Computerised Tomography (CT) Scans

Computed tomography scanning (CT scanning) is used commonly in medicine today. It is similar to conventional radiology as it uses X-rays.

In order to obtain a CT scan patients lie in a CT scanner - similar to a bed inside a "polo" mint. The X-ray tube and the detectors are opposite to each other. Both of these rotate around the patient and information is obtained, usually in slices. The data is constructed by a computer and provides, most commonly, cross-sectional images in a single plane which can be interpreted.

The pictures are obtained by differences in X-ray absorption -compared with conventional radiology these differences are very small allowing different shades of grey and distinction between different tissues e.g. between fat and soft tissues and between brain and cerebrospinal fluid.

There are essentially two types of CT scans:

1. Conventional CT scan - the scan is taken slice by slice and after each slice the scan stops and moves down to the next slice e.g. from the top of the abdomen down to the pelvis. This requires patients to hold their breath to avoid movement artefact.
2. Spiral/helical CT scan - this is a continuous scan which is taken in a spiral fashion. It is a much quicker process and the scanned images are contiguous.

CT scans can also be distinguished according to the plane in which the images are taken. In fact CT scanning has been called CAT scanning meaning "computed axial tomography" describing axial images that are taken - which is the commonest plane. However, other planes of imaging can also be performed e.g. coronal or sagittal. Furthermore, the newer CT scanners can project these images into a 3D image.

When CT scans are produced the operator can choose the section level e.g. brain and also the thickness of the sections. The thickness of the sections are usually between 1 - 10 mm - the thinner the slices the more information in the images - although with the spiral CT scans this is superseded.

• Better detail compared with ultrasonography
• Relatively quick compared to MRI
• Most systems can be scanned e.g. brain to leg

• Requires breath holding which some patients can not manage
• Artefact is common e.g. metal clips
• CT scans of the brain can be affected by bone nearby
• High doses of radiation are involved in CT scanning - one CT scan is thought to be equivalent to having 100 - 250 chest X-rays¹

CT scanning provides images in shades of grey - occasionally the shades are similar making it difficult to discern between two areas. Contrast enhancement can be used to try to over come this problem. Barium is commonly used to outline the gastrointestinal tract and intravenous contrast is used to outline arterial blood vessels.

Intravenous contrast agents are iodine based and there is a risk of anaphylaxis with these. Newer agents are non-ionic and are less likely to cause allergic reactions. However, they are more expensive.

• Injections are usually given rapidly and can cause a feeling of warmth in the arm or even severe pain.
• Contrast can be extravasated which can be severe enough to require skin grafting.
• Nausea and vomiting.
• Urticaria.
• Anaphylaxis with bronchospasm, laryngeal oedema and hypotension.
• Renal failure - contrast is cleared renally and patients with pre-existing renal impairment may develop worsening renal function and even renal failure requiring haemodialysis (see below).

• Asthmatics are at increased risk of atopy and therefore, at an increased theoretical risk of anaphylaxis to intravenous contrast.
• Some hospitals use steroids prior to CT scans with contrast although there is no clear benefit of this policy.²
• Interestingly, a recent survey revealed that although the use of non-ionic contrast media has increased, the use of premedication with steroids is being increasingly used. This is unnecessary as the risk of allergic reactions is low with non-ionic contrast media.³

• Contrast is excreted renally.
• If patients have renal impairment, diabetes mellitus or reduced intravascular volume then they run the risk of accumulating the contrast.⁴
• This can lead to worsening of renal impairment or even renal failure.
• Generally, good hydration prior to contrast will reduce the risk of developing renal impairment.
• Other factors that will reduce the risk is identifying high risk patients early and stopping any nephrotoxic drugs.⁵
• N-acetylcysteine has also been used, it is given orally the day before and the day of the procedure. The efficacy of NAC is unclear although a recent meta-analysis showed that NAC used prophylactically reduces the risk of contrast related nephropathy.