Deformation-Compensated Averaging for Deep-Tissue LED and Laser Diode-Based Photoacoustic Imaging Integrated with Handheld Echo Ultrasound

Averaging is a fundamental necessity for deep photoacoustic (PA) imaging when using low-energy pulsed laser sources or LED’s. Intrinsic (breathing, heartbeat…) or extrinsic (freehand probe guidance) tissue motion, however, leads to phase cancellation of the averaged PA signal when the axial displacement of tissue becomes larger than half the acoustic wavelength at the probe’s centre frequency. Motion-compensated averaging (DCA) is a solution to this problem, and allows the detection of deep structures that are else not visible. In a combined PA and echo-ultrasound (US) system, tissue motion can be quantified in US images that are interleaved with PA images. In this chapter, we exemplarily illustrate the power of this technique when trying to image the optical absorption inside the carotid artery, using a fully integrated PA/US system based on a handheld clinical probe containing a miniaturised laser source. The key components of DCA are discussed and exemplified on volunteer data, and the influence of various parameters on image contrast is investigated. We demonstrate that DCA enables freehand PA detection of blood vessels at a depth of 1.5 cm using only 2 mJ pulse energy, and give some guidelines for image interpretation.