MRI Glossary

This is an incomplete and evolving glossary of random MRI terms that I have found useful to know. Sources include personal communication with experts, textbooks, the internet and personal knowledge. Disclaimer: The definitions are not indepth, but rather a starting point for people. Please let me know if you see any errors or would like any words added.

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Aliasing (Wraparound): occurs in MR imaging whenever the area of anatomy extends beyond the field of view. The areas extending beyond the field of view boundaries are aliased or wrapped back into the image to appear at artifactual locations (for example, in a sagital plane, the tip of someone’s nose can wrap and appear behind their head). Aliasing is a consequence of sampling the MR signal in which any components of the signal that are at a higher frequency than the Nyquist limit will be “folded” in the spectrum so that they appear to be at a lower frequency. For example, say we have a gradient in the x-direction with a frequency ranging from +fmax to -fmax across the field of view. However, the gradient does not stop at the end of the field of view. Therefore parts of the body outside of the field of view will receive a magnetic field that will generate a frequency higher or lower than +fmax or -fmax, respectively. The computer cannot recognize the frequencies above +fmax and below -fmax, so they will be recognized as frequencies within the bandwidth. The higher frequency will be recognized as a lower frequency within the accepted bandwidth or the lower frequency will be recognized as a higher frequency within the accepted bandwidth. There are several ways to eliminate aliasing including (1) using a surface coil that only covers the area within the field of view, (2) increasing the field of view and (3) oversampling of either frequency (no frequency wrap) or phase (no phase wrap), (4) saturation pulses which saturate signals coming from outside the field of view and (5) 3D imaging if aliasing occurs along the slice-select axis then the first and last slices can be discarded.

Arterial spin labeling (ASL): an MRI technique for measuring blood flow. Unlike conventional perfusion imaging which uses a contrast agent like Gadolinium to tag the blood, ASL tags the blood directly. Arterial blood is magnetically tagged before it enters into the tissue of interest (the brain) through the use of an inversion slab in the neck and the amount of labelling is measured and can be compared to a control recording obtained without spin labelling.

Artifact: Any feature that appears in an image which is not present in the original imaged object. Artifacts can result from many different things including subject motion during scanning, presence of field or signal distorting objects (i.e. dental retainers), movement of body fluids during acquisition (flow artifact), partial voluming effects or image wrap-around as a result of an improperly chosen field of view. Examples of artifacts include aliasing, chemical shift, cross talk, magic angle, motion, truncation and zipper.

Attenuation: a reduction of power, for example due to passage through a medium or electrical component. Attenuation in electrical systems is commonly expressed in dB.

Attenuator: Device which reduces a signal by a specific amount, commonly given in dB

Bandwidth: an all-inclusive term referring to the preselected band or range of frequencies which can govern both slice select and signal sampling. There is an inverse relationship between bandwidth and signal-to-noise ratio. If we go to a wider bandwidth, we include more noise and the SNR decreases. If we decrease the bandwidth, we allow less noise to come through and the SNR increases. A decreased bandwidth will also lead to an increased chemical shift artifact.

Bird Cage Coil: An RF volume coil designed to produce a homogeneous B1 field by using multiple parallel conductors symmetrically spaced around the surface of a cylinder, connected by end rings. These are turned into low pass or high pass filter sections by adding capacitors in each conductor, or between each conductor in the end rings, so that at resonance there is a resulting homogeneous B1 field. This type of volume coil is used for brain (head) MRI, or MR imaging of joints, such as the wrist or knees.

Black blood magnetic resonance angiography: an MR technique where flowing blood appears dark. This technique was developed for cardiovascular imaging. The signal from blood is decreased with reference to the myocardium, making it easier to perform cardiac chamber segmentation. With this method, a pair of nonselective and selective 180° inversion pulses are used, followed by a long inversion time to null signal from inflowing blood. A second selective inversion pulse can also be applied with short inversion time to null the fat signal.

