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Intravascular photo-acoustics

Chemically specific optical absorption spectra can be used for tissue identification in ultrasonic imaging of atherosclerosis.

Researchers: Krista Jansen, Gijs van Soest, Geert Springeling, Ton van der Steen

Intravascular imaging has fostered a revolutionary enhancement of our understanding of atherosclerosis in the coronary artery system. Vulnerable atherosclerotic plaques are responsible for a great majority of acute cardiovascular events and sudden cardiac deaths. Rupture of the plaque releases the necrotic material into the bloodstream, causing a thrombus to form, which may block the artery. The current challenge is to indentify these plaques in vivo, before they become symptomatic. 

Presently, intravascular ultrasound, or IVUS, is a common technique in interventional cardiology. It provides grayscale images of the morphology of the plaque. In this project we develop a novel imaging modality, called intravascular photo-acoustic imaging (IVPA), that adds chemical specificity to IVUS. It will allow distinction between different soft tissues, something that IVUS cannot do by itself.  

IVPA relies on generation of acoustic waves by the absorption of nanosecond light pulses. The dissipation of the absorbed optical energy causes instantaneous local thermal expansion of the tissue, which generates a broadband acoustic wave. The acoustic signal can be detected using an ultrasound transducer such as used in IVUS; see figure 1 for a sketch of the principle. An image of the optical absorption, and hence, chemistry, is constructed by measuring the elapsed time between the optical excitation and the detection of the acoustic signal.

Probe design
Part of the research in this project is the development of a hybrid optical/ultrasound catheter for intravascular photo-acoustic imaging. It is conceptually based on an IVUS design, but adds optical access for the delivery of the excitation light pulse. Requirements include: state-of-the-art IVUS imaging capability, able to withstand moderate-energy nanosecond pulsed optical fields, and an outer diameter < 1 mm. Figure 2 is a photograph of a first functioning prototype. The optical and acoustic beams overlap for several millimeters radially from the catheter, providing imaging range.

Imaging pathology
A second research theme in this project is the imaging of human atherosclerosis using IVPA. We will characterize human coronary arteries, obtained at autopsy, at several wavelengths in the visible and infrared spectral range. The specimens will then be processed for histology to compare the IVPA images to the vessel wall condition.