Impact of virtual monoenergetic images on the assessability of lower extremity arteries in (Poly-) trauma photon-counting detector CT.

Abstract

Background: Computed tomography (CT) is state of the art imaging modality in patients after severe trauma. A double-bolus contrast media protocol can be used to combine both arterial and parenchymal contrast in one scan. In case of lower extremity trauma, the CT scans can be extended to include the legs in order to rule out vascular pathologies. Virtual monoenergetic (VME) images derived from a Photon-Counting detector CT (PCDCT) proved to be helpful in vascular diagnostics and could therefore also add information in these extended trauma scans. Aim of this study was to determine the VME reconstructions with sufficient vascular contrast (> 200 Hounsfield Units) and to examine, which VME level has best specificity and sensitivity as well as best subjective vascular visibility.

Methods: In a retrospective analysis, we identified 34 patients undergoing extended whole-body trauma scans including lower extremities on a PCDCT with pathologies of the lower extremity arteries (extravasation, dissection etc.) and paired them with 34 patients without vascular pathologies. We measured CT values and noise of six vascular segments on energy levels 40 to 190 keV and calculated signal to noise (SNR) and contrast to noise (CNR) ratio. Subjective ratings for vascular visibility were performed on a five point Likert scale and two readers analyzed the images for vascular lesions across four energy levels (40, 70, 90 and 110 keV).

Results: CT attenuation was measured above 200 HU below 88 keV in the common femoral artery, below 86 keV in the superficial femoral artery and below 85 keV in the popliteal artery. For the tibial arteries, we found higher CT values above 200 HU at the energy levels below 87 keV. Noise was highest at 40 keV for all energy levels, SNR and CNR for common femoral artery was highest at 40 keV. Sensitivity peaked at 40 keV (78.4%) and showed decreasing values for higher energy levels. Specificity increased with higher keV levels up to 91.7%. 40 keV showed the highest sum of Likert ratings regarding vascular visibility.

Conclusion: PCDCT derived VME reconstructions below 86 keV showed sufficient vascular contrast in extended whole-body trauma scans after double-bolus contrast media administration. Smaller vessel depiction can be hampered by increased noise on lower energy levels resulting in decreased image parameters. Lower energy levels have the potential to improve the detection of vascular pathologies, such as bleeding and vascular occlusions.