P0-1 High Resolution Fundamental and Harmonic Imaging Using a MEMS Fabricated Ultrasonic Transducer

Intravascular ultrasound (IVUS) imaging is increasingly employed to assist in selecting and evaluating therapeutic intervention. Recent work in IVUS backscatter analysis demonstrates the capability of IVUS to characterize specific lesions and identify plaques that lead to various clinical syndromes. Correct identification of plaque types depends on their structure, composition and sufficient image resolution (< 30 mum axially). Tissue harmonic imaging (THI) has shown promise to increase quality of IVUS. However, Current IVUS catheters, with unfocused transducers and narrow bandwidths, might not benefit from advantages due to harmonic imaging. Hence, we are developing miniature focused large bandwidth ultrasonic transducers with high resolutions and harmonic imaging capabilities.We made spherically focused broad bandwidth (120%) PVDF- TrFE transducers using MEMS techniques. We characterized the transducers. We also developed a method for multiple ultrasonic modalities using existing hardware and employing pulse-inversion techniques where a single transducer can be used to produce four modes of images. In this study, we obtained images of the coronary ostium in four imaging modes (Fundamental 20 MHz, Fundamental 40 MHz, Harmonic 40 MHz and Harmonic 80 MHz). We also excited our transducer with 40 MHz monocycle pulse for maximal axial resolution to generate standard mode images. Our standard mode images were compared to histology and commercial IVUS Systems. Axial resolutions with a monocycle pulse were typically less than 19 mum with a bandwidth of ~ 120 %. The lateral resolution characterization was done using a 1 mm transducer with an f-number 2.6. Harmonic signals showed better resolution compared to the fundamental signals. Ex vivo circumferential images of the human coronary ostium at F20, F40, H40 and H80 were acquired. The standard mode ultrasonic image of the human aorta using the MEMS transducer clearly displayed better delineation of the media compared to the existing commercial IVUS systems. When comparing the standard mode 40 MHz ultrasonic image to its histology, our ultrasonic image showed near histological resolution and identified various features in the histology. Such promising results suggest that the high resolution transducers with multi modality imaging capabilities can improve the clinical use of IVUS for cardiovascular diseases.