A heart attack or stroke often triggers from “vunerable” plaque, which is arterial plaque build-up that’s at risk of detaching from the artery wall. Current technology to identify plaque, though effective, cannot determine whether or not it’s vulnerable, making it difficult to assess a patient’s risk. To remedy that problem, researchers from North Carolina State Univ. and the Univ. of North Carolina at Chapel Hill developed a dual-frequency intravascular ultrasound transducer that operates on two frequencies, both transmitting and receiving acoustic signals.

Two ultrasound techniques presently used to identify vulnerable plaque both rely on contrast agents termed “microbubbles.” The first method identifies “vasa vasorum,” clusters of small blood vessels that often permeate arterial plaque and indicate potentially vulnerable plaque. Detection of the vasa vasorum occurs when injected microbubbles flow through the blood vessels, which is highlighted on ultrasound images. The second method relies on the use of “targeted” microbubbles—they attach to specific molecules known to be found in vulnerable plaque, which then become visible on ultrasound images. While these methods are sufficient, it’s difficult for ultrasound technology to detect the contrast agents.

In creating the transducer, the research team, headed by Jianguo Ma, a mechanical engineering Ph.D. student at NC State, designed a small-aperture (measuring 0.6 x 3 mm2), intravascular-ultrasound (IVUS) probe suitable for high-frequency contrast imaging. The design leverages a dual-frequency (6.5 MHz/30 MHz) transducer arrangement that excites microbubbles at low frequencies and detects their broadband harmonics at high frequencies, reducing detected tissue backscatter.

The prototype probe creates a nonlinear microbubble response with over 1.2 MPa of rarefractional pressure at 6.5 MHz. It also can detect microbubble response with a broadband receiving element (30-MHz center frequency; ‒6-dB fractional bandwidth of 58.6%). Nonlinear super-harmonics from microbubbles flowing through a 200-µm diameter micro-tube were detected with a signal-to-noise ratio higher than 12 dB. Thus far, initial phantom imaging tests offer promising results for the device’s use in contrast-enhanced IVUS imaging.