Magnetic Resonance Imaging
What the exam tests
MRI tests physics, pulse sequences, image contrast, artifacts, safety, and contrast agents. Expect T1/T2 weighting, fat suppression, STIR, diffusion, gadolinium safety, and implant screening.
Core rules
- T1 weighting uses a short TR and short TE; T2 weighting uses a long TR and long TE.
- Fluid is dark on T1 and bright on T2; fat is bright on T1 and T2 unless fat suppression is used.
- STIR suppresses fat and is useful for bone marrow edema, especially near metal or in large fields.
- Gadolinium safety: check renal function for certain agents and pregnancy status; avoid inappropriate use in pregnancy.
Worked example
A 30-year-old with knee pain and suspected occult marrow edema needs MRI. Reasoning: a fluid-sensitive sequence such as STIR or T2-weighted fat saturation is best because marrow edema is bright. A T1-weighted image alone may show low signal but is nonspecific and can miss subtle edema. Fat saturation improves conspicuity of edema but can fail near metal artifacts.
Traps
Confusing T1 and T2 fluid signal; using fat saturation near metal; ignoring implant safety; and assuming gadolinium is always safe in renal failure.
Key points
- T1: short TR/TE; T2: long TR/TE.
- STIR is a robust fat-suppressed fluid-sensitive sequence.
- MRI safety screening is mandatory before scanning.
- Gadolinium risks depend on agent and renal function.
- Diffusion-weighted imaging reflects restricted water motion.