The presence of blood vessels within a developing atherosclerotic plaque has been shown to be correlated to increased plaque vulnerability and ensuing cardiac events, however, detecting coronary intraplaque neovascularizations poses a significant challenge in the clinic. In this paper, we demonstrate in vivo a new intravascular ultrasound imaging method using a dual-frequency transducer to visualize contrast flow in microvessels with high specificity. This method uses a specialized transducer capable of exciting contrast agents at a low frequency (5.5 MHz) while detecting their nonlinear superhamonics at a much higher frequency (37 MHz). In vitro evaluation of the approach was performed in a microvascular phantom to produce 3D renderings of simulated vessel patterns and to determine image quality metrics as a function of depth. Furthermore, the ability of the system to detect microvessels is demonstrated both ex vivo using porcine arteries and in vivo using the chorioallantoic membrane of a developing chicken embryo with optical confirmation. Dual-frequency contrast specific imaging was able to resolve vessels of a similar size to those found in vulnerable atherosclerotic plaques at clinically relevant depths. The results of this study adds growing support for further evaluation and translation of contrast specific imaging in intravascular ultrasound for the detection of vulnerable plaques in atherosclerosis
