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, consists 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, conventional 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.