetization of nuclear spins. The precession of transverse magnetization induces a voltage in a coil, which is amplified and demodulated by the receiver. The NMR signal may refer only to this induced voltage.
Nuclear spin quantum number (I): Property of all nuclei related to the largest measurable component of the nuclear angular momentum are quantized (fixed) as integral or half-integral multiples of h/2p, where h is Planck’s constant. The number of possible energy levels for a given nucleus in a fixed magnetic field is equal to 2I + 1.
Nucleon: Particles found in the nucleus of an atom (i.e. neutrons and protons).
Nucleus: The nucleus is the positively charged center of an atom composed of a number of protons and neutrons (generically known as nucleons). Hydrogen, the simplest atom, contains only a single proton. The arrangement of these nucleons, both of which individually possess magnetic moments, determine the total nuclear magnetic moment of the nucleus.
Null point: In the context of inversion recovery imaging, term used to describe the point in time at which the net magnetization passes through zero after the initial 180° pulse.
^31P: Phosphorus-31.
Paramagnetic: Substance with a positive magnetic susceptibility. Individual paramagnetic moments, non-aligned in the absence of an external magnetic field, become aligned in the presence of a magnetic field. Addition of small amounts of paramagnetic substance may greatly reduce relaxation times of water.
Paramagnetism: The component atoms of a paramagnetic material possess permanent magnetic moments. The total moment of an individual atom has contributions from the unpaired orbiting electrons and from the central nucleus. Nuclear paramagnetism is several hundred times weaker than the electronic component. However, the dominance of either will depend upon the particular atomic configuration. The atoms or ions of transition metals are typical examples of an electronic paramagnetic substance.
Partial saturation: When the time between radio frequency (RF) pulses is less than or comparable to the longitudinal relaxation time (T1) then a material is said to be partially saturated. Since there is insufficient time for full T1 relaxation between RF pulses, the signal is currently less than the equilibrium magnetization (M₀) of the tissue. The various extents of saturation amongst a variety of tissues with different T1’s are the basis for generating contrast in MR imaging.
Parts per million (ppm): Measure of the relative difference between an observed quantity and a reference value in units of one-millionth the reference value. For example, a measured frequency of 64.001MHz differs from a reference frequency of 64MHz by [(64.001–64)/64] × 1000000 = 15.625ppm.
PCr: Phosphocreatine.
Peak area: Area encompassed by a spectroscopic peak and the frequency axis. If relaxation effects are taken into account, the peak area is proportional to the concentration of the chemical species producing the peak.
Permanent magnet: Magnet whose magnetic field originates from permanently magnetized (ferromagnetic) material.
Phantom: A test object to assess performance of the scanner. The phantom may contain water doped with paramagnetic Cu2+ or Mn2+ ions in order to control the relaxation times. As an alternative, gels may be included since they may closer approximate to tissue where longitudinal relaxation times (T1) are significantly longer than their transverse counterparts (T₂).
Phase: The components of magnetization constituting the MR signal can be imagined as the hand of a clock. It has a magnitude, the length of the hand, and a phase, how far it has rotated from the 12 o’clock position. In MRI we usually calculate an array of magnitudes only, display them on a two-dimensional grid and call them an image. Alternatively, the phase of the signal may be computed, formatted to a gray scale and be presented as an image. Such practices have been useful in velocity phase encoding, where phase is linearly related to the velocity of blood for example, and field mapping where the phase is directly related to the magnetic field.
Phase angle: Phase difference of two periodically recurring phenomena, expressed in angular measure.
Phase encoding: The two-dimensional Fourier transform (2-DFT) imaging method has emerged as the dominant technique by which MR images are generated. Following slice selection magnetic field gradients are employed to localize the signal into a two-dimensional array of pixels. In the 2-DFT scheme of imaging, one direction, which we can arbitrarily call the horizontal axis, is called the frequency encode axis. The vertical direction is called the phase encode axis. The frequency encoding can be envisaged as a number of phase encode steps collected in rapid succession during one pulse cycle.; Each phase cycle under the 2-DFT scheme employs a different phase encode gradient. Usually 128 × 256 different values would be employed to generate the image. A complete scan would then take 128 × 256 times the TR of the sequence.; For a static object the differences between phase and frequency directions are minimal. However, for breathing or moving objects the fact that the phase encoding points are acquired every TR, rather than the faster rate of the frequency encoding, means this direction is the more sensitive with respect to movement.
