Imaging Procedures
Choosing the sequence
Procedure questions reward a systematic plan: define the clinical question, pick the contrast weighting (T1 for anatomy and post-contrast, T2 for fluid and pathology, proton density for joint detail, STIR or fat-saturated T2 for edema, FLAIR to suppress CSF, diffusion for restricted motion, gradient echo for blood and susceptibility), then set plane, coverage, and resolution. Match the coil to the anatomy and center the patient at isocenter.
Coordinates and tradeoffs
Field of view, matrix, slice thickness, gap, and number of slices interact. Decreasing FOV or increasing matrix raises spatial resolution but lowers signal-to-noise and lengthens scan time. Slices that are too thin with no gap reduce SNR and invite cross-talk; a gap of roughly 10-30% of slice thickness limits cross-talk. Fat saturation requires a homogeneous field and good shim; STIR suppresses fat without relying on shim homogeneity but has lower SNR.
Motion and gating
Breath-hold, respiratory triggering, and cardiac gating control physiologic motion. For cardiac imaging, set the trigger delay from the ECG and remember that the magnetohydrodynamic effect can distort the ECG waveform. Motion artifact propagates along the phase-encoding direction, so swapping phase and frequency directions moves the artifact away from the region of interest.
Worked example
Coverage = number of slices × (slice thickness + gap). A spine protocol needing 150 mm with 24 slices leaves 6.25 mm per slice: 4 mm slices with a 2 mm gap give 144 mm, so add two slices or use 4.5 mm slices with a 1.5 mm gap to reach about 144-150 mm.