ABMP MRI Physics key terms

The rate at which a nuclear spin's magnetic moment precesses about the main magnetic field, equal to the gyromagnetic ratio multiplied by field strength. It sets the resonant frequency for excitation and signal detection.

A nuclide-specific constant relating precession frequency to magnetic field strength, expressed in MHz per tesla. Hydrogen protons have the largest value commonly used in imaging, giving the highest signal per unit of polarization.

The condition in which an applied radiofrequency field oscillates at exactly the precession frequency of the spins, allowing efficient transfer of energy and tipping of magnetization away from equilibrium.

The vector sum of all individual nuclear magnetic moments in a voxel. At equilibrium it lies along the main field; radiofrequency excitation tilts it, and its rotating transverse part generates the measured signal.

The angle by which radiofrequency excitation rotates net magnetization away from its starting orientation. It depends on pulse amplitude and duration, and controls how much longitudinal signal is converted to detectable transverse signal.

The component of net magnetization aligned with the main magnetic field. It is not directly detected but recovers after excitation and supplies the signal available for each subsequent radiofrequency pulse.

The component of net magnetization lying perpendicular to the main field. Its rotation induces current in receiver coils and is the source of the MR signal, decaying as spins dephase.

The recovery of longitudinal magnetization toward its equilibrium value after excitation, governed by energy exchange with the surrounding lattice. Different tissues recover at different rates, producing T1 contrast.

The irreversible decay of transverse magnetization caused by interactions among spins that make each precess at slightly different rates. It is an inherent tissue property and is always shorter than T2 star.

The observed decay of transverse magnetization when static field imperfections and susceptibility differences add to intrinsic T2 losses. It is shortened by local field inhomogeneity and partly reversed by a spin echo.

The interval between successive radiofrequency excitation pulses in a pulse sequence. Short values limit longitudinal recovery and weight the image toward T1 differences; long values permit full recovery.

The interval between excitation and the collection of the echo that forms the image. Longer values allow more transverse decay, weighting the image toward T2 differences.

The flip angle that maximizes signal for a given repetition time and tissue T1. It balances how much longitudinal magnetization is tipped into the transverse plane against how much recovers before the next pulse.

A preparation scheme in which a 180-degree pulse inverts longitudinal magnetization before excitation, so tissues recover from negative values at rates set by their T1. It produces strong T1 weighting and can suppress selected tissues.

Suppression of signal from a chosen tissue by applying an inversion pulse and waiting until its longitudinal magnetization crosses zero before excitation. Commonly used to null cerebrospinal fluid in FLAIR imaging.

A deliberately applied, spatially varying magnetic field that changes precession frequency with position. It is the basis for slice selection, spatial encoding, and diffusion weighting.

Excitation of a limited anatomical slab by combining a gradient along one axis with a narrow-band radiofrequency pulse. The gradient maps frequency to position, so only spins in the chosen frequency range are excited.

The range of frequencies contained in an excitation pulse, which determines slice thickness when combined with a selection gradient. Shorter pulses contain broader frequencies and excite thicker slices.

Spatial encoding in which a gradient applied during signal readout makes precession frequency vary linearly with position along one axis. Fourier transformation of the received signal then resolves position along that axis.

Spatial encoding in which a brief gradient before readout imposes a position-dependent phase shift. Repeating with different gradient amplitudes builds the lines of k-space along one axis.

The spatial-frequency domain in which raw MR data are stored before Fourier transformation. Its center holds low-resolution contrast information and its periphery holds fine detail and edge information.

The mathematical operation that converts raw spatial-frequency data into an anatomical image, and converts image content back into k-space. It underlies essentially all conventional MR image reconstruction.

The decaying radiofrequency signal emitted by transverse magnetization immediately after a single excitation pulse, before any refocusing. Its decay reflects T2 star and field uniformity rather than pure T2.

A signal refocused by a 180-degree pulse applied after excitation, which reverses static dephasing from field imperfections. Its peak occurs at twice the time between excitation and the refocusing pulse.

A signal formed by reversing a readout gradient rather than applying a refocusing pulse. It does not correct field inhomogeneity, so it is sensitive to T2 star and permits short echo times and rapid imaging.

An antenna that transmits excitation pulses, receives the rotating transverse signal, or both. Its geometry and proximity to the anatomy determine signal-to-noise ratio and uniformity of reception.

A set of small surface coil elements whose signals are combined, giving high signal near the body with a wide field of view. It also enables parallel imaging by providing independent spatial information from each element.

The ratio of desired signal amplitude to random background noise in an image. It improves with larger voxels, more signal averages, higher field strength, and closer or better-matched receiver coils.

Repeating an acquisition and summing or averaging the results to reduce random noise. Because noise adds incoherently while signal adds coherently, the ratio improves with the square root of the number of averages.

A strategy that shortens scan time by acquiring fewer phase-encoding lines and using spatial information from multiple coil elements to fill in missing data. The trade-off is reduced signal-to-noise ratio and possible reconstruction artifacts.

The ratio by which phase-encoding steps or acquisition time are reduced in a fast or parallel technique. Higher factors shorten scans further but lower signal-to-noise ratio and increase the risk of unfolding artifacts.

A very fast readout that fills all of k-space with a train of gradient echoes after a single excitation. It enables subsecond imaging but is prone to distortion from field inhomogeneity, especially near air-tissue boundaries.

The rate at which radiofrequency energy is deposited in tissue per unit mass, usually expressed in watts per kilogram. Regulatory limits on it constrain flip angles, pulse types, and scan time, especially at high field.

Nerve excitation caused by rapidly changing magnetic gradient fields, which can induce uncomfortable tingling or muscle twitching. It limits how quickly gradients may be switched, particularly with strong or large-coverage sequences.

The forceful attraction of ferromagnetic objects toward the magnet bore, which can cause injury or damage. Screening of patients and equipment before entry into the magnet room is the primary safeguard.

Wrapping of signal from outside the field of view into the opposite side of the image, caused by undersampling along a phase-encoding or frequency direction. It is corrected by larger fields of view, oversampling, or special reconstruction.

Displacement of signal from fat relative to water because hydrogen nuclei in different molecular environments precess at slightly different frequencies. It appears as bright or dark rims at fat-water boundaries and grows with field strength.

Replicated or smeared copies of moving structures, or periodic banding, appearing along the phase-encoding direction. It results from inconsistent data between phase-encoding lines, such as motion or flow during acquisition.

A quantitative measure of how freely water molecules move within tissue, derived from diffusion-weighted acquisitions. Low values indicate restricted diffusion, as in densely cellular tissue, and high values indicate freer motion.

A spatial map of main-field deviations computed from phase images acquired at different echo times. It is used to correct distortion, shim the field, and support quantitative susceptibility and fat-water separation methods.