Blood oxygen level dependant effect (BOLD): A change in MRI-measurable signal caused by changes in the amount of oxygenated hemoglobin available in the venous circulation of the brain. Oxygenated hemoglobin has a smaller magnetic susceptibility than deoxygenated hemoglobin. Neural activation causes an increase in blood flow which is much higher than the corresponding oxygen consumption, leading to an increased amount of oxygenated hemoglobin in the vasculature around the activation area. Oxygenated hemoglobin has a higher T2* than deoxygenated hemoglobin, due to its smaller magnetic susceptibility. As T2* increases, higher signal is measured on T2*-weighted gradient-echo images, yielding a positive signal of increased venous circulation.

Carr-Purcell (CP) sequence: a 90° RF pulse followed by repeated 180° RF pulses to produce a train of spin echoes. Useful for measuring T2.

Carr-Purcell-Meiboom-Gill (CPMG) sequence: Modification of Carr-Purcell RF pulse sequence with 90° phase shift in the rotating frame of reference between the 90° pulse and the subsequent 180° pulses in order to reduce accumulating effects of imperfections in the 180° pulses. Useful for measuring T2.

Chemical shift: The change in the Larmor frequency of a given nucleus when bound in different sites in a molecule, due to the magnetic shielding effects of the electron orbitals. Chemical shifts make it possible to differentiate different molecular compounds and different sites within the molecules using high-resolution MR spectra. The amount of the shift is proportional to magnetic field strength and is usually specified in parts per million (ppm) of the resonance frequency. The actual frequency measured for a given spectral line may depend on environmental factors such as effects on the local magnetic field strength due to variations of magnetic susceptibility.

Chemical shift artifact: bright or dark outlines predominantly at fat / water (and silicone) interfaces. The artifact is due to the differences in resonance frequencies between fat and water. It occurs in the frequency encoding direction where a shift in the detected anatomy occurs because fat resonates at a slightly lower frequency than water. It can also occur in the phase encoding direction in sequences where more than one k-space line is acquired in a single TR. This frequency difference results from the different electron environments of the protons of water and of fat (due to the effect of the electron cloud to greater or lesser degree shielding the nucleus from the external static magnetic field (Bo)). The Larmor frequency which determines the frequency at which a particular nucleus resonates is established at the nucleus, and therefore different tissues will have slightly different Larmor frequencies depending on their chemical composition. The difference in chemical shift between water and fat is approximately 3.5 parts-per-million (ppm) which at 1.5 Tesla corresponds to a frequency difference between that of fat and water of approximately 220 Hz. Chemical shift artifact increases with decreasing gradient strength and narrowing bandwidth.

Coil: an electrical device composed of a single or multiple loop(s) wire that can either generate a magnetic field from current flowing through the wire (gradient coil, RF coil) or detect a changing (oscillating) magnetic field as an electric current induced in the wire (RF coil). Several types of coils are used in MRI and they include: (1) Gradient coils: (a) imaging gradient coil, (b) shim coil and (2) Transmit and/or Receive RF coils: (a) single phase or quadrature (receive or transmit), (b) surface or volume coil (Helmholtz or solenoid), (c) single or phased-array, (d) bird cage coil.

Complex data: Numerical data with a real and an imaginary component.

Cross-talk: an artifact introduced into images by interference between adjacent slices of a scan. It occurs due to imperfect slice excitation leading to a slice profile that is not ideal. This artifact can be reduced by using a gap between slices, interleaving slices and utilizing optimized (but longer) rf pulses.

Crusher: When a pulse sequence contains nonideal refocusing RF pulses (for example flip angles not equal to 180 degrees), stimulated echoes can be produced in addition to spin echoes. These signals can carry inconsistent spatial information encoded in their phase, leading to errors in the acquired k-space data. A crusher gradient is a correction gradient that preserves the desired signal while eliminating the unwanted signal by manipulating the phase of the signals. In quantitative T2 measurement using CPMG sequence, crushers are used largely to remove signal that was outside of the slice selected by the initial 90 degree pulse. In this case, removing stimulated echoes with crushers may not be a good idea since the signal will then decay more quickly, making measurement difficult. Crushers are routinely used in clinical sequences to get rid of moving spins, ie to make vessels black.

Cryostat: A device that maintains a constant low temperature (as by means of liquid helium). Requires vacuum chambers to help with thermal isolation.

