Development of a Near-Full-View Angle Coverage Photoacoustic Tomography System and Its Application Towards Optical Fluence Distribution Imaging
Abstract
Photoacoustic tomography (PAT) is a widely explored hybrid imaging modality combining advantages of ultrasound and optical imaging. However, on the acoustic detection side, limited-view angle coverage and limited-detector bandwidth are common key issues in PAT systems that result in unwanted artifacts. While analytical and simulation studies of limited-view artifacts are extensive, experimental setups capable of comparing limited-view to an ideal full-view case are lacking. Due to the lack of PAT systems capable of artifact-free full-view imaging, applications for such a system have also been left unexplored.
A custom ring-shaped detector array was assembled and mounted to a 6-axis robot, which was used to rotate and translate the array to achieve up to 3.8π steradian view angle coverage of an imaged object. To optimize the system, a method to minimize negativity artifacts and phantom imaging were used, followed by demonstrative imaging of a star contrast phantom, a synthetic breast tumor specimen phantom, and a vascular phantom. Finally, this system was applied towards imaging of the relative optical fluence distribution emitted by a fiber bundle into a series of two-layered absorbing and scattering agarose phantoms. Resulting 3D images were compared to Monte Carlo simulations using two software packages, the diffusion equation, and extracted reduced scattering coefficient values.
Optimization of the angular scans found ~4000 effective detectors spread over a spherical surface were needed for high quality images, while 15-mm steps were used to increase the field of view. Example phantoms were clearly imaged with all discerning features visible and minimal artifacts. When measuring fluence distributions, reasonable visual correspondence was found compared to simulated distributions. Image volumes compared well with similar trends, while shape analysis revealed a better match to TracePro than Monte Carlo eXtreme. Reduced scattering coefficient recovery appeared successful for values below 0.5 mm-1.
A near full-view closed spherical system has been developed, paving the way for future work demonstrating experimentally the significant advantages of using a full-view PAT setup. When applied to one previously unexplored application, initial results to measure fluence distribution appeared to have potential, although some barriers remain and further development is needed.