Computed Tomography
What the exam tests
CT questions cover physics, contrast timing, protocol selection, radiation dose, and artifacts. Expect CT numbers, windowing, multiphasic liver/pancreas protocols, renal stone CT, and dose-reduction principles.
Core rules
- Hounsfield units are calibrated to water (0 HU) and air (-1000 HU); acute blood is about 40-60 HU before contrast.
- Contrast phases: unenhanced; late arterial/pancreatic (~40-50 s); portal venous (~60-70 s); delayed/excretory (minutes).
- Bolus tracking triggers acquisition at a threshold in a monitor vessel; patient size and cardiac output affect timing.
- Radiation dose follows ALARA; use automatic exposure control, lower kVp when appropriate, and limit phases.
Worked example
A 64-year-old with weight loss and jaundice has a suspected pancreatic head mass. Reasoning: the best protocol is a thin-section pancreatic parenchymal phase (about 40-50 s) with neutral oral contrast to distend the stomach and duodenum. A pancreatic adenocarcinoma is typically hypoenhancing and may cause ductal dilation. Portal venous phase alone can obscure a small hypervascular neuroendocrine tumor or reduce conspicuity of a hypoenhancing mass.
Traps
Using the wrong contrast phase; giving positive oral contrast when it would obscure a pancreatic or biliary mass; misinterpreting pseudoenhancement of renal cysts; and ignoring that patient motion can mimic dissection or fracture.
Key points
- Pancreatic mass protocols use a late arterial/pancreatic phase.
- Portal venous phase is standard for most abdominal staging.
- CT numbers help characterize fat, fluid, blood, and calcification.
- Dose optimization is examinable and clinically required.
- Bolus timing varies with cardiac output and patient size.