Phased array coil: Two or more RF coils, connected to separate pre-amplifiers are used simultaneously to image a larger area, with higher SNR, than is possible with a single coil.
pHi: Intracellular pH.
Photon: Elementary quantity, or quantum, of radiant energy.
Pi: Inorganic phosphate.
Pixel: Shorthand form for picture element. Each pixel within an MR image has a thickness, say 5mm, and this can lead to partial volume effects. Under these circumstances the term voxel (volume element) is perhaps more suitable.
Planck’s constant: Constant of proportionality (6.626 × 10–27 erg-second) that relates the amount of energy emitted or absorbed by a photon to its oscillation frequency.
Planar imaging: MR signals are generated from the volume enclosed within, or in close proximity to, the receiver coil. By employing slice selective RF pulses, signals may be limited to planes within this volume. Additional field gradients are then employed to further spatially localize these signals in order to generate images. Planar imaging requires a decision as to the best imaging plane prior to the acquisition of the data. However, if sufficient numbers of narrow slices are produced, they can be employed to produce further oblique images.; As an alternative the signal from the volume may be manipulated without slice selective pulses. This is the volume scan which, once the data has been acquired, can be reformed to generate any arbitrarily oriented image plane.
PME: Phosphomonoester.
Population: Total number of nuclei or electrons in different energy levels. At thermal equilibrium the populations of the energy levels will be given by the Boltzmann distribution.
Power: Rate of energy flow, expressed in watts (W).
ppm: Parts per million.
Pre-amplifier: The MR signal is very weak and can be easily swamped by noise. The pre-amplifier is an exceptionally low noise subsystem whose role is to magnify the basic MR signal (and noise) from the receiver coil without degrading it with additional noise. This initial amplification is the most critical and subsequent stages of the signal processing are more tolerant of lower quality components.
Precession: The precession of nuclear magnets lies at the center of the classical description of the nuclear magnetic resonance phenomenon. The static magnetic field (B₀) has a twisting effect upon the individual nuclear paramagnets. These magnets trace out a conical path around the direction of the field B₀ and this is known as precession. The nuclear precessional frequency is given by γB₀, and this is known as the Larmor frequency.
Precession angle (β): Angle between the axis of gyration of a spinning body and the static magnetic field.
Pre-emphasis: The close proximity of the gradient coil system to the structures of the magnet ensure that there is always a limited rise-time to the required changes in the gradient waveform. The changing currents in the windings of the gradient coils induce currents within the bore of the magnet via magnetic interaction. This induced, or eddy current in the magnet structure generates additional magnetic field gradients which are in opposition to those actually required. This non-ideal response can be reduced by pre-emphasizing the initial sections of the input gradient drives.
Proton: Positively charged nucleon.
Proton density (ρ): Number of protons per unit volume.
Proton density-weighted image: Spin echo technique with a long TR and a short TE. Also, a gradient echo technique using very small flip angles (=10_) and long TR values (>100ms).
Proton relaxation enhancement (PRE): Enhancement of the signal intensity of hydrogen spectra or images using contrast agents.
Pulse sequence: An MR pulse sequence consists of a series of radiofrequency (RF) and the gradient pulses spaced by well-defined time intervals. The RF pulses generate the MR signal. When applied simultaneously with a magnetic field gradient the RF pulses are slice selective, that is, MR signal is produced from only a slice of tissue. The signal can be further encoded with spatial information by the application of magnetic field gradients alone.; Pulse sequences repeat themselves with the periodicity of TR (seconds). Additional commonly occurring intervals are also defined; TE, the echo time and TI, the inversion time.; The MR pulse sequence may also include locking or synchronization to physiological features such as the heart beat or respiratory motion.
Pulse, 90° (π/2 pulse): RF pulse designed to rotate the macroscopic magnetisation vector 90° about its axis, as referred to the rotating frame of reference. If the spins are initially aligned with the magnetic field, this pulse will produce transverse magnetization.
Pulse, 180° (π pulse): RF pulse designed to rotate the macroscopic magnetization vector 180° about its axis, as referred to the rotating frame of reference. If the spins are initially aligned with the magnetic field, this pulse produces an inversion.
Pulse repetition time: See TR.