Dephasing: The loss of magnetization in the transverse plane, typically due to the fact that different magnetic dipoles of different nuclei are precessing about the main magnetic field, Bo, at slightly different precessional frequencies and therefore lose phase coherence.

Diamagnetic: A substance that will slightly decrease a magnetic field when placed within it (its magnetization is oppositely directed to the magnetic field, i.e., with a small negative magnetic susceptibility)

Dipole: A magnetic field characterized by its own north and south magnetic poles separated by a finite distance

Dipole-dipole interaction: Interaction between a spin and its neighbors due to their magnetic dipole moments. This is an important mechanism contributing to relaxation rates. In solids and viscous liquids this can result in broadening of the spectral lines.

Echo time (TE): is the amount of time in milliseconds between the application of the 90° pulse and the peak of the echo signal in spin echo and gradient echo pulse sequences.

Eddy current: an induced spurious electrical current produced by time-varying magnetic fields. Electric currents can be induced in a conductor by a changing magnetic field or by motion of the conductor through a magnetic field. Eddy currents can cause artifacts in images and may seriously degrade overall magnet performance. Eddy currents are especially bad for magnetic resonance spectroscopy.

Fat suppression: the suppression of fat signal is used in MRI when the fat signal causes artefacts or otherwise obscures a tissue of interest. Hydrogen protons from lipid and from water behave differently during an MR imaging acquisition, and fat suppression techniques are based on these differences. Two major properties are involved: First, there is a small difference in resonance frequency, between lipid and water protons, which is related to the different electronic environments. This so-called chemical shift allows frequency-selective fat saturation. Second, the difference in T1 between adipose tissue and water can be used to suppress the fat signal with inversion-recovery techniques. Fat suppression techniques include: Short inversion-Time Inversion Recovery (STIR), Spectrally-Selective RF Pulses, Composite RF Pulses and Regional Saturation Bands.

Ferromagnetic: A substance, such as iron, that has a large positive magnetic susceptibility.

Field of view: the region over which MR data is collected. The dimensions are controlled by the application of frequency encoding and phase encoding gradients.

Flip angle: the angle to which the net magnetization is rotated or tipped relative to the main magnetic field direction via the application of an RF excitation pulse at, or close to, the Larmor frequency.

Fourier transform: a mathematical procedure which analyzes and separates amplitudes and phases of the individual frequency components of the complex time varying signal. Fourier transform analysis allows spatial information to be reconstructed from the raw MRI data (k-space).

Frequency encoding: Method of spatially localizing an MR signal in one dimension by using a magnetic field gradient along that dimension to vary the frequency dependent on position. The gradient is applied during the time the echo is received (ie during the read out).

Gadolinium: a paramagnetic contrast enhancement agent utilized in MR imaging. When injected during the scan, gadolinium will tend to change signal intensities by shortening T1 in its surroundings. Although toxic by itself, it can be given safely in a chelated form such as Gd-DTPA, which still retains much of its strong effect on relaxation times.

Gauss (G): a unit of magnetic field strength that is approximately the strength of the earth’s magnetic field at its surface (the earth’s field is about 0.5 to 1G). The value of 1 gauss is defined as 1 line of flux per cm2. As larger magnetic fields have become commonplace, the unit gauss (G) has been largely replaced by the more practical unit tesla (T), where 1 T = 10,000 G

Gaussian noise: Noise distributed in a normal (Gaussian) pattern. In such a distribution, approximately 65% of all points fall within one standard deviation (s) of the mean

Ghosting artifact: Displaced duplications of an image feature in the phase encoding direction. Ghosting artifacts are in the most cases caused by movements (e.g., respiratory motion, bowel motion, arterial pulsations, swallowing, and heartbeat). Ghosting artifacts can be reduced by respiratory and cardiac triggering, the use of breath holding pulse sequences, flow compensation or presaturation pulses, depending on their origin

Gradient: a variation in the magnetic field with respect to distance.

