Imaging study · Moderate dose

CT scan

Also known as: computed tomography, CAT scan, CT angiography, CTPA, CT colonography

A rotating X-ray tube and computer build cross-sectional slices of the body in seconds, showing bone, organs, blood vessels and bleeding in fine detail; the workhorse of emergency imaging.

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How it works

An X-ray tube spins around the body while it moves through the ring, collecting thousands of projections that a computer reconstructs into thin cross-sectional slices. The slices are stacked to view any plane or a 3D model. Iodine contrast injected into a vein lights up blood vessels and shows how organs and tumours take up blood.

What it shows

  • Brain: bleeding, stroke (once established), tumours, injury
  • Chest: lung nodules and cancer, blood clots in the lung arteries (CTPA), fibrosis, infection
  • Abdomen and pelvis: appendicitis, diverticulitis, kidney stones, tumours, abscesses, bowel obstruction
  • Bones: complex fractures, the spine and pelvis
  • Blood vessels: aneurysms, dissection, coronary artery narrowing (CT coronary angiography)

Best for

  • Emergencies where speed matters: trauma, stroke, suspected bleeding, severe abdominal pain
  • Lung disease and cancer staging
  • Kidney stones (no contrast needed)
  • Detailed bone anatomy for surgical planning

Not the right tool for

  • Early stroke (may look normal for hours), spinal cord, ligaments and cartilage (MRI)
  • Repeated scans in children and young adults where dose accumulates
  • People with severe kidney disease or contrast allergy (contrast may be withheld)

What to expect

Modality
CT (computed tomography)
Preparation
Fasting for a few hours before contrast; kidney function checked; a cannula placed. Oral contrast is sometimes drunk an hour before abdominal scans.
Duration
The scan itself takes seconds to a few minutes; the visit 20-30 minutes.
Radiation
A head CT is about 2 mSv, a chest CT 5-7 mSv and an abdomen-pelvis CT around 10 mSv: several years of background radiation, though modern scanners use far less than older ones.
Contrast
Iodine-based, injected into a vein for vessels, organs and tumours; causes a warm flush and a metallic taste. Metformin may be paused around the scan.
Risks
A small increase in lifetime cancer risk with each scan, weighed against the benefit; contrast reactions (rare) and a temporary drop in kidney function in those already at risk. Avoided in pregnancy unless essential.

Common uses

RegionTypical question
HeadHead injury, sudden severe headache, first seizure, suspected stroke or bleed
ChestLung cancer staging, pulmonary embolism (CTPA), fibrosis, complicated pneumonia
AbdomenSevere pain, suspected appendicitis, pancreatitis, bowel obstruction, cancer staging
HeartCT coronary angiography for stable chest pain; calcium scoring

Studies this page covers

CT headCT chestCT abdomenCT pelvisCT angiographyCT pulmonary angiographyCT colonographyLow-dose lung CTCT urogramCT perfusionHigh-resolution chest CTMusculoskeletal CT

Named studies of the master test taxonomy that this modality page covers; each links to its test category.