CAMRT MRI Exam key terms

The rate at which nuclear spins precess in a magnetic field, equal to the gyromagnetic ratio multiplied by field strength. At 1.5 T, hydrogen protons precess at about 63.9 MHz.

A constant specific to each nucleus describing the ratio of its magnetic moment to its angular momentum. It determines the Larmor frequency at a given field strength; for hydrogen it is about 42.58 MHz/T.

The condition in which radiofrequency energy matches the Larmor frequency of a spin system, allowing net absorption of energy and tipping of magnetization away from its equilibrium alignment.

The angle by which net magnetization is rotated away from the longitudinal axis by an RF pulse. It depends on pulse amplitude and duration; 90 degrees creates transverse magnetization and 180 degrees inverts it.

A brief burst of electromagnetic energy at the Larmor frequency applied to excite spins. Its shape and duration determine the flip angle and the slice profile when combined with a gradient.

The recovery of longitudinal magnetization toward equilibrium after excitation, caused by energy transfer from spins to their surroundings. Tissues with short T1 recover quickly and appear bright on T1-weighted images.

The decay of transverse magnetization caused by interactions between neighbouring spins, leading to loss of phase coherence. It is always shorter than or equal to T2 star and reflects true tissue properties.

The observed decay of transverse magnetization when both spin-spin interactions and magnetic field inhomogeneity contribute. It is shorter than true T2 and is the basis of gradient echo contrast.

The time between successive excitation pulses in a pulse sequence. Longer values allow more longitudinal recovery, giving stronger T1 weighting when combined with a short echo time.

The interval between excitation and the collection of the echo signal. Longer values increase T2 weighting; short values minimize T2 decay and favour proton density or T1 contrast.

A pulse sequence in which a 180-degree refocusing pulse reverses dephasing from field inhomogeneity, producing an echo at a chosen time. It reduces susceptibility effects and provides true T2 weighting.

A sequence that refocuses transverse magnetization by reversing a magnetic field gradient rather than using a 180-degree pulse. It is fast, sensitive to field inhomogeneity, and allows short echo and repetition times.

A preparation method in which a 180-degree pulse inverts longitudinal magnetization before excitation. The inversion time chosen determines fluid suppression, as in FLAIR and STIR sequences.

The delay between the inverting 180-degree pulse and the excitation pulse in an inversion recovery sequence. It is selected so that a chosen tissue crosses zero magnetization and appears dark.

Spatial localization achieved by applying a magnetic field gradient during the RF pulse so that only spins in a narrow frequency band are excited. Slice thickness depends on gradient strength and pulse bandwidth.

The width of the excited imaging plane, determined by the RF pulse bandwidth divided by the slice-select gradient strength. Thinner slices improve through-plane resolution but reduce signal-to-noise.

Spatial localization along one axis by applying a gradient during readout so that position maps to precessional frequency. A Fourier transform converts the received signal into a spatial profile.

Spatial localization by applying a gradient briefly before readout so that position maps to accumulated phase. Each repetition samples one line of k-space, so the number of phase steps sets scan time.

The raw data matrix in which MRI signals are stored before Fourier transformation. Its centre contains low-spatial-frequency contrast information and its periphery contains high-spatial-frequency edge detail.

The mathematical operation that converts k-space data from the frequency and phase domain into a displayable image. It is the final reconstruction step in conventional two-dimensional imaging.

The ratio of true signal amplitude to random background noise in an image. It rises with voxel volume, field strength, and number of signal averages, and falls with higher bandwidth.

The number of times each k-space line is acquired and averaged. Signal-to-noise ratio improves with the square root of the number of averages, while scan time increases proportionally.

The range of frequencies sampled during readout. A wider bandwidth reduces chemical shift and susceptibility artifacts and shortens minimum echo time, but admits more noise and lowers signal-to-noise ratio.

Misregistration at fat-water interfaces caused by the small difference in precessional frequency between fat and water protons. It appears as bright and dark bands along the frequency-encoding direction.

Local signal loss and geometric distortion near materials that alter magnetic field homogeneity, such as metal implants or air-tissue boundaries. It is worse with gradient echo sequences and longer echo times.

Wrapping of anatomy from outside the field of view into the opposite side of the image when the sampled object exceeds the field of view. It occurs along the phase-encoding direction and is corrected by oversampling or saturation bands.

The physical dimensions of the imaged region. It relates to pixel size through the acquisition matrix; a smaller field of view improves spatial resolution but increases aliasing risk and lowers signal-to-noise.

The ability to distinguish small adjacent structures, set by pixel dimensions and slice thickness. Finer resolution requires more encoding steps, which lengthens scan time and reduces signal-to-noise ratio.

A reconstruction technique that uses the differing spatial sensitivity of multiple receiver coils to skip phase-encoding lines. It shortens scan time or reduces distortion, at the cost of lower signal-to-noise.

Techniques that reduce signal from adipose tissue, including chemical shift saturation, inversion recovery such as STIR, and Dixon methods. It improves conspicuity of edema and contrast enhancement.

A T2-weighted sequence using a long inversion time to null cerebrospinal fluid signal. It improves detection of periventricular and cortical lesions that would otherwise be obscured by bright fluid.

An inversion recovery sequence with a short inversion time chosen to null fat signal while preserving fluid brightness. It is highly sensitive to edema and useful when field homogeneity is poor.

A paramagnetic chelate that shortens T1 relaxation of nearby water protons, increasing signal on T1-weighted images. It is used to detect blood-brain barrier breakdown and characterize lesions.

A rare fibrosing disorder associated with exposure to older linear gadolinium agents in patients with severe renal impairment. Risk assessment of renal function guides contrast use and agent selection.

Increased signal on post-contrast images where gadolinium accumulates, reflecting regional perfusion and blood-brain barrier integrity. Comparison with pre-contrast images confirms true enhancement rather than intrinsic T1 shortening.

The rate at which radiofrequency energy is deposited in tissue, expressed in watts per kilogram. It is monitored because excessive heating risks burns, and it rises with field strength and with fast, high-flip-angle sequences.

The sudden loss of superconductivity in the magnet coil, releasing cryogen gas and collapsing the magnetic field. It requires immediate evacuation of the room and activation of emergency procedures.

The main magnet design in clinical MRI, in which a niobium-titanium coil cooled by liquid helium carries current with no resistance, producing a strong, stable, and homogeneous static field.

The stray static magnetic field extending beyond the scanner room. It is contained by passive or active shielding, and its extent defines the controlled access area and pacemaker exclusion zones.

Design measures that confine the magnetic fringe field or block external radiofrequency interference. Passive shielding uses steel; active shielding uses opposing coils; RF shielding uses a copper-lined room.

One of four designated areas around an MRI suite, graded from freely accessible to the scanner room itself. Zones three and four are restricted to screened personnel and patients under direct supervision.

The forceful attraction of ferromagnetic objects into the magnet bore, which can injure patients or staff and damage equipment. Prevention relies on screening, signage, and strict control of zone four.

External electromagnetic noise that degrades MR images when it enters the scan room. It is prevented by a continuous RF shield, filtered penetrations, and careful door and waveguide maintenance.