Sequence physics

• TR controls T1 weighting (short TR = more T1); TE controls T2 (long TE = more T2).
• T1w = short TR / short TE. T2w = long TR / long TE. PD = long TR / short TE.
• Larmor: 42.58 MHz/T → 63.9 MHz @1.5 T, 127.7 MHz @3 T.
• SNR ∝ voxel size × √(NEX) × √(scan time); rises with field strength, falls with bandwidth.
• STIR TI ≈ 0.69 × T1(fat) to null fat; B0-insensitive → best near metal.
• Fat & water opposed-phase TE ≈ 2.2 ms @1.5 T (in-phase ≈ 4.4 ms).

Vendor decoder

Artifacts

• Phase-direction: motion/flow ghosts, wrap-around, Gibbs → swap phase/frequency to move off ROI.
• Frequency-direction: chemical shift type 1 → raise bandwidth.
• Chemical shift type 2 (India-ink): opposed-phase GRE; used to prove microscopic fat.
• Susceptibility/metal: SE over GRE, short TE, high BW, STIR not spectral fat-sat.
• Gibbs: pseudo-syrinx in cord; don’t call a real one.
• Zipper/herringbone: RF/hardware; check door, shielding, spikes.

Gadolinium contrast

• Standard dose 0.1 mmol/kg.
• ACR Group I (linear, e.g. gadodiamide) = highest NSF risk → avoid if eGFR<30.
• Group II (macrocyclic: gadobutrol, gadoteridol, gadoterate) = few/no unconfounded NSF cases → usable at any eGFR.
• Pregnancy: only if essential. Lactation: may continue breastfeeding (<0.04% excreted).
• Premedicate prior allergic-like reactions; recognise extravasation (elevate, monitor for compartment syndrome).

MR safety

• The magnet is always on. Ferromagnetic projectile = the lethal hazard.
• Zones I–IV; Zone IV = magnet room. The controlled access area is bounded by a fringe-field contour marked on the site plan: 0.9 mT (9 G) per IEC 60601-2-33 Ed 4.0 (2022) and the ACR Manual on MR Safety (2026); the older 0.5 mT (5 G) contour lies further out and remains conservative and widely used. Know which one your site uses.
• Cardiac arrest in Zone IV: remove the patient; never bring a crash cart/O₂ cylinder in.
• Quench: cryogen boil-off displaces oxygen → evacuate, don’t re-enter.
• Screen for cardiac devices, cochlear implants, aneurysm clips, neurostimulators, orbital metal (metalworker → orbital XR).
• Prevent RF burns: no skin-to-skin loops, pad cables, ear protection for acoustic noise.

Search patterns

• Always systematic; beware satisfaction of search (stop after first finding).
• Brain: DWI/ADC every time; check skull base, IACs, pituitary, vessels, sinuses.
• Spine: cord signal, conus level, every disc, and glance kidneys/aorta on sagittals.
• Prostate: scan the skeleton on T1 + DWI for mets.
• Knee: menisci, cruciates, cartilage, marrow oedema, extensor mechanism.

Reporting systems

• PI-RADS v2.1: PZ driven by DWI, TZ driven by T2; category 1–5.
• LI-RADS v2018: APHE + washout + capsule + threshold growth → LR-5; LR-M = probably malignant non-HCC.
• BI-RADS: 0–6; assess background enhancement, mass vs non-mass, kinetics.
• Rectal (ESGAR): mrCRM ≤1 mm = threatened margin; report EMVI, T/N stage, tumour height.
• MS (McDonald): dissemination in space + time.

Must-not-miss cases

• Stroke: DWI bright + ADC dark; DWI-FLAIR mismatch dates it <4.5 h.
• Cord compression / cauda equina: emergency; assess conus & canal.
• Bucket-handle meniscus: double-PCL sign.
• HCC: arterial hyperenhancement + washout (LR-5).
• Pituitary apoplexy: sudden headache + visual loss + haemorrhagic mass.

The 10-second troubleshooting reflexes

• Artifact repeats identically & is geometric → hardware/RF (door, shield, spike).
• Artifact changes with breathing/pulsation → physiologic (coach, gate, sat bands, swap phase).
• Artifact tied to metal/air/interface or worse at 3 T → physics (bandwidth, TE, sequence family, fat-sat method, shim).
• Fat suppression patchy → use STIR or Dixon; shim; centre at isocentre.
• Poor eGFR + need contrast → reach for a Group II macrocyclic agent.
• Anything ferromagnetic near the bore → stop; the field never switches off.