Pulse shape: Profile of amplitude versus time for magnetic gradient pulses or the profile of amplitude versus frequency for RF pulses. RF pulse shape corresponds to the slice profile.
Q: Q is the symbol for the quality factor of a tuned resonant circuit. A receiver coil, which is tuned to the nuclear Larmor frequency, with a high Q will resonate over a smaller range of frequencies than that of a low Q system. The signal-to-noise (SNR) of the detected MR signal is related to the Q of the coil; a high Q coil being most desirable in this respect. The proximity of a patient, or any conducting medium, to a tuned coil system will degrade its quality factor (and also SNR). The realistic measurement of Q will require ‘loading’ of the coil with a suitable conducting material (or piece of anatomy).
Quadrature coil: A quadrature coil can be considered as two separate coils in close proximity to one another. Ideally, they do not couple together so that the noise detected in each coil is uncorrelated. When these two noise signals are combined or averaged then the noise level in increased by √2.; The two coils are arranged to be perpendicular (90°) to one another. If the two signals are combined, taking into account this 90° in the receiver electronics, then overall signal is improved by 2. The net advantage of the quadrature coil is an improvement which if it were sought by signal averaging alone, would increase the scan time by a factor of 2.
Quadrature coils: Phase-sensitive detector or demodulator that detects the components of the signal in phase and at 90°.
Quadrature excitation: Transmission of RF energy that produces a single rotating magnetic field vector (i.e. circular polarization) resulting in a more uniform distribution of RF and less RF power deposition.
Quadrature detection: Phase-sensitive detector or demodulator that detects the components of the signal in phase and 90° out of phase with a reference oscillator.
Quench
Quenching: Loss of superconductivity in the current-carrying coil causing loss of magnetic field in a superconducting magnet. Change from superconductivity releases heat, boiling off cryogenic liquid.
R1: Longitudinal relaxivity or efficiency, per unit concentration of solute, of an agent that alters T1 relaxation rates. R1 is 1/TI and is expressed in units of (mm-s)–1. See also relaxivity.
R2: Transverse relaxivity or efficiency, per unit concentration of solute, of an agent that alters T2 relaxation rates. R2 is 1/T2 and is expressed in units of (mM-s)–1.
Radiofrequency (RF): The relationship between frequency w and the magnetic field B is given by the Larmor equation (w = gB/2p). For protons (1H) at 1 Tesla the frequency is 42.6 MHz and, for the range of currently available magnets (say up to 12 Tesla for smaller bore systems), This puts the NMR frequency in the radiofrequency (RF) band of the electromagnetic spectrum. The RF band is usually defined to lie in the frequency range of 10–100 MHz. NMR is often referred to as an RF spectroscopy technique. The related technique of electron paramagnetic resonance (EPR) operates in the microwave portion of the electromagnetic spectrum.; The energy involved with each quantum of photon of RF radiation is low compared to typical molecular bond energies and this underlies the intrinsic safety aspects of MR when compared to X- and g-rays towards the higher end of the electromagnetic spectrum.; The widespread use of RF in radio and television systems can lead to problems unless measures are taken to shield the MRI scanner from all interfering sources.
Radiofrequency (RF) coil: Used for transmitting RF pulses and/or receiving NMR signals. For MRI, saddle-shaped coils or coils with solenoid configurations are most frequently used.
Radiofrequency (RF) pulse: Timed burst of RF energy of amplitude B₁ is delivered to an object by the RF transmitter. For RF frequencies near the Larmor frequency, the RF pulse will result in rotation of the macroscopic magnetization vector in the rotating frame of reference or a more complicated nutational motion in the stationary frame of reference. The angle of rotation will depend on the strength B1 and duration of the RF pulse.
Rapid imaging: Methods employed to reduce the total scan time are termed rapid imaging techniques. In general, these methods reduce one or more of the following: (i) number of excitations; (ii) pulse repetition time; and (iii) number of phase-encoding steps.
RARE (rapid acquisition with relaxation enhancement): Multiple RF or gradient echo sequence with echo-encoding, multiple phase steps.
Readout gradient: Magnetic field gradient applied for frequency encoding of the object being imaged.
Receiver: Detector for an RF signal. Receivers consist of electronic demodulators, preamplifiers and amplifiers.
