Localization of subsurface photoacoustic fiducials for intraoperative guidance

Intraoperative guidance is often used during procedures to provide surgeons with information linking the current surgical scene to some preoperative plan or model. This allows surgeons to visualize structures or targets of interest that are not visible in the intraoperative imaging modality. A common method to enable this type of technology is the use of fiducials that can be located both preoperatively and intraoperatively. In this work, we focus on the registration of preoperative computed tomography with intraoperative three-dimensional ultrasound. We examine the use of titanium fiducials activated intraoperatively by the photoacoustic effect. The photoacoustic effect is generated when the metal fiducials are illuminated by laser light, resulting in an acoustic signal at each fiducial that can be observed by a conventional ultrasound transducer. A transrectal ultrasound transducer is translated to generate a three-dimensional ultrasound volume. The set of fiducial points are segmented from the three-dimensional ultrasound volume and registered with the corresponding set of fiducial points segmented from the computed tomography volume. The target registration error metric is used to validate the registration between these two coordinate systems. The resulting target registration error was 1.55 ± 0.62mm. It was observed in this experiment that there were certain acoustic reverberation and refraction artifacts that occurred at distances coinciding with the size of the fiducial. Further work with different shapes and sizes of fiducials may be necessary to quantify and analyze how they affect the generation of a photoacoustic signal.