Imaging Procedures
Core concepts
Procedure questions emphasize protocol selection, patient positioning, contrast timing, and image review. Know common CT protocols: noncontrast head, CTA head and neck, chest without and with contrast, CT pulmonary angiography, abdomen/pelvis in arterial, portal venous, and delayed phases, and spine or musculoskeletal protocols. Understand scan direction, breath-hold instructions, field of view, reconstruction thickness, multiplanar reformats, and when oral or rectal contrast is used. Familiarity with bolus tracking and test-bolus timing is essential for vascular studies.
Traps
Many questions hinge on phase timing. Portal venous abdomen is typically about 70–80 seconds after IV contrast; arterial is about 35–40 seconds; delayed phases are often 5–10 minutes. CT pulmonary angiography is timed to the pulmonary trunk, not the aorta. A noncontrast head is generally acquired before a contrast-enhanced head when both are requested. Do not confuse oral contrast with IV contrast timing. Also watch for pitfalls in breath-hold coaching: scanning the chest during inspiration but the abdomen often during expiration or quiet breathing per protocol.
Worked example / rule of thumb
For a CTA chest to evaluate pulmonary embolism, place the region of interest in the pulmonary trunk and use bolus tracking with a threshold around 100 HU, then trigger the scan. If bolus tracking fails, a test bolus or fixed delay per protocol may be used. Rule of thumb for routine abdomen/pelvis: portal venous phase at roughly 70–80 seconds gives good hepatic and venous enhancement, while arterial phase highlights hypervascular lesions and arteries.
Image review
Check coverage, motion, contrast opacification, and artifacts before the patient leaves. Repeat only when clinically necessary and justified.