Performance of a high-resolution hybrid gamma/optical camera for intraoperative imaging

Objectives The development of portable gamma cameras offers new applications for intraoperative imaging [1]. Hybrid gamma-optical systems have been proposed for improving surgical utility [2]. Dual modality imaging in the operating theatre could aid surgeons in localizing the site of uptake in procedures such as sentinel node detection. A novel handheld small field of view (SFOV) hybrid camera has been developed and evaluated. This study investigates the imaging performance characteristics of the system.

Methods The compact hybrid gamma camera comprised a columnar scintillator backed with a CCD and a co-aligned optical camera. Performance characteristics including spatial resolution, energy resolution and sensitivity, were investigated based on NEMA protocols adapted for use with high resolution SFOV systems. Image contrast and accuracy of image alignment were quantified with custom designed phantoms.

Results Full-width half-maximum system spatial resolution was 1.3mm at the non-magnifying point and 5.0mm through 100mm of scattering material. A simulated node 2mm in diameter containing 2MBq 99mTc was detectable (signal-to-noise ratio 25) situated 10mm from a simulated injection site 8mm in diameter with an activity of 32MBq at a source-to-camera distance of 55mm. All fused images showed good alignment of the two modalities. Different methods of fusion (i.e. pixel replacement vs. pixel addition) highlighted a potential trade off in contrast between activity and its surroundings and fine detail in the optical image. Initial clinical studies showed that the site of uptake could be visualized in the patient.

Conclusions Performance characteristics of a hybrid camera have been described and initial images demonstrate that it is ideally suited for intraoperative imaging. Anatomical context provided by the optical camera aids the physical localization of radiopharmaceutical uptake. These studies encourage us to carry out further evaluation in the surgical theatre setting.