Characterization of Carotid Atherosclerotic Stenosis on Photon-Counting Computed Tomography

Sammanfattning: Introduction and aim Atherosclerotic stenosis located at the carotid bifurcation accounts for 10% − 20% of all Transient Ischemic Attacks (TIA) and ischemic strokes, which occurs when Atherosclerotic stenosis, or so-called plaque, ruptures. The current method to evaluate carotid atherosclerotic stenosis is to measure the degree of stenosis. Strong markers for plaque rupture lies in the plaque content, and not in the degree of stenosis. Conventional Computed Tomography (CT) does not have the soft tissue resolution to differentiate the material content of the plaque. Photon Counting Computed Tomography (PCCT) has the potential to perform better plaque imaging due to its improved noise reduction, energy resolution, and image reconstruction algorithm. The aim of this project is to investigate if the PCCT images can differentiate materials in carotid atherosclerotic stenosis (ex vivo and in vivo), by using histopathological images as a reference. Materials and method A photon-counting CT, Siemens Naeotom Alpha, was used for scanning in this project. One patient with symptoms of atherosclerotic stenosis was examined on the PCCT using the clinical protocol. Carotid endarterectomy was performed and the removed plaque was encased in paraffin before being imaged once again at the PCCT. Histopathology analysis stained the interesting materials in ex vivo. The result of the staining was used to define the Region Of Interest (ROI) in the ex vivo and in vivo PCCT images. One-way ANOVA was performed to determine if statistically significant differences in the Hounsfeild Unit (HU) of each material in Virtual Monoenergetic Images (VMI) (40190 keV in steps of ten) existed, the CI = 95.0%. If a statistical significance was found two-sided t-test found the exact materials that were significantly separated. Bonferroni correction was performed to reduce the risk of type 1 errors, resulting in the CI = 99.2%. A visual comparison of the in vivo images was also performed between conventional CT and PCCT systems. Result The one-way ANOVA test yielded statistical significance for all monoenergetic energy levels in the ex vivo measurements. The consecutive t-tests, however, showed no statistical significance for any energy or material after correcting for multiple tests. The one-way ANOVA performed on the in vivo measurement did not show any statistical significance for any energy. The appearance of in vivo images from the conventional CT and the PCCT system was very different. The calcium blooming was less prominent and structures that were invisible on the conventional CT system could be distinguished on the PCCT system. Conclusion The materials could not be statistically significantly differentiated in the in vivo PCCT images. Ex vivo PCCT provided a larger difference in HU for different materials, but not significantly so. However, due to limited measurements, no certain conclusions could be drawn. The visual difference between in vivo images acquired by a PCCT and a conventional CT was considerable which could be valuable in the clinic due to the importance of good image quality when evaluating carotid atherosclerotic stenosis.