Receiver coil: The detector of the MR signal is the receiver coil which would usually be defined to be fairly closely fitting to the anatomy in order to improve sensitivity. The receiver coil is tuned to the Larmor frequency which is defined by the field strength. The coil connects to a low noise pre-amplifier in order to boost the intrinsically low signals prior to meeting the outside world.; The receiver coil may physically be the same coil as the transmitter. In such cases additional electronics will be included in the transceiver system to switch between the high power transmit and low power receive phases of its operation.
Refocusing: Restoration of phase coherence by application of magnetic gradient(s) or RF pulses.
Relaxation rates: Reciprocal of the relaxation times, measured in inverse seconds.
Relaxation: The initial equilibrium polarization of nuclear magnets or spins takes a finite time to evolve following positioning within the magnet. This time period is known as relaxation. Any non-equilibrium state of the nuclear magnetization can only exist for a transitory period since relaxation processes are always working to maintain or restore the magnetization along the B₀ field. These non-equilibrium states could be achieved by rapidly changing the magnitude or direction of B₀; in this case rapid means fast compared with the timescale of the relaxation processes. A more elegant approach is to apply RF pulses to realign magnetization away from its preferred orientation along B₀. The relaxation behavior of nuclear spins, and in particular that of water (1H), underpins the diversity of image contrast available in MRI.
Relaxivity: Efficiency of relaxation enhancement expressed in units of (mM-s)–1.
Repetition time (TR): The time between consecutive repetitions of a pulse sequence is labeled as TR. It is usual to say that if TR exceeds the five times the longitudinal relaxation time T1 then full relaxation has occurred and the spin system has no ‘memory’ of its previous history. Variations in the TR of a particular sequence can be employed to vary the T1 contrast in the resultant image.
Rephasing: By applying a 180° pulse, out of phase spins are returned back into phase.
Resistive magnet: Magnet whose magnetic field originates from current flowing through an ordinary, non-superconducting conductor.
Resolution: The smallest distance between distinct objects or features in an image. For a fixed field of view image higher resolution requires a longer data acquisition period. The resolution of the acquired data is a function of the amplitude of the gradient pulses and their duration. The appearance of the processed image may be improved by interpolating the data to a finer matrix; however, no additional detail results. Routine resolution in the plane of the image may be in the range 1–2mm and it might have a depth or slice thickness, of perhaps 5mm.
Resonance: A mechanical system such as a child’s swing has a natural frequency at which it will oscillate if left to its own devices. If such an oscillatory system is driven by some periodic force then it will undergo forced oscillations. When the driver is at the natural frequency then resonance is said to have occurred; the amplitude of the oscillation builds up rapidly as energy is absorbed from the driver, in this case the exhausted parent. At other frequencies, slower and faster than the natural frequency, there is a flow of energy back and forth between oscillator and driver with no net gain by either system.
Resonance frequency: Frequency at which resonance phenomenon occurs, given by the Larmor equation for MRI.
RF spoiled FAST: Gradient echo technique that provides superior T1 contrasts. The technique removes effects of steady-state contributions without gradients by RF spoiling.
RF spoiling: Component of several fast scanning techniques having ‘digital’ RF systems. RF (as opposed to gradient) spoiling randomizes the phase of sequential RF pulses such that any residual transverse magnetization does not contribute to the subsequent signal.
Rotating frame of reference: Frame of reference that is rotating around the axis of the static magnetic field B₀ (with respect to a stationary [‘laboratory’] frame of reference) at a frequency equal to the nuclear precessional frequency. Although B1 is a rotating vector, it appears stationary in the rotating frame during resonance, leading to simpler mathematical formulations.
Saddle coil: RF coil design (resembling a saddle) frequently used when the static magnetic field is coaxial with the axis of a magnet (i.e. typically used solenoidal type of superconducting magnets).
Sampling: The MR signal is a continuous voltage waveform that must be sampled or digitized into discrete steps for input into a computer prior to processing into an image. The sampling occurs at intervals in time during the evolution of the signal. During an MRI data acquisition the signal may typically be sampled every 10ms. The voltage, which may now have been amplified to somewhere between –10 and +10V is digitized, for example, into one of 16384 discrete levels.
Saturation: Non-equilibrium state in which equal numbers of spins are aligned against the magnetic field so that there is no net magnetization. Saturation can be produced by repeated RF pulses at the Larmor frequency with a TR much less than T1.
