Computed tomography

What is computed tomography?

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

In order to obtain a computed tomography 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 are constructed by a computer and provide, 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 - for example, between fat and soft tissues and between brain and cerebrospinal fluid.

What are the types of computed tomography?

There are essentially two types of computed tomography scans:

• Conventional CT scan - the scan is taken slice by slice and after each slice the scan stops and moves down to the next slice - for example, from the top of the abdomen down to the pelvis. This requires patients to hold their breath to avoid movement artefact.

• 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.

Computed tomography scans can also be distinguished according to the plane in which the images are taken. In fact computed tomography scanning has been called 'computed axial tomography (CAT)' scanning, describing axial images that are taken - axial being the most common plane. However, other planes of imaging can also be performed - for example, 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 (for example, brain) and also the thickness of the sections. The thickness of the sections is usually between 1-10 mm - the thinner the slices, the more information in the images - although, with the spiral CT scans, this is superseded.

Advantages of computed tomography scanning

• Better detail compared with ultrasonography.

• Relatively quick compared with MRI scanning.

• Most body systems can be scanned - for example, brain to leg.

Disadvantages of computed tomography scanning

• Requires breath holding which some patients cannot manage.

• Artefact is common - for example, metal clips.

• Computed tomography scans of the brain can be affected by bone nearby.

• High doses of radiation are involved in CT scanning - chest CT scan is equivalent to 350 chest X-rays; CT abdomen to 400 chest X-rays and CT pulmonary angiography 750 chest X-rays.^{1 2} However, the consensus is that clinically, the dangers of such exposure in adults are small and less of a risk than withholding CT if it is required for diagnostic purposes.³

• The situation regarding exposure to unborn babies during pregnancy is however different. There is a risk of childhood cancer and leukaemia in mothers who have CT imaging during pregnancy.² However, some of the studies are small and difficult to interpret due to confounding factors. Imaging to aid potentially fatal conditions during pregnancy should not be withheld.⁴

• In children, it is preferable to use modalities which do not involve exposure to ionising radiation, such as ultrasound and MRI, if possible.³

Computed tomography scanning with contrast³

Computed tomography 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 overcome this problem. Barium is commonly used to outline the gastrointestinal tract; intravenous contrast is used to outline arterial blood vessels.

Some intravenous contrast agents are iodine-based and there is a risk of anaphylaxis with these and worsening of acute kidney injury. Barium sulfate and gadolinium-based are non-ionic and are less likely to cause allergic reactions. However, they are more expensive.

Side-effects of intravenous contrast

• 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 'Contrast-induced renal impairment', below).

Asthma and hypersensitivity to contrast agents

• Those with asthma are at increased risk of atopy and are, therefore, at an increased theoretical risk of anaphylaxis to intravenous contrast.

• Steroid premedication before a contrast CT scan is generally recommended for patients with a history of adverse reactions to contrast media. This has been shown to reduce the recurrence of moderate and severe hypersensitivity reactions. ⁵

Contrast-induced renal impairment

• Contrast is excreted renally.

• If patients have chronic kidney disease, diabetes mellitus or reduced intravascular volume then they run the risk of accumulating the contrast.⁶

• This can lead to worsening of renal impairment or even renal failure.

• Recent eGFRs (within the last 6 months) should be reviewed prior to scans with contrast. There is a small but increased risk of acute kidney injury associated with an eGFR less than 30 ml/min/1.73 m2. ⁷

• If the eGFR is unavailable, it is important to ask some screening questions to assess the patient:
  
  - Do they have kidney disease or a kidney transplant?
  - Have they seen or are waiting to see a kidney specialist, or a kidney surgeon or urologist?
  - Do they have symptoms of acute illness likely to cause acute kidney injury such as diarrhoea, vomiting, fever, hypovolaemia, infection or difficulty passing urine?
  
  If there is a history, then carry out an eGFR test, if not- the National Institute for Health and Care Excellence (NICE) suggests proceeding for the scan if the patient is stable.
  

• Generally, good hydration prior to contrast will reduce the risk of developing renal impairment.

• Other factors that will reduce the risk are identifying high-risk patients early and stopping any nephrotoxic medicines.⁸

• Consider temporarily stopping ACE inhibitors or ARBs in a patient with an eGFR less than 30ml/min/1.73 m2

• Metformin is usually withheld before a computed tomography scan.

• N-acetylcysteine (NAC) 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.