Gadolinium Contrast Medium (MRI Contrast agents)
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Dual energy CT scans are a relatively new form of CT scanning that use separate X-ray energies to make images. Images can be generated:
There are a wide and growing range of indications for selecting dual energy CT scanning over standard CT. The main current indications are:
The prerequisites for a dual energy CT scan are identical to a standard CT scan. Pregnancy should be excluded and, if iodinated contrast is required, the renal function should be known. This particularly applies if the patient is diabetic, has renal disease, is on nephrotoxic drugs or is aged over 60 years (see RANZCR contrast guidelines). Any history of previous contrast reaction should be recorded on the request form.
Given the precautions about iodinated contrast, there are no absolute contraindications for dual energy CT. If contraindications to intravenous iodinated contrast exist, consideration should be given to carrying out an alternative study or optimising the relevant factors; for example, by pre-hydrating the patient or having anaesthetic cover available. Discussion of this type of case with the radiologist is often helpful.
Relative contraindications for dual energy CT scans are no different to those of single energy CT scanners (see Computed Tomography).
The major adverse effects of a dual energy CT scan are related to the radiation exposure and the use of intravenous iodinated contrast (not always required).
Radiation (see Radiation Risk of Medical Imaging for Adults and Children). Dual energy CT produces similar doses overall to the standard CT machines in use. Scanners that use two exposures or dual layer X-ray detectors tend to use a slightly higher dose of radiation. Newer machines with in-built dose-saving technology can use similar radiation doses to standard CT devices. With ‘dual source’ dual energy CT scanners, the combination of a more efficient arrangement and dose-saving technologies means that the dose can actually be less than older, standard CT scanners.
Contrast reactions. Not all dual energy CT uses contrast, but there obviously is a risk of contrast reaction for those that do (see RANZCR contrast guidelines and Iodine-containing contrast medium (ICCM)).
Most patients can leave straight after any venous cannula has been removed. However, it is recommended that patients at higher risk of contrast reaction; for example, previous reaction or atopic individuals, are observed for 30 minutes post-contrast administration, and that the venous cannula is left in situ for that time. (see RANZCR contrast guidelines).
CT angiography can be carried out on a standard CT scanner, but image quality and bone subtraction might not be as good. MRI and ultrasound may provide further evaluation of blood vessel structure and anatomy. Non-invasive renal stone and gout evaluation cannot be carried out by any other technique. Development of reconstruction systems for standard CT scanners has produced improved metal artefact suppression using iterative reconstruction techniques. However, the combination of dual energy CT and iterative metal artefact suppression is likely to be superior. Research is ongoing on this point.
Page last modified on 26/7/2017.
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