Saturation pulse: Technique for decreasing flow artifacts by selectively saturating spins located outside the image volume. For example, blood or CSF spins flowing into the image volume are so heavily saturated that they do not cause flow artifacts arising from flow-related enhancement.
Saturation recovery (SR): Pulse sequence characterized by two sequential 90° pulses, a saturation pulse and a detection pulse, with signal collected after the detection pulse. In this manner, magnetization builds up exponentially from zero to a value determined by the interpulse interval (time between saturation pulse and detection pulse). Short TR spin echo pulse sequences in clinical use are occasionally referred to as SR sequences.
SE: See spin echo imaging.
Selective excitation: A RF pulse, by virtue of its limited duration, contains only a restricted range of frequencies, and as such can only excite a portion of the MR spectrum. Narrow excitation bandwidths are achieved with longer RF pulses. A pulse of duration t_p seconds will have an approximate bandwidth in the region of 1/t_p Hertz, a 1ms (10–3s) pulse has a bandwidth in the region of 1kHz (103Hz). By carefully tailoring the exact pulse shape and its carrier or center frequency it is possible to selectively excite a given portion of the MR spectrum.
Sensitive volume: Region from which an MR signal is acquired because of strong magnetic field inhomogeneity elsewhere.
Shielding: The interference caused by external RF sources such as hospital paging and computer systems can introduce artifacts and seriously degrade the signal-to-noise ratio of resultant MR images. The MR scanner is protected from its RF environment by Faraday shielding. ECG leads and other patient monitoring equipment that may breach the Faraday shield will have to be RF filtered to prevent interfering signals propagating through the wires.
Shim coils: Electric coils used to correct for inhomogeneities in the static magnetic field of an MR system.
Shimming: The process of improving the quality or homogeneity of the static polarizing field B₀ is known as shimming. The intrinsic inhomogeneity of a magnet in a particular hospital site can be improved at the time of installation with additional current carrying coils within the construction of the magnet itself. These shim coils may take currents of several amperes in order to generate additional corrective magnetic fields. As an alternative small pieces of ferromagnetic material can be accurately positioned within the bore of a magnet to produce a similar compensating and shimming effect.
SI: International standard of physical units and measures which supersedes the meter–kilogram–second (MKS) and centimeter–gram–second (CGS) systems.
Signal averaging: Repetition of signal acquisition in an imaging plane. As more signals are averaged, signal-to-noise ratio increases (as the square root of the number of averages) and artifacts are reduced. However, scan time increases to obtain the additional signals.
Signal-to-noise ratio (SNR, S/N): Describes the relative contributions to a detected signal of the true signal and random superimposed signals (‘noise’). SNR can be improved by increasing the number of signals averaged by increasing field strength, or by increasing the size of the imaging voxels.
Sinc pulse: RF pulse modulated by a sinc function, i.e. sin(x)/x.
Slice: Physical extent of the planar region being imaged.
Slice profile: Spatial distribution of sensitivity of the imaging process in the direction perpendicular to the plane of the slice.
Slice selection: When an RF pulse is applied at the same time as a magnetic field gradient the frequency excitation can be expected to be heavily spatially dependent. If all pulses within an MR sequence are slice selective then during the necessary relaxation delays additional slices can be excited.
Slice-selective excitation: Exclusive excitation of protons in one slice. Slice-selective excitation is performed by applying a gradient magnetic field G_z and a narrow bandwidth or slice-selective RF pulse. The range of frequencies in a slice-selective pulse, via the Larmor equation relationship, corresponds to a specific range of magnetic field strengths along the slice-selection gradient G_z.
Slice thickness: Thickness of an imaging slice. Since the slice profile may not be sharply edged, a criterion such as the distance between the points at half the maximum value (FWHM) or the equivalent rectangular width (the width of a rectangular slice profile with the same maximum height and same area) is generally used.
Software: Set of instructions and programs (supervising ‘executive’ programs, data acquisition programs, data-processing programs such as image reconstruction, display programs) that controls the activities of the computer.
Solenoid coil: Coil wound in the form of a cylinder. When a current is passed through the coil, the magnetic field within the coil is nearly uniform.
Spatial frequency: Signal frequency related to the spatial coordinate.
Specific absorption rate (SAR): Electromagnetic radiation can deposit energy by inducing small currents in the electrolytes of the body. The absorbed energy manifests itself principally in the form of heat. The specific absorption rate is defined as the energy deposited per second into a kilogram of tissue. It has the units of watts per kilogram (W/kg).
