ARRT MRI cheat sheet

GFR <30 mL/min/1.73 m²: avoid group I gadolinium; group II agents only at lowest dose if essential.

NSF: highest risk with linear group I agents in severe renal impairment; macrocyclic group II agents carry the lowest risk.

Metformin: no routine withholding for gadolinium; withhold 48 h after iodinated contrast only if eGFR <30 or AKI.

Pregnancy: noncontrast MRI is preferred; gadolinium crosses the placenta and is reserved for essential indications.

Extravasation: stop injection, elevate limb, apply cold compress, document volume and site, monitor for compartment syndrome.

Anaphylactoid reaction: stop agent, assess airway, give oxygen, IM epinephrine 0.3 mg (1:1000) adult for bronchospasm or hypotension.

Metal screening: pacemaker, aneurysm clip, cochlear implant, ocular metal, shrapnel, tattoos; confirm MR labeling.

Sedation: continuous SpO2 and HR, BP every 5 min, observe respiration and level of consciousness.

T1 weighting: TR <800 ms and TE <30 ms; fluid is dark, fat is bright.

T2 weighting: TR >2000 ms and TE >80 ms; fluid is bright.

STIR: inversion recovery with TI about 150 ms at 1.5 T; fat is nulled and edema is bright.

FLAIR: long TI about 2000 ms; CSF is nulled and periventricular lesions are bright.

MRCP: heavily T2-weighted thick-slab or 3D respiratory-triggered sequence; stationary fluid is bright.

TOF MRA: flow-related enhancement of unsaturated blood; no gadolinium required; 3D TOF for intracranial arteries.

Chemical shift: fat-water misregistration along the frequency-encoding direction; increases as receive bandwidth decreases.

Aliasing: FOV smaller than anatomy; correct with larger FOV, saturation bands, or phase-encoding oversampling.

Cardiac cine: ECG gating, breath-hold, steady-state free precession; bright-blood contrast.

Larmor: ω = γB0; 1H γ = 42.58 MHz/T; 1.5 T = 63.9 MHz; 3 T = 127.7 MHz.

T1 is longitudinal recovery to 63%; T2 is transverse decay to 37%; T2* includes static B0 inhomogeneity.

TR/TE control weighting: short TR and short TE = T1; long TR and long TE = T2; long TR and short TE = proton density.

Ernst angle: cos θ = e^(−TR/T1); maximum signal for a given TR and T1.

k-space: center lines encode contrast and SNR; peripheral lines encode resolution and edge detail.

SNR ∝ voxel volume × √NEX × √Nacq; voxel volume = FOV_x × FOV_y × slice thickness / matrix.

Receive bandwidth: higher bandwidth lowers SNR, shortens minimum TE, and reduces chemical shift and susceptibility artifact.

SAR ∝ B0² × flip angle² × duty cycle; fast spin echo sequences increase SAR.

Inversion nulling: TI = T1 × ln2 ≈ 0.693 × T1.

Static field: the 5 gauss line marks the restricted boundary; loose ferromagnetic objects become projectiles.

Zones: I public, II screened, III restricted, IV magnet room.

Whole-body SAR limits: 2 W/kg normal mode and 4 W/kg first level averaged over 15 minutes.

RF burns: avoid skin-to-skin loops, pad cables, and keep transmit coil cables off the patient.

Acoustic noise: gradients can exceed 99 dB; hearing protection is required in the magnet room.

Quench: helium displaces oxygen; evacuate, open doors, and monitor oxygen before reentry.

MR conditional: safe only under the stated field strength, SAR, and coil conditions.

Pacemaker: MR conditional devices only under validated conditions; otherwise the static field is contraindicated.