Mechanical failure of the fibrous cap of a vulnerable atherosclerotic plaque may lead to sudden plaque rupture and thus precipitate arterial thrombosis. Because ultrasound correlates strongly with mechanical features of tissues it might provide information on the stability of fibrous caps. The acoustic properties of the normal vessel wall and plaques, particularly fibrous caps of lipid-rich plaques, were evaluated in the aortic roots of six normal C57BL mice and 12 atherosclerosis-prone apoE-deficient (apoE-/-) mice by scanning acoustic microscopy (SAM). After processing, the attenuation of high-frequency (1.1 GHz) focused ultrasound was measured in unstained tissue sections by SAM followed by quantification of the amount and type of collagen in picrosirius red stained sections by means of polarized light microscopy (PLM). The acoustic and optical images were superimposed and ultrasonic attenuation was measured in different tissue components. Pertinent plaque features, particularly fibrous caps covering lipid-rich pools, were clearly visualized by both SAM and PLM. Collagen appeared green in thin fibrous caps and bright orange in thick caps by PLM. The attenuation of ultrasound was significantly higher in the collagen fibers with orange color compared to those with green color (17.2 versus 6.6×103 dB/mm). SAM has shown the possibility to characterize the types of collagen by high frequency intravascular ultrasound in vivo and it might improve our understanding of the vulnerable plaque and its sudden rupture from micro-mechanical point of view.
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