Ultrasonic-heating-encoded photoacoustic tomography with virtually augmented detection view

Photoacoustic (PA) imaging of arbitrarily shaped or oriented objects may miss important features because PA waves propagate normal to structure boundaries and may miss the acoustic detectors when the detection view has a limited angular range. To overcome this long-standing problem, we present an ultrasonic thermal encoding approach that is universally applicable. We exploit the temperature dependence of the Grueneisen parameter and encode a voxel using heat generated by a focused ultrasonic transducer. The PA amplitude from the encoded voxel is increased while those from the neighboring voxels are unchanged. Consequently, the amplitude-increased PA waves propagate in all directions due to the round cross section of the encoded region and thus can be received at any viewing angle on the cross-sectional plane. We built a mathematical model for the thermally encoded PA tomography, performed a numerical simulation, and experimentally validated the ultrasonic thermal encoding efficiency. As a proof of concept, we demonstrate full-view in vivo vascular imaging and compare it to the original linear-array PA tomography system, showing dramatically enhanced imaging of arbitrarily oriented blood vessels. Since ultrasonic heating can be focused deeply, this method can be applied to deep tissue imaging and is promising for full-view imaging of other features of biomedical interest, such as tumor margins.