Computed Tomography (CT)

Authors: Mr Jason McKenzie*
                            Prof Stacy Goergen *

What is computed tomography?

Computed Tomography (CT) is a high resolution technique using X-ray technology to generate images of any area of the body. Patients are placed on a moveable X-ray table which then enters the CT gantry. Depending on the equipment used, one or sometimes two Xray tubes rotate around the patient at high speed and up to 320 axial slices of the body can be obtained per rotation. This produces a large amount of data which is analysed using powerful computer algorithms to produce images which can be viewed in any anatomical plain or reconstructed into three dimensional rotating images. All body structures can be visualised and this can be enhanced with the use of intravenous iodinated contrast medium.

The rapid acquisition of data from CT means that accurate images and information can be obtained without being degraded by patient movement, be it physiological (e.g. cardiac or respiratory) or unexpected (e.g. uncooperative patient).

What are the generally accepted indications for computed tomography?

Computed tomography is a useful and accurate cross-sectional imaging test ideally suited for investigating possible pathology in body cavities where the organs of interest may not be accessible to superficial imaging techniques (e.g. ultrasound). These cavities include the skull, thorax, abdomen and pelvis.

CT is a good examination in a variety of conditions including:

• acute head injury;
• suspected subarachnoid haemorrhage;
• ureteric calculus;
• acute cervical spine trauma where there is a higher than average likelihood of fracture or dislocation;
• suspected acute appendicitis in a non-pregnant patient.

It has excellent capabilities to differentiate various soft tissue structures, and can provide excellent bone detail. Compared to other cross-sectional techniques, such as magnetic resonance imaging, it can be done so rapidly that it is minimally affected by patient movement.

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. Other techniques may be more appropriate in these patients.

Before referring your patient to have a CT scan, it is important to rule out early 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 (e.g. assessment of liver metastatic disease or CT vascular imaging) you should check that their renal function is normal. If their renal function is insufficient, the use of intravenous 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 before CT scanning – only those with certain risk factors, such as diabetes, advanced age, current or past history of abnormal kidney function, single kidney or kidney surgery, which 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 before contrast administration. Please contact the radiology department if you are still unsure whether contrast injection is required or whether you need to check renal function before their procedure.

If the patient is found to have impaired renal function, an informed decision will be made by the radiologist, after discussion with you and the patient, about the clinical indication for the scan. Sometimes the risk of contrast administration is outweighed by the potential information provided by the CT, 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?

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

Patients who have a known allergy to the intravenous 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. If there are no other imaging alternatives, it may be possible to do scans in patients with minor allergies, but this should be discussed with the radiologist.

Renal impairment may also prohibit your patient from having IVCM. You should check the patient’s creatinine and estimated glomerular filtration rate before referral. An estimated glomerular filtration rate of less than 30 mL/min increases the risk of acute renal injury (for further information see the Health Consumer section of InsideRadiology: Iodine-containing contrast medium).

Hyperthyroidism or goitre may be a contraindication to the use of IVCM, as it can 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.

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 220 kg. 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 carried out. A common diameter is approximately 70 cm.

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 IVCM.

When using 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, constant revision of scanning protocols and ensuring that only areas of clinical concern are examined, 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 20 µSv. A brain CT results in an approximate effective dose of 2200 µSv, 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 (2400 µSv).

The effective dose from a brain CT (2200 µSv) 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)).

More recent scanners have ‘low dose’ imaging techniques, which may be appropriate for your patient. These reduce X-ray dosages even more significantly, sometimes even lower that a simple chest X-ray.

Please see “Radiation Risk of Medical Imaging for 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 might not be evident on scans carried out 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.

Is there any specific post procedural care required following computed tomography?

No post procedural care is required.

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

CT scanning may not be the most suitable test to investigate your patient in a number of clinical situations. Other imaging options include magnetic resonance imaging, which is particularly good for the brain, spine and the musculoskeletal system. Ultrasound is also useful for imaging the non-bony parts of the musculoskeletal system, soft tissues and vascular structures, providing the area of clinical concern is accessible by the ultrasound probe from the skin surface. Ultrasound and magnetic resonance imaging do not involve ionising radiation.

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)
   www.mja.com.au/public/issues/180_11_070604/dic10124_fm.html
2. USA Food and Drug Administration:
   www.fda.gov/radiation-emittingproducts/radiationemittingproductsandprocedures/medicalimaging/medicalx-rays/ucm115317.htm
3. Impactscan.org:
   www.impactscan.org/

References

Thomson JEM, Tingey DRC (1997) Radiation doses from computed tomography in Australia, ARL/TR 123

*The author has no conflict of interest with this topic.

Page last modified on 15/9/2017.