Differential phase photoacoustic imaging for high-resolution position sensing

Photoacoustic methods that may be translatable to in vivo applications generally have limited frequency and wave vector response. As a result, the point spread function of array-based scanners at moderate frequencies may have dimensions of about 1 mm
3
; multiple objects within this volume cannot be resolved. Small structures, such as entangled arteriole-venule pairs, can be routinely imaged in vitro with photoacoustic microscopy, but this technology is more difficult to implement in a bedside setting. The goal of this research is to detect and sense the position of unresolved absorbers with spectral contrast, spaced by less than the conventional axial or lateral image resolution, in images acquired with an array-based imaging system. We propose a differential phase photoacoustic method to estimate the distance between two unresolved objects. Two absorbers were spaced 115 µm apart, around eight times less than the lateral image resolution. Using the proposed method, absorbers were separated and their mutual distance was successfully recovered.