Combined ultrasound and photoacoustic imaging

Abstract

Biomedical imaging applications that provide solely morphological information of targeted tissues are often inadequate for effective diagnosis and therapy guidance. Ultrasound (US) imaging, which has been widely utilized for both preclinical and clinical applications within cardiovascular medicine for several decades, is one such modality capable of providing effective anatomical assessment of imaged tissues with inherently limited ability to distinguish cellular and molecular composition. Recent technological developments have led to a rapid expansion of preclinical research seeking to utilize and advance an imaging modality with the potential to fill this void—photoacoustic (PA) imaging. The PA effect refers to a phenomenon in which pressure waves are generated as a result of rapid thermal expansion following absorption of pulsed electromagnetic energy. PA imaging therefore utilizes an optical source for signal excitation and a US transducer to receive the generated PA pressure, which is a function of the local laser fluence, the wavelength-dependent optical absorption, and the efficiency of thermal expansion of the tissue. Due to the similarities in form of the detected signals, combined US and PA imaging can be performed using shared instrumentation and can yield coregistered, complimentary information. PA imaging can be performed at significantly greater imaging depths compared to pure optical imaging methods and at spatial resolutions similar to those achievable using US imaging and provides the ability to assess composition of an imaged medium based on unique optical absorption spectral characteristics. Furthermore, the addition of PA imaging provides an improved ability for therapy guidance, including temperature monitoring. These advantages have led to the investigation of combined US/PA imaging for applications ranging from the characterization of atherosclerotic plaque composition to the monitoring of cardiac ablation to the assessment of hemodynamic function. This chapter provides an introduction to the fundamental principles of PA imaging, the major components and techniques utilized for effective combined US/PA imaging, and the various applications most relevant to cardiovascular medicine which are subject to ongoing investigation, concluding with an assessment of the future outlook for this hybrid imaging technique.