CT scan vs X-ray: which test answers which question?

If the question is "is this bone broken or this joint aligned?", a plain X-ray is usually the right first test. If the question is "what is happening inside the abdomen, head, or chest?", or if the X-ray leaves the answer uncertain, a CT scan is more likely to settle it.

The two tests are not rivals so much as stages. Many pathways start with an X-ray because it is fast, widely available and gives a low dose, then move to CT only when the first picture cannot answer the question or the clinical picture is serious enough to justify going straight to CT.

A plain X-ray passes a single beam through the body onto a detector. Dense tissue such as bone absorbs more of the beam and appears white; air appears black. Because the picture is flattened, structures that sit behind one another overlap, and a small abnormality can hide behind a rib or the spine.

A CT scanner sends thin X-ray beams from many angles as you pass through a ring. A computer reassembles the measurements into slices. Overlap is removed, soft tissue contrast is much better, and the radiologist can scroll through the body layer by layer. Contrast dye given through a vein can highlight blood vessels, tumours and inflammation.

Both tests use ionising radiation, and the aim is always to use the lowest dose that answers the question. Radiology departments follow a principle called justification: the test should have a realistic chance of changing management. The Royal College of Radiologists publishes referral guidance that clinicians use to decide when imaging is appropriate and which type.

Dose matters more for some people than others. Children are more sensitive, pregnancy needs special consideration, and people who have had many scans over the years may be asked about previous imaging. None of this means a necessary CT should be avoided. An untreated bleed or an undiagnosed clot carries far greater risk than the dose from a single scan.

These are the usual decision rules clinicians apply. Your own consultant may weigh them differently.

• Suspected broken bone in a limb with a clear injury: X-ray first.

• Persistent pain after a fall with a normal X-ray: CT (or MRI) may be arranged to look for a hidden fracture.

• Severe abdominal pain, suspected bleeding or head injury with red flags: CT, often urgently, because internal organs cannot be assessed on plain film.

• Cough or breathlessness: chest X-ray first; CT if the X-ray shows something that needs characterising or if a clot is suspected.

• Joint wear (arthritis): a weight-bearing X-ray shows joint space narrowing and is usually enough; CT adds little.

• Planning complex surgery: CT can give a three-dimensional bone map that an X-ray cannot.

A normal X-ray does not always mean nothing is wrong. It means nothing that shows on that test was found, and early or small fractures can be invisible. Equally, a CT is not automatically "more accurate" for everything; for a ligament or cartilage injury, MRI is usually the better test and CT would add radiation without answering the question.

Another misconception is that you should ask for a CT "to be safe". Unnecessary scans can produce incidental findings that lead to more tests and worry without benefit. A good referral matches the test to the problem.

• What question is this scan meant to answer, and what will we do differently depending on the result?

• Would an X-ray, ultrasound or MRI answer it with less radiation?

• Will contrast dye be needed, and do my kidney function or allergies matter?

• How soon will the report be available and who will explain it?