CT intravenous contrast media
Intravenous contrast media used in CT (also simply called IV contrast) is used to improve contrast resolution (to aid ease of differentiation between tissues).
Once IV contrast has been injected, it flows around the vascular system in a predictable fashion:
- pulmonary artery
- aorta (approximately 20 seconds)
- spleen and portal system (approximately 60 seconds)
- inferior vena cava (approximately 100 seconds)
Appropriate IV access
In some cases, it is important to inject the contrast quickly to keep the bolus tight. This requires a pump that injects up to 300 psi. In order to achieve that pressure, the patient needs to have a relatively large cannula in a large vein. Tests that require a large cannula include CT angiograms, e.g. CT pulmonary angiogram, CT aorta, limb CT angiogram. As time progresses, the column of contrast becomes more dilute, vascular definition decreases and venous artefact increases. In many centres, pump-injection is not permitted through central lines.
Other examinations are less dependent on a tight bolus of contrast and the pump injects at slower (but controlled) rates. An example would be a routine CT of the abdomen and pelvis.
Some examinations don't require pump injection because delayed post-contrast imaging is all that is required, e.g. post-contrast CT head. Smaller peripheral cannula may be acceptable.
IV contrast is an iodinated liquid that is injected into a vein. There is a very small risk that some people may have anaphylactic reactions to the contrast media and in those cases, cardiac arrhythmia may result in cardiac arrest. This risk has decreased over recent years with newer contrast agents but is still real. If a patient has had an allergic reaction in the past to contrast, he or she may be pre-medicated with steroids and/or antihistamines. Shellfish allergies were once considered a contraindication for IV contrast, but this is no longer the case.
Iodinated IV contrast can contribute to worsening renal function, especially in patients with acute or chronic renal failure. The exact mechanism is not clear, but assessment of U&Es with an eGFR prior to a contrast-enhanced CT scan will be mandatory in many centres. The risk of renal dysfunction from IV contrast is now thought to be lower than it was once considered.
In patients with acute renal failure who require a CT scan with contrast, the clinical team will need to balance the risks and benefits and consider other options for investigation or ways to mitigate the risk, e.g. dialysis following the procedure. Patients who are currently on long-term dialysis can receive contrast without risk.
If a cannula is not sited correctly or tissues, contrast may extravasate into the surrounding tissues. This can be a serious complication and result in skin and soft tissue necrosis.
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