Combined subharmonic and ultraharmonic intravascular ultrasound imaging

Abstract

Abnormal proliferation of the vasa vasorum has been implicated in the rupture of atherosclerotic plaques. Imaging the vasa vasorum could help clinicians identify rupture-prone plaques and guide the choice of therapy. We hypothesized that subharmonic and ultraharmonic modes can be combined to improve the performance of contrast-enhanced intravascular ultrasound (IVUS) imaging. To test this hypothesis, vessel phantoms perfused with phospholipid-shelled ultrasound contrast agent (Targestar-P®, Targeson, Inc., San Diego, CA) were visualized using either subharmonic, ultraharmonic, or combined subharmonic and ultraharmonic modes. Flow channels, as small as 0.8 mm and 0.5 mm in diameter, were imaged using commercial peripheral and coronary imaging catheters at 12 MHz and 30 MHz transmit frequencies, respectively. Subharmonic and ultraharmonic imaging modes attained contrast-to-tissue ratios (CTRs) of 18 ± 2 dB and 20 ± 2 dB at 12 MHz transmit frequency, and 9 ± 2 dB and 13 ± 1 dB at 30 MHz transmit frequency, respectively. Combining subharmonic and ultraharmonic modes enhanced the CTRs to 33 ± 3 dB and 25 ± 2 dB at 12 MHz and 30 MHz transmit frequencies, respectively. These preliminary findings support the continued investigation of combined subharmonic and ultraharmonic IVUS for vasa vasorum imaging.