Computed tomography

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As a medical student, it's highly unlikely you're going to be asked to interpret a CT scan. However, it is useful to understand what they're used for and what certain things look like on CT.

It's also known as CAT scan - computer-assisted tomography. This is largely an American term.

What is it, MedRevise?

It is a very big X-ray.

With plain X-ray, you get a single shot of X-ray beams and you get a single image. With CT, you get a couple of hundred X-rays and the computer puts them all together to give you a set of images.


As with any medical procedure, there are risks and benefits. The main risk is radiation. This predisposes to cancer.

If you have a 4-month old child, you would have to have a very good reason to do a CT scan. As they are growing, there's lots of mitosis going on and a much higher chance that you do damage to a gene and pre-dispose somebody towards cancer.

An 80 year-old lady with a background of MI, severe COPD and end-stage lung cancer is very amenable to CT. Even if she develops a neoplasm, it is unlikely to kill her.


This is not radiology in a paragraph but it will give you a vague idea of what you're looking at.

  • Black is air
  • White is bone, contrast or metal
  • Grey is everything else

Essentially, if they have given contrast, it will appear as bright white on the scan. Metal, like hip replacements, appear white as well. Soft tissues tend to appear grey, empyema appers grey, fat appears grey, colon get the picture.

Note: YOU ARE LOOKING FROM THE BOTTOM! When you see a CT scan, imagine you are at the end of the bed. The front is at the top, the back is at the bottom, the patient's left is on the right and the patient's right is on the left. Capiche?


When you want to look at vascular structures within the brain, contrast is given. Contrast is given intravenously or intrarterially and the flow of blood can be seen.

This can be used to rule out conditions such as intracranial haemorrhage, bowel ischaemia, pulmonary embolism and aortic aneurysm. Anything that involves bleeding or blockage of a blood vessel may be amenable to contrast.

This is contraindicated in renal failure (acute or chronic) - if you have an eGFR <20-30 depending on trust, you cannot have contrast.

Where do we CT and why?

CT head

Stroke and intracranial haemorrhage is the main reason to do an urgent CT head. Brain cancer or MS can often be ruled in as a diagnosis with CT. Fractures can be seen in trauma. CT angiography is often used to look at the structure of the neurovasculature.

CT head and neck can be used to rule out fractures (skull, cervical spine, facial, orbital), cord compression or to look for tumours or abscesses.

CT chest

The main thing chest CT is used for is pulmonary embolism and cancer. PE can be diagnosed with CT pulmonary angiography (CTPA) as is highly specific and sensitive for this condition. Bronchiectasis is often diagnosed using high=resolution CT.

CT abdomen/pelvis

These can be ordered seperately or together but are commonly ordered together.

Surgeons often use this to diagnose diverticulitis, bowel obstruction (where this is unclear on AXR), appendicitis (again, if the diagnosis is unclear), bowel cancer, bowel ischaemia (with contrast) and many more conditions.

Ovarian tumours, fibroids and other gynaecological pathology can be seen using this modality but this is largely done using transabdominal and transvaginal ultrasound.

CT chest/abdo/pelvis

This is used for staging of cancer by and large. Anybody who has a probably diagnosis of cancer should get a staging CT to look for metastases. It takes time for histology to come back so if you're fairly sure about the diagnosis, you would go ahead with these if the cancer you're talking about often metastasises.