Spectral line: Frequency or narrow band of frequencies, depending on resolution, that corresponds to a particular chemical shift.
Spectral width: Width of frequency (in Hz) that is selected for a particular NMR spectrum to be observed.
Spectrometer: MR apparatus that actually produces the NMR phenomenon and acquires the signals. Components of a spectrometer include the magnet, the probe, the RF circuitry, and the gradient coils.
Spectroscopy: In the general sense spectroscopy is the separation of a signal into its component frequencies. The decomposition of white light into seven distinct colors is the common example. Spectroscopic techniques now exist throughout the electromagnet spectrum from the lower radiofrequencies (NMR), electron paramagnetic resonance (EPR), infrared, optical and ultraviolet and beyond. The methods are employed as probes to molecular and atomic structure.
Spectrum: Array of the frequency components of the MR signal. Nuclei with different resonant frequencies will show up as peaks at their corresponding frequencies.
Spin: Intrinsic angular momentum of an elementary particle (nuclei, electrons, etc.) that is also responsible for the magnetic moment of the particle. Spins of nuclei have characteristic fixed values. Pairs of neutrons and protons align, thus canceling out their spins, so that only nuclei with odd numbers of neutrons and/or protons will have a net non-zero rotational component characterized by an integer or half-integer quantum nuclear spin number (I).
Spin density: Density of resonating spins in a given volume. Spin density is one of the principal determinants of the strength of the MR signal (i.e. the higher the spin density the higher the signal received).
Spin echo imaging: Imaging of a spin echo formed by sequence of RF pulses and gradient reversals. The standard spin echo sequence uses an RF pulse sequence consisting of a 90° excitation pulse followed by a 180° echo-rephasing pulse.
Spin lattice relaxation times: See T1.
Spin–spin coupling: Interaction between nuclei in the same molecule, resulting in a splitting of a single resonance line into two or more lines. For example, a carbon nucleus with one directly bonded proton will be a doublet.
Spin–spin relaxation time: See T2.
Spin tagging: Nuclei retain their magnetic orientation for a short time (on the order of the corresponding T1 value) even in the presence of motion or chemical exchange. If nuclei have their spin orientation changed, the altered spins serve as a ‘tag’ to trace the motion of any fluid.
Spin warp imaging: Most common form of Fourier transform imaging in which phase-encoding gradient pulses with constant duration but varying amplitude are applied to one of three spatial dimensions; the second dimension is frequency encoded and the third dimension is defined by slice selection.
SPIO: Superparamagnetic iron oxide.
Static magnetic field: Constant magnetic field in an MR system (B₀).
Statistical noise: Point-to-point variation in signal intensity caused by random fluctuations (e.g. Brownian motion) at each voxel. This is contradistinction to non-random fluctuations from macroscopic motion and flow.
STIR (short T1 inversion-time recovery): Inversion recovery technique that produces images in which T1- and T2-dependent contrasts are additive. STIR imaging is used to suppress the signal of short T1 tissues such as fat.
Superconducting magnet: Device that creates a magnetic field by use of electric current flowing through a superconductor.
Superconductor: Substance whose electric resistance disappears, usually at temperatures near absolute zero. A frequently used superconductor in MR system magnets in niobium–titanium, embedded in a copper matrix.
Superparamagnetic: The magnetic susceptibilities of superparamagnetic materials are similar to ferromagnetic materials and are much larger than paramagnetic materials. Unlike ferromagnetic materials, superparamagnetic materials do not exhibit residual magnetism when the external magnetic field is removed.
Suppression: It is not uncommon in MR for there to be very strong signals in close proximity to more interesting but weaker areas. In MRI the intense lipid signals, exacerbated by the use of a surface coil, can dominate the image. In MRS the strong water resonance, representing 100m 1H, totally overwhelms the weaker spectroscopically interesting metabolite resonances. In both cases signal suppression techniques can be employed to reduce the amplitude of the offending signals.
Surface coil: RF that is placed close to the surface of the object being imaged. Surface coils increase signal-to-noise ratio for regions close to the coil, permitting increased resolution, and sometimes help decrease motion artifacts.
Susceptibility imaging: Gradient echo sequence that relates fluctuations of the actual tissue susceptibility to the measured field deviations.
