Computed Tomography (CT)

Authors: Mr Jason McKenzie*
                            Prof Stacy Goergen *

What are the prerequisites for having computed tomography done?

Before referring a patient for a CT scan, it is important that you are certain that a CT scan is the best test to confirm or exclude the condition(s) you suspect. If you are not certain, contact a radiologist and ask their advice, especially before requesting CT scanning in a child or young person because of their greater radiosensitivity.

Before referring your patient to have a CT scan, it is important to rule out pregnancy in a female of child bearing age as this is usually an absolute contraindication to CT.

If the test you are requesting requires the patient to have an iodinated contrast injection, you should check that their renal function is normal. If their renal function is insufficient, the use of IV contrast can cause further injury to their kidneys and in severe cases this may necessitate short or long term dialysis. Not all patients need to have their renal function tested prior to CT scanning, but certain factors such as diabetes, advanced age, current or past history of abnormal kidney function, single kidney or kidney surgery, all increase the chance of kidney function impairment. A number of other risk factors can also increase the risk of contrast induced renal failure (See Iodine-Containing Contrast Medium). If any of these are present, it is prudent to test renal function prior to contrast administration.

If the patient has renal function impairment after testing, an informed decision can be made by the radiologist after discussion with you and the patient about the clinical indication for the scan. It is true that sometimes the risk of contrast administration is outweighed by the information provided, even in a patient with renal impairment. Knowing in advance that their kidney function is impaired, and to what degree, may allow measures to be taken before and after the scan, such as intravenous hydration and minimisation of the contrast volume administered, that will reduce the likelihood of severe or prolonged contrast induced nephropathy.

What are the absolute contraindications for a computed tomography?

Due to the relatively high radiation dose involved in CT scans, it is important to avoid scanning patients who are pregnant. Radiation exposure to a foetus can cause developmental problems. Thus, CT should only be performed for pregnant patients in critical situations and only after discussion of the potential risks.

Patients who have an allergy to the IV contrast media (IVCM) used in CT scans should not be referred for scans where IVCM is required to attain a diagnosis. These tests include CT angiograms and most abdominal and chest scans.

Renal impairment may also prohibit your patient from having IVCM. You should check the patient’s creatinine and eGFR prior to referral*.

Hyperthyroidism or goitre may be a contraindication to the use of IVCM as it may induce thyrotoxic crisis in these patients*.

Patients with phaeochromocytoma may experience a hypertensive crisis if intravenous contrast is administered, so if this is suspected clinically and biochemically, a non-contrast scan is prudent.

Patients with myasthenia gravis have a small increased risk of worsening of their myasthenia, including respiratory muscular weakness, when iodinated contrast is administered* and thus contrast should be used with caution and patients with myasthenia monitored after contrast administration.

Check with the radiology department whether your patient will require IVCM and what their policies are regarding its use in the above conditions.

*See Iodine-containing contrast medium (ICCM) for further information regarding the risks of contrast medium.

What are the relative contraindications for computed tomography?

All scanners will have weight limits that are specified by the manufacturers. Newer scanners have higher limits than their predecessors with some able to accommodate 220kg. Despite these weight recommendations, there may be limitations to the patient’s size. The gantry of the scanner is a fixed diameter and if the patient cannot fit through the gantry, the scan cannot be performed. A common diameter is about 70cm.

You should check with the radiographers at the radiology department you refer to regarding weight limits and gantry size if you have concerns about a particular patient being too large to be scanned.

What are the adverse effects of computed tomography?

There are two components of CT scanning that have potential risks. These are the radiation dose involved in scanning and the use of IV contrast media (IVCM).
When employing radiation for any type of examination, radiographers adhere to the ALARA (As Low As Reasonably Achievable) Principle. This principle, along with calibration of the scanner and constant revision of scanning protocols, results in the lowest dose possible to acquire diagnostic images.

The risk of radiation exposure is a product of the dose. An average chest X-ray gives a patient an effective dose of 20µSv. A brain CT results in an approximate effective dose of 2200µSv, which is close to the amount of background radiation that an average Australian receives in a year from the atmosphere, even if they do not have any diagnostic imaging of any kind (2400µSv). The effective dose from a brain CT (2200µSv) is estimated to give an additional lifetime risk of radiation induced fatal cancer of 1 in 9000. This is equivalent to smoking 175 cigarettes or travelling 8800 km in a car. It must be remembered that the average expectation of dying from cancer (independent of any radiation exposure) is 1 in 4 (these figures are taken from Radiation Doses from Computed Tomography in Australia, ARL/TR 123 (1997)).

Please see Radiation Risk of Medical Imaging in Adults and Children, for a detailed discussion of the potential risks.

Intravenous iodinated contrast media are generally safe but can be associated with:

  • mild, moderate, or severe (anaphylactic) allergic reaction;
  • contrast induced renal impairment in susceptible people;
  • worsening or precipitation of thyrotoxicosis;
  • the need for special precautions in some patients with diabetes taking metformin.

Please see Iodine-containing contrast medium (ICCM) for detailed information about all of these risks.

As with many other drugs and procedures, the small risks associated with intravenous iodinated contrast media must be balanced against the often considerable additional information that is provided to the radiologist interpreting the scan.

The radiologist is best placed to determine whether contrast is needed, but his or her ability to make this determination depends greatly on the clinical information provided. For example, if the main question is whether or not acute ureteric obstruction from a calculus is present, contrast is usually unnecessary and can even obscure the calculus so that it is not seen by the radiologist. On the other hand, if a kidney cancer is suspected, it is important that contrast is administered as small cancers may not be evident on scans performed without it.

Therefore, it is vital that you provide as much detailed information about the patient’s clinical situation and your clinical question(s) on the request for the CT scan. Telephone the radiologist if you have any doubts about whether or not intravenous contrast is likely to be needed.

Are there alternative imaging tests, interventions or surgical procedures to computed tomography?

While CT is quite a good examination for a variety of conditions, it may not be the most suitable test to investigate your patient specifically. Other imaging options may include MRI, which is particularly good for brain and spine imaging and the musculoskeletal system.

Ultrasound is useful for imaging the non-bony parts of the musculoskeletal system and it is also good for soft tissue and vascular imaging. Ultrasound and MRI do not involve ionising radiation.

Conditions such as acute head injury, suspected subarachnoid haemorrhage, ureteric calculus, and acute cervical spine trauma where there is a higher than average likelihood of fracture or dislocation, are all best imaged with CT scanning, as is suspected acute appendicitis in a non pregnant patient.

If there is any doubt about which test is appropriate, you should contact the radiologist at the radiology department to which you refer.

Useful websites about computed tomography:

  1. Medical Journal of Australia (subscription required)
  2. USA Food and Drug Administration:
*The author has no conflict of interest with this topic.

Page last modified on 26/7/2017.

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