Gradient coil: Current carrying coils designed to produce a desired magnetic field gradient (so that the magnetic field will be stronger in some locations than others). Gradient coils cause an intentional perturbation in the magnetic field homogeneity (usually in a linear fashion), which allows one to decipher spatial information from the received signal and localize it in space. This perturbation in magnetic field is several orders of magnitude smaller than the external magnetic field. Three orthogonal gradient coils are used, corresponding to the X, Y and Z axes. The gradients are refered to as the slice select gradient, the phase-encoding gradient and the frequency-encoding or readout gradient.

Gyromagnetic ratio: a constant for any given nucleus that relates the nuclear MR frequency and the strength of the external magnetic field. It represents the ratio of the magnetic moment (field strength) to the angular momentum (frequency) of a particle. The value of the gyromagnetic ratio for hydrogen (1H) is 4,258 Hz/Gauss (42.58 MHz/Tesla).

Hemodynamic response: Changes in blood flow, blood volume, and blood oxygenation as a result of local neural activity

Homogeneity: uniformity of the main magnetic field. An important criterion of the quality of the magnet.

Inhomogeneity: lack of homogeneity or uniformity in the main magnetic field.

Interleaving: a method of MRI slice acquisition whereby odd slices are collected first, followed by even slices (or vice versa). Eliminates cross-talk between slices.

J-coupling: an indirect scalar interaction between two nuclear spins which arises from hyperfine interactions between the nuclei and local electrons. J-coupling contains information about bond distance and angles. Most importantly, J-coupling provides information on the connectivity of molecules. In NMR spectroscopy, it is responsible for the appearance of many signals in the NMR spectra of fairly simple molecules. Also called indirect dipole-dipole coupling.

K-space: the Fourier transform of the MR image measured. Its complex values are sampled during an MR measurement, in a premeditated scheme controlled by the pulse sequence. Conventional MR pulse sequences such as spin-echo and gradient-echo imaging fill a single line of k-space with each data measurement (one line per TR). A different phase encoding step is used to fill out another parallel line of k-space. The full set of measurements completes a Cartesian grid of points in k-space with the same number of rows and columns as the final image. Other options for k-space filling include radial filling (back-projection imaging) or spiral filling (spiral imaging). The center of k-space corresponds to image contrast while the periphery of k-space corresponds to fine details.

Larmour frequency: the frequency of precession for nuclear spins in the presence of magnetic field. The frequency varies directly with magnetic field strength, and is normally in the radio frequency (RF) range for the field strengths used in NMR or MRI. The frequency of precession of the nuclear magnetic moment is directly proportional to the product of the magnetic field strength and the gyromagnetic ratio. For protons (hydrogen nuclei), the Larmor frequency is 42.58 MHz/tesla.

Longitudinal magnetization: the component of the macroscopic magnetization vector along the static magnetic field

Longitudinal relaxation: Also known as T1 relaxation. The return of longitudinal magnetization to its equilibrium value after excitation due to the exchange of energy between the nuclear spins and the lattice.

MR angiography (MRA): MR image visualization of selected vascular structures, such as the Circle Of Willis or the carotid arteries. MRA is used to evaluate arteries for stenosis, occlusion and aneurysms. Fine vessels and superior blood supply are not well seen with MRA due to limitations of the physical properties of the smaller vessels and due to loss of signal at skull apex. A variety of MR acquisition techniques exist, including contrast-enhanced, time of flight (relying on the flow of unsaturated blood into a magnetized presaturated slice – the difference between the unsaturated and presaturated spins creates a bright vascular image without the invasive use of contrast media) and phase contrast (utilizes the change in the phase shifts of the flowing protons in the region of interest to create an image).

Magic angle: a precisely defined angle with value of approximately 54.7°. Two nuclei with a dipolar coupling vector at an angle of approximately 54.7° to a strong external magnetic field will have zero dipolar coupling. The magic angle is a root of a second-order Legendre polynomial, P2((cos(theta))=0, and so any interaction which depends on this second-order Legendre polynomial vanishes at the magic angle. The magic angle artifact refers to the increased signal on sequences with short echo time (TE) in MR images seen in tissues with well-ordered collagen fibers in one direction (e.g., tendon or articular hyaline cartilage). This artifact occurs when the angle such fibers make with the magnetic field is equal to the magic angle. A bright signal from this artifact is commonly seen in the rotator cuff and occasionally in the patellar tendon and elsewhere.

