Characterization of vulnerable plaques by multiphoton microscopy.

Cardiovascular disease is the primary cause of death in the United States; the majority of these deaths are caused by the rupture of vulnerable plaques. An important feature of vulnerable plaques is the thickness of the fibrous cap that covers the necrotic core. A thickness of less than 65 microm has been proposed as a value that renders the plaque prone to rupture. This work shows that multiphoton microscopy (MPM) can image the plaque with microm resolution to a depth deeper than 65 microm. The fibrous cap emits primarily second harmonic generation due to collagen, in contrast to the necrotic core and healthy artery, which emits primarily two-photon excited fluorescence from elastin. This gives a good demarcation of the fibrous cap from underlying layers, facilitating the measurement of the fibrous cap thickness. Based on a measure of the collagen/elastin ratio, plaques were detected with a sensitivity of 65% and specificity of 81%. Furthermore, the technique gives detailed information on the structure of the collagen network in the fibrous cap. This network ultimately determines the mechanical strength of the plaque. A mechanical model based on this information could yield a measure of the propensity of the plaque to rupture.