Dual Energy CT Scan

Author: Dr John Coucher*

What is a dual energy CT?

Dual energy CT scans are a newer form of CT scanning that use separate X-ray energies to make images. Images can be generated:

• by the simultaneous use of two X-ray tubes (‘dual source’);
• by using an X-ray detector with separate layers to detect two different energy ranges (‘dual layer’);
• by using a single scanner to scan twice using two different energy levels (electronic kVp switching); or
• by using an X-ray tube with two separate filters made of different materials between the tube and patient (‘twin beam’).

Some machine vendors refer to this as Spectral CT.

What are the generally accepted indications for a dual energy CT scan?

There are a wide and growing range of indications for selecting dual energy CT scanning over standard CT. The main current indications are:

• CT angiography. Dual energy scans can amplify the iodine signal of contrast agents, improving the delineation of arteries. They can also better distinguish iodine from calcium, therefore allowing better bone subtraction around vessels; for example, at the skull base.
• CT of the kidney, ureter and bladder (CT KUB). Dual energy CT KUB scans can reliably distinguish urate from non-urate calculi.
• CT imaging around metal implants. Dual energy CT can significantly reduce the streak artefact normally associated with metal implants and allow better visualisation; for example, around spinal rods or hip replacements.
• CT imaging of tumours. Dual energy scans can improve visualisation of vascular tumours in the abdomen and elsewhere.
• CT pulmonary angiography. Dual energy scans can reduce the amount of contrast required, improve detection of peripheral emboli, and detect areas of lung with reduced blood flow.
• Gout imaging. Dual energy CT can specifically detect the monosodium urate crystals responsible for gout.

What are the prerequisites for having a dual energy CT scan done?

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.

What are the absolute contraindications for a dual energy CT scan?

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.

What are the relative contraindications for a dual energy CT scan?

Relative contraindications for dual energy CT scans are no different to those of single energy CT scanners (see Computed Tomography).

What are the adverse effects of a dual energy CT scan?

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. With most 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)).

Is there any specific post procedural care required following a dual energy CT scan?

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).

Are there alternative imaging tests, interventions or surgical procedures to a dual energy CT scan?

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. Dual energy CT can create ‘virtual non-contrast’ (VNC) images from post-contrast images therefore reducing radiation dose if non-contrast images are also required. Photon-counting CT is a further technical development that uses a novel detector to directly quantify and convert multiple photon energies into electrical signals.

Useful resources about a dual energy CT scan

• Kambadakone, A, Marin, D. (eds), Dual Energy CT and Beyond. Radiologic Clinics of N. America. 2023. 61(6).
• Tatsugami, F. et al., Dual-Energy CT: Minimal Essentials For Radiologists. Japanese Journal of Radiology. 2022. 40:547-559.
• Parakh, A. et al., Dual Energy CT Images: Pearls and Pitfalls. Radiographics. 2021. 41:98-119.

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

Page last modified on 19/8/2024.