Carotid plaque characterization with histology and quantitative ultrasound

Most atherosclerotic plaques are heterogeneous, making it difficult to classify them in the clinic as calcified or lipidic plaques. We propose a novel approach to characterize localized plaque regions with different tissue types. We will compare and correlate calcified, fibrous and lipid regions within heterogeneous plaque using a one-to-one registration with histology and quantitative ultrasound imaging (QUS). Atherosclerotic plaque excised during a carotid endarterectomy procedure was imaged ex vivo using a VisualSonics Vevo 770 ultrasound system. Attenuation coefficient images were obtained from three-dimensional (3D) radiofrequency data collected and rendered into a 3D volume. Fixed plaque tissue was then longitudinally sectioned with thickness of 5 µm, and sections separated by 100 µm were utilized. Two-dimensional histopathology images were digitized using a PathScan Enabler IV, registered and reconstructed into a 3D volume using Matlab. Different regions such as the lumen, calcified, lipid and fibrous regions were segmented by a pathologist and digitally color-coded into the 3D histology volumes. Similar tissue types from the 3D histology volume were then compared to the estimated 3D attenuation coefficient obtained using quantitative ultrasound methods. Our results indicate that calcified, lipid and fibrous regions in the two volumes demonstrate good correlation. Calcified regions delineated on the histology volume correspond to high attenuation coefficient regions (2.45 dB/cm/MHz) in the 3D attenuation coefficient volume, while lipid regions tend to have lower attenuation coefficient values (1.50 dB/cm/MHz). Fibrous regions in histology, correlate to the lowest attenuation coefficient value (0.83 dB/cm/MHz). Our work demonstrates a direct correlation between histology and ultrasound quantitative imaging characterization of plaque. Using 3D histology volumes, different tissue composition within the plaque can be better identified and characterized.