Magic angle spinning: a technique in solid-state NMR spectroscopy, which employs the magic angle effect to remove or reduce dipolar couplings, thereby increasing spectral resolution.

Magnetic susceptibility: the extent to which a material becomes magnetized when placed within a magnetic field. Differences in magnetic susceptibilities at tissue borders are a frequent cause of MRI artifacts.

Myelin water imaging: imaging of myelin based on the short T2 component of the T2 relaxation distribution.

Motion artifact: occurs when the object being imaged during the sequence moves, which results in inconsistencies in the phase and amplitude, leading to blurring and ghosting. Causes of motion artifacts can also be mechanical vibrations, cryogen boiling, large iron objects moving in the fringe field (e.g. an elevator), as well as sample motion. These artifacts appear in the phase encoding direction when the motion is in the phase encoding direction. One way to minimize motion artifacts is to set the frequency encoding direction in the direction of the motion.

Nuclear Overhauser effect: A change in the steady state magnetization of a particular nucleus due to irradiation of a neighboring nucleus with, which it is coupled by means of a spin spin coupling interaction. This interaction must be the primary relaxation mechanism of these nuclei. Such an effect can occur during decoupling and must be taken into account for accurate intensity determinations during such procedures.

Nyquist frequency: Frequency of a signal beyond which aliasing will occur in the sampling process. This frequency is equal to one half the sampling rate. For example, if we have a gradient in the x-direction (Gx) with a maximum f (fmax) at one end of the field of view, and a minimum frequency (-fmax) at the other end of the field of view – these are the Nyquist frequencies. Any frequency higher than the maximum frequency allowed by the gradient cannot be detected correctly. When the signal is collected in quadrature, then the Nyquist frequency is 1/dwell time

Parallel imaging: a reduced data set in the phase encoding direction(s) of k-space is acquired to shorten acquisition time, combining the signal of several coil arrays. During the reconstruction process the missing information is recovered. We can recover the missing k-space lines using the B1 profiles of the different coils in the phased array (SMASH), or use the B1 profiles after the Fourier transform to unwrap the aliasing caused by the under-sampled k-space (SENSE). Parallel imaging speeds data collection and therefore decreases total imaging time, with some loss in signal-to-noise ratios compared to conventional imaging and longer post-acquisition reconstruction times.

Paramagnetic: A substance with a small but positive magnetic susceptibility. Typical paramagnetic substances usually possess an unpaired electron and include atoms or ions of transition elements, rare earth elements, some metals, and some molecules including molecular oxygen and free radicals. Paramagnetic substances are considered promising for use as contrast agents in MR imaging.

Phantom: an artificial object of known dimensions and properties that is used to test or monitor an MRI systems homogeneity, imaging performance, orientation aspects, etc.

Phase: In a periodic function (such as rotational or sinusoidal motion), the position relative to a particular part of the cycle. An angular relationship describing the degree of synchronism between two sinusoidal waveforms of the same frequency.

Phase coherence: a term describing the degree to which precessing nuclear spins are synchronous

Phase encoding: the process of locating an MR signal by altering the phase of spins in one dimension with a pulsed magnetic field gradient along that dimension prior to the acquisition of the signal. As each signal component has experienced a different phase encoding gradient pulse, its exact spatial reconstruction can be specifically and precisely located by the Fourier transformation analysis. Spatial resolution is directly related to the number of phase encoding levels (gradients) used.

Phased array coil: multiple small surface coils that are positioned on either side of the anatomy of interest. This type of coil allows for faster scanning with finer details.

Pixel: the smallest discrete part of a digital image display

Precession: gyration of the axis of a spinning body so as to trace out a cone. Caused by the application of a torque tending to change the direction of the rotation axis and continuously directed at right angles to the plane of the torque. The magnetic moment of a nucleus with spin will experience such a torque when inclined at an angle to the magnetic field, resulting in precession at the Larmor frequency.

