ABMP MRI for Radiation Therapy flashcards

32 cards. Read the term and the definition here. The deck at the top of this page is optional.

Larmor frequency at 3 T

For hydrogen-1, f = 42.58 MHz/T × 3 T ≈ 127.7 MHz. This is the precession frequency and determines RF coil tuning and chemical shift magnitude.

T1 relaxation time

The time constant for recovery of longitudinal magnetization after an RF pulse; tissues with short T1 (fat) recover faster and appear bright on T1-weighted images.

T2* decay

Transverse signal decay that includes both spin-spin dephasing (T2) and static field inhomogeneity; gradient echo sequences are T2*-weighted and sensitive to susceptibility.

SAR definition

Specific absorption rate, RF power deposited per unit mass (W/kg). It increases with field strength squared and flip angle squared and limits fast 3 T sequences.

Parallel imaging tradeoff

Reducing phase-encoding lines by acceleration factor R shortens scan time but lowers SNR roughly by the square root of R and can produce aliasing if coil sensitivity is inadequate.

MR conditional labeling

Device is safe only under the specific conditions stated in its labeling, such as field strength, coil type, and SAR limits; outside those conditions safety is unknown.

5-gauss line

The contour where the static magnetic field falls to 5 gauss; it defines the fringe field boundary that must be controlled to prevent projectile and pacemaker hazards.

RF burn risk

Elongated conductors such as ECG leads or implanted wires can concentrate RF energy and cause localized heating; risk rises with SAR and conductor length.

Peripheral nerve stimulation

Rapid gradient switching induces electric fields that can stimulate nerves, producing tingling or muscle twitching; dB/dt limits constrain fast sequences.

Quench response

Sudden loss of superconductivity releases cryogen; evacuate the room, monitor oxygen, and do not re-enter until oxygen levels are safe and the field is verified.

SNR formula

SNR increases with voxel volume and the square root of NEX and phase-encoding steps, and decreases with the square root of receive bandwidth.

Chemical shift artifact

Fat-water misregistration along the frequency-encoding direction; magnitude is proportional to field strength and inversely proportional to receive bandwidth.

Susceptibility artifact

Signal loss and geometric distortion near metal or air-tissue interfaces; worsens with longer TE, higher field strength, and echo-planar readout.

Phase-encoding motion ghosting

Motion during acquisition produces replicated ghosts along the phase-encoding direction; swapping phase and frequency directions relocates the artifact.

Gradient nonlinearity distortion

Spatially varying gradient fields cause geometric warping that is worst at the FOV periphery and off-isocenter; distortion correction is needed for radiotherapy geometry.

MR simulation tabletop

A flat, indexed tabletop with MR-compatible immobilization and external lasers is required so the simulation setup matches the treatment position.

DICOM coordinate origin

In MR DICOM images, patient coordinates are referenced to the scanner isocenter; correct slice position and orientation metadata are essential for planning geometry.

4D-MRI at simulation

Respiratory-correlated MR acquisition used to characterize tumor motion for thoracic and abdominal targets; breath-hold instructions should match treatment delivery.

Simulation scan protocol

Large FOV, thin slices, distortion correction enabled, and avoidance of EPI-based sequences to preserve geometric accuracy for radiotherapy planning.

Rigid registration DOF

Rigid registration has six degrees of freedom: three translations and three rotations. It preserves distances and is standard for CT-MR fusion.

Mutual information metric

A similarity measure based on shared information between image intensities; it does not assume a linear intensity relationship and is standard for CT-MR fusion.

Target registration error

The distance between corresponding landmarks after registration; used to quantify fusion accuracy and to justify planning margins.

Deformable registration validation

Nonrigid transforms must be verified with phantoms and physician review of propagated contours because local errors can be large and are not captured by rigid metrics.

DICOM registration object

A DICOM object storing the spatial or deformable transform between image sets; it should be archived with the plan for traceability.

Synthetic CT from MRI

An MR-derived density map generated by bulk assignment or deep learning; it enables dose calculation when a planning CT is unavailable.

MRI soft-tissue advantage

Superior contrast for brain, prostate, and head-and-neck targets improves GTV delineation and reduces interobserver contouring variability compared with CT alone.

Distortion in margins

Residual MR geometric distortion, typically under 1-2 mm within the imaging FOV, must be quantified and included in PTV margins.

Bone on MRI

Cortical bone and air appear dark on most sequences, so synthetic CT must accurately represent bone density for heterogeneous dose calculation.

Electron return effect

In a magnetic field, secondary electrons curve back toward tissue-air interfaces, increasing surface and interface dose; dose algorithms must model it.

Online adaptive radiotherapy

Daily MR-guided recontouring and replanning based on the anatomy of the day, with plan QA completed before beam delivery.

Cine MR tracking

Real-time imaging used to track target motion for gating and beam-hold delivery; imaging-to-beam latency must be characterized during commissioning.

Occupational dose limit

Radiation workers are limited to 20 mSv per year averaged over five years, with no more than 50 mSv in any single year; the public limit is 1 mSv per year.