Intravascular ultrasound contrast imaging at 25 MHz using radial modulation: A promising approach for coronary vasa vasorum imaging

In this study, we characterized a radial modulation (RM) intravascular ultrasound (IVUS) system, implemented using a 20 MHz commercial catheter, for contrast enhanced vasa vasorum imaging. RM is a dual band approach applicable for high frequency microbubble (MB) imaging, where a low frequency (LF) ultrasound pulse is used to manipulate the MB radius while a synchronized high frequency (HF) pulse successively measures MB backscatter in expanded and compressed states. Subtracting the two HF signals yields high resolution imaging with contrast enhancement and tissue cancellation. Lipid-encapsulated perfluorocarbon MBs were circulated in hollow agar-based tissue mimicking phantoms doped with sigmacell. The IVUS catheter was housed against the wall of the tube and rotated at 2.6 and 30 frames per second, allowing to simultaneously image MB and tissue scattering. RM and B-mode images were analyzed at increasing depths to determine contrast to tissue ratio (CTR) and contrast to tissue ratio improvement (CTRI) relative to the same image in B-mode. The effects of phase synchronization, MB concentration and LF pressure amplitude on the CTR and CTRI were measured. Our prototype IVUS system could produce RM images at 30 frames per second that selectively enhanced contrast signal and suppressed tissue and blood signal. Tubes with 200 μm diameter perfused with microbubbles and embedded in the tissue phantom to mimic microvessels could be selectively imaged at high resolution. The optimal synchronization phase was found to correspond to the oscillation phase of resonant MB. CTR and CTRI respectively decreased from 12 and 14 dB near the catheter to 5 and 3 dB at 5 mm, respectively. This corresponded to LF pressures dropping from 295 kPa at 2 mm down to 105 kPa at 5 mm. It was demonstrated that our prototype IVUS system allows microbubble specific imaging at high resolution. Contrast signal could be detected up to a distance of 5 mm, sufficient for coronary imaging. Further- improvements could be expected by using monodispersed MBs and developing a more efficient LF pressure delivery system.