Quadrature coil: consists of two coils oriented 90 degrees relative to each other. In transmission mode it generates two orthogonal B1 fields, which superimpose to generate a circularly polarized field (ie rotating field, as opposed to an oscillating field generated by a single coil). In the receive mode, each coil will record the MRI signal, but these signals will have phase difference of 90 degrees. The signals are then combined together (after correcting for the phase difference) using a combiner, and then the combined signal is send to the receiver where it is split again into real and imaginary parts. This process of splitting the signal into the real and imaginary parts at the receiver stage (together with the demodulation and some filtering) is called the quadrature detection, and is done regardless whether we use a quadrature or a linear coil.

Radio frequency (RF): an electromagnetic wave with a frequency that is in the same general range as that used for the transmission of radio and television signals. The RF pulses used in MR are commonly in the 1-100 megahertz range. The principal effect of RF magnetic fields on the body is power deposition in the form of heating, mainly at the surface; this is a principal area of concern for safety limits.

Radiofrequency pulse: Burst of RF magnetic field delivered to an object by an RF transmitter. For RF frequency near the Larmor frequency (3T: 128MHz, 7T: 300Mhz, 9.4T: 400MHz), it will result in rotation of the macroscopic magnetization vector in the rotating frame of reference. The amount of rotation will depend on the strength and duration of the RF pulse.

Repetition time (TR): is the amount of time that exists between successive pulse sequences applied to the same slice. It is delineated by initiating the first RF pulse of the sequence then repeating the same RF pulse at a time t. Variations in the value of TR have an important effect on the control of image contrast characteristics. Short values of TR (1000 ms) are common in images exhibiting T1 contrast. For a T2 weighted image the TR must be long (approximately 5 times T1). TR is also a major factor in total scan time.

Resonance: A large amplitude vibration in a mechanical or electrical system caused by a relatively small periodic stimulus with a frequency at or close to a natural frequency of the system

Resonance frequency: frequency at which resonance phenomenon occurs; given by the Larmor equation for MRI/NMR

Saturation: A nonequilibrium state in MR, in which equal numbers of spins are aligned against and with the magnetic field, so that there is no net magnetization. Can be produced by repeatedly applying RF pulses at the Larmor frequency with interpulse times short compared to T1.

Shielding: a way to confine the strong magnetic field surrounding a magnet. Most commonly a material with high permeability is used (passive shielding) or secondary counteracting coils outside the primary coils are employed (active shielding). For clinical systems active shielding is almost always used.

Shim coils: coils positioned near the main magnetic field that carry a relatively small current that is used to provide localized auxiliary magnetic fields in order to improve field homogeniety

Shimming: Correction of inhomogeneity of the magnetic field produced by the main magnet of an MR system due to imperfections in the magnet , the presence of the sample itself and/or the presence of external ferromagnetic objects. This can be accomplished through a combination of passive (mechanical) shimming (e.g., adding or removing steel from the magnets poles) and active shimming (the use of shim coils) to fine-tune the magnetic field.

Signal to noise (SNR, S/N): The ratio between the amplitude of the received signal and background noise, which tends to obscure that signal. SNR, and hence image quality, can be improved by such factors as increasing the number of excitations, increasing the field of view, increasing slice thickness, etc. SNR also depends on the electrical properties of the patient being studied and the type of receiving coil used.

Sinc pulse: An RF pulse shaped like Sin(x)/x.

Slew rate: The rate at which a gradient may be turned on or off. The faster the slew rate the more possible it is for an MRI system to run fast imaging sequences, and the shorter the TE value that can be achieved in a spin echo sequence.

Solenoid coils: A coil of wire wound in the form of a long cylinder. When a current is passed through the coil, the magnetic field within the coil is relatively uniform. Solenoid RF coils are commonly used when the static magnetic field is perpendicular to the long axis of the body. These coils are usually used in lower field magnets (for example open scanners) which have a vertical magnetic field orientation (rather than horizontal orientation in higher magnetic field scanners)

Specific absorption rate (SAR): the RF power absorbed per unit of mass of an object SAR is measured in watts per kilogram (W/kg).

