ABMP MRI for Radiation Therapy key terms

The rate at which nuclear spins precess in a magnetic field, equal to the gyromagnetic ratio times field strength; at 1.5 T protons precess near 63.9 MHz and at 3 T near 127.7 MHz.

A constant specific to each nucleus describing how strongly its magnetic moment responds to an applied field; for hydrogen protons it is about 42.58 MHz per tesla.

The recovery of magnetization along the main magnetic field after excitation, characterized by the time constant T1; tissues with short T1 recover quickly and appear bright on T1-weighted images.

The decay of magnetization perpendicular to the main field after an RF pulse, described by the time constant T2; signal fades faster when local field variations are included, giving T2*.

The interval between successive excitation pulses in a pulse sequence; it is set by the operator and strongly influences T1 weighting and total scan duration.

The delay between the excitation pulse and the collected signal echo; choosing it controls how much T2 decay is expressed in the resulting image contrast.

A magnetic field gradient applied during an RF pulse so that only spins in a narrow frequency band, and therefore a thin slice, are excited; its amplitude sets slice thickness.

A gradient applied briefly before readout that labels spins by position along one in-plane axis, allowing spatial information to be separated during image reconstruction.

A gradient applied during signal readout so that spins at different positions along one axis emit at different frequencies, providing one dimension of spatial localization.

The raw data matrix recorded during a scan, where each point holds spatial-frequency information; its center carries contrast and its edges carry fine detail, and it is Fourier transformed to form the image.

A brief burst of oscillating magnetic field at the Larmor frequency that tips nuclear spins away from alignment, producing the signal detected by the receive coil.

The amount by which an RF pulse rotates the net magnetization, commonly 90 degrees for spin echo or less for gradient echo sequences; it shapes contrast and signal strength.

A pulse sequence in which a 180-degree refocusing pulse reverses dephasing caused by field inhomogeneity, producing an echo with strong T2 weighting and reduced susceptibility artifact.

A spin echo variant that collects several echoes per repetition by applying multiple refocusing pulses, greatly shortening scan time and commonly used for T2-weighted planning images.

A sequence that produces echoes by reversing a magnetic field gradient instead of using a refocusing pulse; it is fast and allows short echo times but is more sensitive to field inhomogeneity.

An antenna-like device that transmits excitation pulses, receives the weak MR signal, or both; surface and phased-array coils improve signal-to-noise for the anatomy they cover.

The ratio of true tissue signal to random background noise in an image; higher values give cleaner images, and it rises with field strength, larger voxels, and more signal averages.

The smallest distance at which two structures can still be distinguished in an image, set mainly by voxel size, field of view, and matrix dimensions.

The physical extent of the imaged region, equal to the matrix size times the pixel size; it must cover the anatomy of interest to avoid wraparound artifacts.

Wraparound of tissue outside the field of view into the opposite side of the image, arising because phase-encoding sampling cannot distinguish positions beyond the defined extent.

Misregistration of fat and water signals along the frequency-encoding direction because their protons precess at slightly different frequencies, producing bright and dark bands at fat-water boundaries.

Local signal loss and geometric distortion near materials that alter magnetic field homogeneity, such as air-tissue interfaces, surgical clips, or implanted metal.

Blurring, ghosting, or misregistration caused by patient movement, breathing, or pulsation during acquisition; it is reduced with breath-holding, gating, faster sequences, or motion-robust reconstruction.

Adjusting additional magnetic fields to make the main field more uniform across the imaging volume, which reduces distortion and signal loss from susceptibility effects.

A scheduled set of tests and measurements that verify scanner performance, including field homogeneity, geometric accuracy, signal-to-noise, and coil function, documented against accepted tolerances.

The use of MRI, often with radiotherapy-specific coils and flat couch tops, to acquire images in the treatment position for target and organ-at-risk delineation and reference for planning.

A mask, cradle, or vacuum cushion that holds the patient in a reproducible position so simulation images and daily treatment setups match as closely as possible.

The process of spatially aligning two image sets, such as MR and CT, by computing a transformation that maps corresponding anatomy onto a common coordinate system.

The display and combined use of two registered image sets so that information from each, for example MR soft-tissue contrast and CT electron density, is available in one view.

Alignment that allows only translation and rotation, preserving all distances within each image; it is standard for brain and other anatomies where shape does not change between scans.

Alignment that allows local stretching and compression so that one image can be warped onto another; it suits anatomy that changes shape, such as prostate or head and neck between scans.

A similarity measure that quantifies how much knowing the intensity in one image reduces uncertainty about intensity in the other; it enables registration between different modalities such as MR and CT.

A measure of overlap between two outlined regions, calculated as twice their shared volume divided by the sum of both volumes; values near one indicate close agreement.

The number of electrons per unit volume in tissue, which determines photon attenuation and is derived from CT, not MRI; MRI must be converted or registered to CT for dose calculation.

Assigning a fixed density to a region, such as a whole organ or implant, when its true density is unknown or unreliable, so the planning system can compute dose.

Positional error in MR images caused by field inhomogeneity, gradient nonlinearity, or susceptibility, which can shift apparent anatomy and must be corrected or accounted for in planning.

The structured questioning and checking of a patient before entering the scanner room to identify implants, foreign bodies, and conditions that make MRI unsafe.

A device shown to pose no known hazard in a specified MRI environment under stated conditions of field strength, spatial gradient, and heating limits; those conditions must be verified before scanning.

The radiofrequency power absorbed per unit mass of tissue, measured in watts per kilogram; regulatory limits exist because excess absorption can heat tissue.

The strong attraction of ferromagnetic objects toward the magnet bore, which can cause them to fly into the scanner and injure patients or staff; it underlies strict room-access rules.

A sudden loss of superconductivity in the magnet coils, releasing large volumes of helium gas; the room must be evacuated and oxygen levels monitored until ventilation restores safe conditions.

Delivery of radiation using a system that images the patient with MRI at the treatment position, allowing soft-tissue targeting and daily or real-time adaptation of the plan.

Re-optimizing a treatment plan immediately before or during a fraction using the day's imaging, so that dose follows the anatomy actually present at treatment.

Rapid repeated MR acquisition that shows motion over time, used during MR-guided treatment to track structures such as the prostate or bowel and to trigger beam gating.

The difference between the patient position intended by the plan and the position actually achieved at treatment, detected and corrected by image guidance.