Development of non-contact photoacoustic tomography

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Introduction and Objective: Photoacoustic tomography (PAT) has been studied as a method to detect cancer for more than 20 years. The idea is based on the principle that there is an abundance of hemoglobin in and around tumors providing higher optical absorption in these regions compared to nearby healthy tissues. Several groups have constructed PAT breast scanners, but all use a cumbersome water tank to acoustically couple tissues to the piezoelectric detectors. Elimination of the water tank has many potential benefits including reduced scan time, higher resolution PAT images, flexible subject positioning, and sterility due to a lack of contact with the subject. The objective of this work will be to develop and optimize non-contact PAT (ncPAT) to realize these benefits. Approach:  Non-contact PAT uses a holographic detection technique called double-pulse electronic speckle pattern interferometry (dpESPI) that avoids the use of a water tank and piezoelectric detectors. With dpESPI it is possible to measure nanometer scale surface displacements resulting from photoacoustic waves arriving at the surface. Several important parameters relate to the performance of the holographic setup and need further research within the context of PAT. These include the angle between the reference and object light paths, spatial and temporal sampling of the holograms, conversion of surface displacements to pressure estimates, and finally reconstruction of the dpESPI pressure estimates into photoacoustic images representative of optical absorbers (tumours). Research Plan: Our 5-year plan includes first developing a COMSOL simulation tool for a dpESPI system (Aim 1, Year 1). Second, using the dpESPI system, we will measure the relationship between reference-object angle and displacement sensitivity and compare to simulation results (Aim 2, Year 2). Third, we will measure the relationship between field of view, spatial sampling interval and displacement sensitivity and compare to simulation results (Aim 3, Year 3). Fourth, we plan to measure the effect of probe laser intra-pulse time interval and inter-pulse time interval on the characteristics of photoacoustic sampling (Aim 4, Year 4). Last, we plan to experimentally capture photoacoustic data using dpESPI from phantoms, reconstruct into 3D PAT images and compare to simulations and similar data captured using a conventional water tank PAT system with piezoelectric detectors (Aim 5, Year 5). Resources: All resources are in place including access to conventional PAT equipment, dpESPI equipment, wet lab facilities, and computing resources. Research Team: Dr. Carson is an expert in PAT. Over last 3 years, his group has developed a non-contact, holography-based system for ncPAT. Trainees will include one PDF and one PhD student. Conclusion and Significance: This research will result in improved understanding of ncPAT technology and how it can be adapted to ultimately capture images from human tissues.