From the September 2019 issue of HealthCare Business News magazine

Virtual unenhanced (aka “virtual non-contrast”) imaging
Many scan protocols call for an initial non-contrast scan followed by a contrast-enhanced scan. This allows the radiologist to differentiate between tissues that were intrinsically bright on the image prior to contrast administration from those which became bright only after contrast administration. However, this comes at extra expense, time, and radiation dose to the patient. With spectral CT, once the iodine and water contributions have been decomposed, virtual unenhanced images can be generated which mimic a non-contrast scan. In some applications this virtual unenhanced image may be able to replace an actual non-contrast scan in multiphase pre- and post-contrast protocols, saving time, radiation dose and cost.

Virtual monoenergetic imaging
X-ray beams consist of individual photons of varying energy, although with a user selected peak energy level (i.e. 120 kVp). This is referred to as a polyenergetic or polychromatic beam. The image generated is therefore a composite of all of the different energy X-ray photons contained in the beam. Spectral CT allows the generation of virtual monoenergetic images, as if the beam consisted entirely of a single X-ray energy. This can benefit in several ways.

Lower energy images can be generated that enhance the appearance of iodinated contrast. These have been shown in multiple studies to improve lesion conspicuity in the liver, both for hypervascular and hypovascular masses. Additionally, they can be used in angiography to enhance vascular contrast. This can be leveraged to reduce contrast dose, which can both reduce cost as well as improve patient safety (lower risk of contrast induced nephropathy), and can also reduce the need for repeat scans due to a suboptimal contrast bolus. CT pulmonary angiography occasionally suffers from transient contrast interruption which can result in a non-diagnostic scan, for example. In these cases patients are often re-dosed with contrast and rescanned, resulting in both double the contrast load as well as double the radiation dose. With virtual monoenergetic imaging, lower energy images can be reconstructed that enhance what contrast there is and can salvage an otherwise non-diagnostic scan, reducing cost as well as contrast and radiation dose to the patient.

On the other hand, higher energy images can be generated which reduce metal artifact, such as in the presence of orthopedic hardware. This is of significant benefit in the evaluation of joint prostheses, where traditional CT scans suffer from severe artifacts. The difference is so dramatic, in fact, that in our own institution, many orthopedists now specifically request scans to be done with dual energy. We also use higher energy virtual monoenergetic images in addition to metal artifact reduction to improve CT myelography in the presence of spinal hardware, and have not looked back.

September 2019 Magazine

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