collimation
Restricting the X-ray beam to the region of interest. In CT, collimation determines beam width along the z-axis (e.g., 64 × 0.625 mm = ~40 mm collimation). In radiography, collimation determines the rectangular field at the detector. Reduces dose to anatomy outside the diagnostic region.
Why it matters to buyers: Wider collimation scans more volume per rotation; narrower gives finer z-axis detail at smaller per-rotation coverage. Automatic in modern CT — selected by protocol and pitch.
Why it matters to engineers: Collimator calibration drifts over time; annual physicist QA verifies. Out-of-spec collimation = excess patient dose or missed anatomy at edge of field.