Development of a scanning photoacoustic tomography system for tumor margin assessment in breast conserving surgery

Photoacoustic tomography (PAT) has excellent sensitivity for hemoglobin and lipids, which make up much of human breast tissue. Our group has focused on intraoperative PAT applied to tissues obtained during breast-conserving surgery (BCS). In BCS, the tumor is excised with a margin of healthy tissue to ensure tumor removal. Margin detection can be difficult and re-excision surgeries are required in 10 to 25% of cases. Our first-generation intraoperative PAT system was capable of 3D imaging specimens up to 11 cm in diameter and several centimeters thick. The system used a semi-circular ring of low frequency transducers, resulting in a 2.5 mm spatial resolution. The current objective is to improve spatial resolution using higher frequency transducers. An array was constructed with 41 circular transducers positioned on two concentric circular rungs with a single point of focus. An optical window at the center allowed illumination. The array was tested with imaging phantoms consisting of written words on a clear plastic bag, 108 µm polyester monofilament arranged as parallel lines with spacing varying from 1 mm to 8 mm, and finally with porcine tissues. The array was positioned above and perpendicular to the imaging area and raster scanned. Signal averaging was implemented, and images were reconstructed with universal back projection. Image analysis demonstrated a 400 μm spatial resolution, but with low penetration depth and low sensitivity. Results suggest the transducers could improve spatial resolution of the first-generation intraoperative PAT system by 6-fold.