T1 or T1 (‘T-one’): Spin lattice, thermal, or longitudinal relaxation time measured in milliseconds. T1 reflects the characteristic time constant for spins to align themselves with the external magnetic field. Starting from zero magnetization in the z direction, the z magnetization will grow to 63% of its final maximum value M₀ in a time T1 (i.e. M_z/M₀ = 1 – exp [–t/T1]).
T1 shortening: Decrease of the spin lattice relaxation time or increase in relaxation rate caused by MR contrast agents or macromolecular binding.
T1 weighted: MR sequence such as short TR/short TE spin echo or inversion recovery designed to distinguish tissues with differing T1 relaxation times.
T2 or T₂ (‘T-two’): Spin–spin or transverse relaxation time. T2 reflects the characteristic time constant for loss of transverse magnetization M_xy and MR signal. Starting from a non-zero value of the magnetization of the xy plane M₀, the x magnetization, decays to 37% of its initial value in a time T2 (i.e. M_xy/M₀ = exp[–t/T2]).
T2* (‘T-two star’): (1) Effective spin–spin relaxation time (faster than T2, it includes effects of static field inhomogeneities, intrinsic to tissue or B₀ imperfections).; (2) Observed time constant of the free induction decay.
T2 shortening: Decrease of the spin–spin relaxation time caused by molecular diffusion, paramagnetic molecules, ferromagnetic particles, or local magnetic susceptibility effects.
T2 weighted: MR sequence such as spin echo with a long TR/long TE designed to distinguish tissues with differing T2 relaxation times.
T1, T₂, T₂*: The time constants associated with longitudinal (T1) and transverse (T₂) relaxation. These relaxation processes are usually assumed to be exponential in character and are given by the analytical solution to the Bloch equations. In cases where this is not true then a sum of exponential relaxation processes may physically be a realistic model to allow characterization of the relaxation behavior.; Following a 90° RF pulse there is no component of magnetization aligned along the applied field B₀; after a period of T1 there will be 63% growth of the magnetization along B₀ towards the equilibrium M₀. During T₂ transverse components of magnetization decay by 63% from their value towards zero. Usually five times the relaxation time, either T1 or T₂ is taken to be the time necessary for the respective components of nuclear magnetization to fully relax.; T₂* is employed to denote the actual observed decay in transverse relaxation. The distinction is required since T₂* depends upon the experimental conditions under which the signal is observed to be decaying. In the simplest case, in the absence of imaging field gradients following a single RF pulse for example, this merits that the field inhomogeneity (dB₀) can also contribute to the decay of the MR signal.
TE: Echo time. Time between the center of the 90° pulse and the center of the spin echo. For multiple echoes, TEs are designed numerically as TE1, TE2 and so on.
Tesla: Named after Nikolai Tesla (1870–1943), the Tesla is the SI unit of magnetic flux density and is given the symbol T. Commercial MR scanners usually operate in the range of 0.2–2.0T. One Tesla equates to 10000 Gauss, the equivalent CGS unit.
Three-dimensional Fourier transform (3-DFT): Mathematical technique that constructs a 3D image from acquired MR data. Two dimensions are constructed using the standard 2-DFT technique. Positional information for the third dimension is obtained in the same manner as the second dimension of the 2-DFT (i.e. by using phase encoding gradients in the z direction).
TI: Inversion time (used in inversion recovery). Time between the inverting (180°) RF pulse and the subsequent exciting (90°) pulse.
Time of flight signal loss: Phenomenon of the decreased signal resulting from protons travelling through a selected slice too quickly to acquire the 90° excitation pulse and subsequent 180° rephasing pulse. These protons are therefore unable to emit signal.
Tip angle: Angle between bulk magnetization vector before and after an RF excitation pulse.
Tissue characteristics: Tissues parameters that determine MR signal intensity, such as spin density, relaxation times, chemical shift and motion.
TR: Repetition time. Time between the beginning of one pulse sequence and the beginning of the succeeding pulse sequence at a specified tissue location. See also imaging cycle.
Transmitter: Portion of the MR systems that produces the RF current and delivers it to the transmitting coil.
Transmitter coil: A coil system capable of handling high RF current. The current, as it flows around the coil, generates a time dependent of RF magnetic field; this is generally referred to as B₁ to compare with the static polarizing field B₀. The current in the transmitter coil is switched on and off, or pulsed, under computer control in order to generate the required RF pulse sequences.
