Transcranial Photoacoustic Imaging Using a Virtual Array

Abstract Human brain mapping has become one of the most exciting contemporary research areas, with major breakthroughs expected in the following decades. Modern brain imaging techniques have allowed neuroscientists to gather a wealth of anatomic and functional information about the brain. Among these techniques, by virtue of its rich optical absorption contrast, high spatial and temporal resolutions, and relatively deep penetration, photoacoustic tomography (PAT) has attracted substantial attention, and is playing an increasingly important role in brain studies. In particular, PAT complements other brain imaging modalities by providing high-resolution functional and metabolic imaging. More importantly, PAT?s unique scalability enables scrutinizing the brain at both microscopic and macroscopic scales, using the same imaging contrast. However, to bring PAT to studying human subjects or non-human primates, one critical issue must be addressed. That is, the human or non-human primate skulls severely distort the photoacoustic (PA) signals, giving rise to substandard images. Time-reversal based reconstruction is considered one of the most accurate and sophisticated reconstruction algorithms for transcranial PAT. Time-reversal based methods are capable of correcting for both the phase and amplitude distortions due to the skull when imaging the brain. However, this method is slow, which presents a significant hurdle to the application of real-time functional imaging. Here, we propose to develop an innovative virtual array approach for achieving accurate and real-time transcranial PAT to image the brain cortex of non-human primates and humans. Instead of using the actual PA signals for image reconstruction, we propose to use virtual signals received by a virtual array situated inside the skull, which is considerably less affected by the skull. This can be implemented in real-time as a propagator (also known as the propagation operator) can be pre-computed. The specific tasks to be completed during this grant period are: First, develop and validate the virtual array algorithm for transcranial PAT using synthetic data, second, verify our new method experimentally on a full-ring-array based PAT system, using head-mimicking phantoms coupled with an ex vivo monkey skull. At the end of this project we would have confirmed the effectiveness of the proposed virtual array approach. The results from this project will provide a route for probing brain cortex functions in a highly efficient and accurate manner and pave the way for future applications of this novel PAT technique.