There were features on the dual energy CT angiogram, which included reduce attenuation on the affected side on the low energy mono energy reconstruction and vessel wall thickening on curved MPR reconstruction which could help in diagnosing vertebral artery dissection
Discussion:
Vertebral artery dissection can often be quite difficult to detect and manage, CT findings can be subtle, especially in cases of normal variants (hypoplastic side) that can mimic dissection.
Often MRI is needed to make the diagnosis, but the use of MRI is not always feasible due to the difficulty of acces
s, prolonged scanning time. In these 4 cases, all the patients had proven dissection on MRI.
The patients had 45 Kev mono-energy reconstructions of the CT angiogram, automatically generated for all the scans (the CT scanner that is used in our facility is detector-based spectral CT). These are virtual monoenergetic images (VMIs), which simulate the images that could be acquired with a true monoenergetic x-ray source.1
On the CT scans, all patients had marked reduction in contrast attenuation on the affected side (dissected vertebral artery), which may be due to reduced flow on that side. This finding is more obvious on the dual energy CT, as the energy is closed to the K-edge of Iodine leading to augmentation of any differences in brightness. low-energy VMIs reconstructed close to the k-edge of iodine at 33.2 keV show substantially higher attenuation of structures containing iodine and can be used to optimize enhancement of hypervascular lesions and parenchyma as well as blood vessels.2
Other causes of reduced brightness could be attributed to atherosclerotic clot or extrinsic compression of the vessel, although not always possible (likely in one of the cases, due to the vessel occlusion, MPR was not done) , but MPRs showing thickening of the vessel wall with irregularity, can aid in the diagnosis on the spectral CT.
for cases in which a dissection is not clinically suspected but is seen on contrast- enhanced images, virtual unenhanced images can show an intramural haematoma without the need for additional CT.3
VMI provides several advantages, such as reducing beam hardening and metal artifacts and optimizing image quality. VMI generated at lower energy levels can intensify the signal from contrast media, making it possible to reduce the volume of contrast media administered while still obtaining adequate enhancement in the lumen of vessels. Radiologists can also optimize the energy level at which to synthesize the best VMI based on the diagnostic purpose, as well as other factors such as the patient’s habitus.4
As the virtual monoenergetic energy level decreases, the attenuation of iodine increases. This concept can be used both to retrospectively salvage a poorly timed bolus or create a virtual arterial phase from an examination that has already been performed and to prospectively acquire CT angiograms with a reduced iodinated contrast material load.5-8
If we have both findings (reduced contrast attenuation on 45 Kev mono-energy and the thickening/irregularity of the vessel wall on the affected side), we can be more confident in diagnosing vertebral artery dissection on spectral CT.
We must acknowledge that these are limited data set from a single hospital, where further data is needed for validation.