Superconductor: a substance whose electrical resistance essentially disappears at temperatures near absolute zero. A perfect superconductor can carry an electrical current without losses. A commonly used superconductor in MR imaging system magnets is niobium-titanium, embedded in a copper matrix to help protect the superconductor from quenching.

Superconducting magnet: a magnet whose magnetic field originates from current flowing through a superconductor

Surface coil: a type of receiver coil which is placed directly on or over the region of interest for increased magnetic sensitivity. These coils are specifically designed for localized body regions, and provide improved signal-to-noise ratios by limiting the spatial extent of the excitation or reception.

Susceptibility artifact: The loss of MR signal in voxels or regions with varying magnetic susceptibility (magnetic non-uniformities) due to greater T2* decay. Susceptibility artifacts are more obvious in pulse sequences weighted more heavily by T2* effects, such as gradient-echo imaging.

Tesla: unit of magnetic flux density. One tesla is equal to 10,000 gauss.

T1 relaxation: Also known as longitudinal or spin-lattice relaxation. The characteristic time constant for the longitudinal magnetization to return to its equilibrium value after excitation due to the exchange of energy between the nuclear spins and the lattice. The magnetization will grow after excitation from zero to a value of about 63% of its final value in a time of T1.

T2 relaxation: Also know as transverse or spin-spin relaxation time. The time constant for loss of phase coherence among spins oriented at an angle to the static magnetic field due to interactions between the spins with resulting loss of transverese magnetization and MR signal. After time T2, transverse magnetization has lost 63% of its original value.

T2* relaxation: the time constant for loss of phase coherence among spins oriented at an angle to the static magnetic field due to a combination of magnetic field inhomogeneities and spin-spin relaxation. Results in a rapid loss of transverse magnetization and the MRI signal.

Transmit/Receive Coil: A transmit coil sends or transmits an RF pulse. A receive coil receives the MRI signal. Some coils both transmit and receive (head and body coils) – such a coil requires a T/R switching circuit to switch between the two modes. Other coils only receive, like surface coils. These days most head coils are receive only.

Transverse magnetization: the component of the net magnetization vector at right angles to the main magnetic field. Precession of the transverse magnetization at the Larmor frequency is responsible for the detectable NMR signal. In the absence of externally applied RF energy, the transverse magnetization will decay to zero with a characteristic time constant of T2, or more strictly T2*.

Truncation artifact: Also known as Gibbs ringing. This artifact occurs at high contrast interfaces (for example skull/brain, cord/CSF, meniscus/fluid) and causes alternating bright and dark bands. Due to undersampling of high spatial frequencies (omission of higher frequency terms in the Fourier transform). Often seen when zero filling is used to replace unsampled higher frequencies.

Tuning: the process of adjusting the transmitter and receiver circuitry so that it provides optimal signal performance at the Larmor frequency. A properly tuned scanner will produce images with a higher signal-to-noise ratio, and therefore improved diagnostic versatility. In practice, the only element the user would tune is the RF coil. All other electronic circuits are “tuned” by the manufacturer during the production process (or even the design process).

Voxel: volume element; the element of the three-dimensional space corresponding to a pixel, for a given slice thickness

Water suppression: the elimination or reduction of water signal from the image by application of a narrow-band frequency-selective pulse centered around the resonant frequency of the tissue. Also called water saturation. In magnetic resonance spectroscopy, suppression of water is necessary as the signal from water is 10,000 time higher than the signal from brain metabolites

Wraparound artifact: see Aliasing

Zipper artifact: radiofrequency noise often from an external source (patient monitoring system, scan room door is open) appears as intense thin bright lines through the image. In principal any signal that is not phase encoded will generate the zipper artifact, e.g. spurious signal produced by the imperfect refocusing pulses in the CPMG sequence, which is insufficiently spoiled by the crushers.

Zero filling: Substitution of zeroes for unmeasured data points in order to increase the matrix size of the new data prior to Fourier transformation of MR data. This can be equivalent to performing an interpolation in the transformed data, resulting in pixels smaller than the actual resolution of the image.

References include:

American College of Radiology
Chester F. Carlston Center for Imaging Science
MRI: The Basics
MRI Tutor
MR Technology Information Portal
Revise MRI
Wiki Radiography