Hybrid Photoacoustic and Laser-Induced Ultrasound Computed Tomography

We present a lab-built prototype for a 3D hybrid photoacoustic/laser-induced ultrasound computed tomography device for breast phantom imaging. The device, inspired by a hemispherical geometry, allows some flexibility of combining laser-induced ultrasound transmitters, light sources for photoacoustic imaging and sensitive, purpose-built ultrasound detectors in standardised mounting points. Our initial results employ tailored 1 MHz centre frequency laser-induced ultrasound transmitters to achieve sound speed mapping in a breast-sized phantom. The phantom, a two-layer breast analogue constructed using everyday lab supplies, con-sists of polyvinyl chloride plastisol and agarose-based tissue mimicking materials. Using transmission-mode and reflection-mode acquisition protocols, respectively, 2D maps of the sound speed and ultrasound reflectivity are reconstructed. Both sets of data compare well with the ground truth of the phantom. Simultaneously, con-ventional photoacoustic imaging of the phantom is also performed. A comparison of image quality in photoacoustic images using three different sound speed models, a single-speed, two-speed and full sound speed map model, shows the advantages of measuring the sound speed map, especially when imaging more complex objects such as the human breast. This proof-of-concept work shows promise for implementation with a higher density of ultrasound detection elements, where the benefits of laser-induced ultrasound in otherwise cross-talk-prone situations may become clear.