Transverse magnetization (M_xy): Component of the macroscopic magnetization vector in the right angles to the static magnetic field (B₀). Precession of the transverse magnetization at the Larmor frequency produces the detectable MR signal, which decays to zero (with a characteristic time constant of T2 or T2*) when the externally applied RF magnetic field is switched off.
Transverse relaxation: The equilibrium magnetization is aligned along the z-axis, defined by the static magnetic field B₀ and there are no components in the transverse or xy-plane. Following an RF pulse any transverse magnetization must decay to zero and this occurs at the molecular level by transverse relaxation mechanisms.
Tuning: The adjustment of the frequency to a specific volume where resonance can occur is known as tuning. The concept is familiar when selecting a specific radio station. In MR terms the RF system must be matched to the Larmor frequency. Individual coils within the MR system, be they high powered transmitter coils or receiver coils, must also be tuned to the Larmor frequency for optimum performance.
TurboFLASH: An ultrafast imaging technique that employs a separate magnetization preparation (MP) period before a standard FLASH sequence with a short TR (5–10ms).
Two-dimensional Fourier transform (2-DFT): MR technique in which data are reconstructed mathematically into a 2D image. Brightness of pixels is proportional to the intensity of MR signal from the corresponding region of the imaged object.
Vector: Mathematical quantity having both magnitude and direction. Vectors are represented by an arrow whose length is proportional to the magnitude and an arrowhead indicating the direction.
View (projection): Data obtained from sampling of the free induction decay or spin echo. For the generation of an image, multiple views must be acquired (e.g. 128 or 256).
Volume averaging: MR technique in which signals are received from the whole object being imaged rather than from a slice. Advantages of volume imaging include improvement in signal-to-noise ratio.
Volume imaging: In the absence of any magnetic field gradients all parts of the body within the transmitter coil are excited by an RF pulse. If the receiver coil surrounds a large volume of this excited region then the signal can be manipulated for volumetric imaging.; Volume imaging can be achieved with frequency encoding and two phase encode axes – a three dimensional Fourier transform (3-DFT) technique. A volume scan will take n times longer than its planar equivalent, where n is the number of points required in the third dimension. Unless n is large then a multislice dataset may be competitive in terms of coverage of the required anatomy.
Voxel: Shorthand form for the volume element of an image. Voxel emphasises that the smallest image element not only has an in-plane dimension but also has a thickness or depth.
Water-suppression techniques: Elimination of a water signal by saturation of the water resonance or by selective excitation of the non-water region.
Wavelength: Distance between points of corresponding phase of a periodic wave.
Weighting: A variety of tissue characteristics interact with any selected pulse sequence to influence the signal intensity of reach tissue and thereby determine image contrast. When one tissue characteristic is the major determinant of the image contrast, the image is said to be weighted by that tissue characteristic.
x,y,z: These refer to the three principal axes in a Cartesian coordinate system, that is the usual rectangular system that can be applied to rooms, buildings and the like. x, y and z are said to be in the laboratory frame of reference if they are attached to the system hardware associated with the MR scanner. Convention has it that the applied magnetic field defines the z-axis of the coordinate system, Since the majority of systems today are based upon the superconducting solenoid magnet this means that z usually runs from head to foot in the patient. The x and y axes are not only perpendicular to z but also at right angles with respect to each other.
x*, y*, z*: These also refer to a Cartesian system of coordinates but the inclusion of the superscript prime indicates that these axes refer to a rotating frame of reference. The choice of the ‘rotation’ is entirely arbitrary but is selected to simplify the description of the movements of nuclear magnetization. This is true for the visualization in the mind and also the underlying mathematics. In MR the rotating frame is usually taken to be attached to the precession of a particular group of nuclei in the magnetic field. As such the z and z* axes are one and the same and the x, y and x*, y* axes are simply rotating with respect to one another. The literature is apt to be rather loose about the coordinate system or frame of reference in which nuclei or magnetization are being discussed.
Zero filling: Dummy data points (usually zero) added to the actual sample values to increase digital resolution before the Fourier transform is processed.
Zeugmatography: A term coined from Greek roots to mean MRI. Positively the last word in Magnetic Resonance Imaging.

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Glossary of physics terms

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References