Virtual Monoenergetic Imaging of Thoracoabdominal CTA on a Photon-Counting Detector CT: Assessment of Image Quality and Leveraging Low-keV Series for Salvaging Suboptimal Contrast Acquisitions

Abstract Purpose To assess the possibility of image improvement of ECG-gated, high-pitch computed tomography angiography(CTA) of the thoracoabdominal aorta before transaortic valve replacement(TAVR) on a novel dual-source photon-counting detector CT(PCD-CT) in the setting of suboptimal low-contrast attenuation. Methods Continuous patients who underwent an ECG-gated, high-pitch CTA of the aorta on a PCD-CT with a contrast decrease of at least 50% between the ascending aorta and the common femoral arteries(CFA) were included. Patient characteristics were documented. Virtual monoenergetic imaging(VMI) reconstructions with three keV settings were generated. CT-values and noise were measured for five vascular segments of the aorta and the CFA. Signal-to-noise(SNR) and contrast-to-noise ratios(CNR) were calculated. Two independent board-certified radiologists rated the images with the focus on vascular attenuation, vessel sharpness and image quality using a 5-point Likert scale. Results Fifty-five patients (mean age 77.4±8.5 years; 15 women) were included. The SNR was significantly higher at 40 and 45 keV VMI compared to reference 70 keV(p<.001 and p=.005, respectively). The same was shown for the CNR(p<.001 and p=.0049, respectively). Subjective image evaluation showed a significant increase in vessel attenuation in the lower keV reconstructions while the overall image quality decreased only slightly. Furthermore, 50%(8/16) of primarily non-diagnostic scans were considered diagnostic when using low keV reconstructions (p>.05). Conclusion ECG-gated CTA of the aorta in high-pitch mode on PCD-CT with suboptimal contrast enhancement at the level of the CFA can be salvaged by using low keV VMI. This implies the possibility of radiation dose reduction by eliminating the need for repeat scans.