Purpose: Conventional positron emission tomography can be used only preoperatively to identify lymph node metastases, and hence, these growths are difficult to locate intraoperatively. Previously, an intraoperative laparoscopic coincidence imaging system, with an external fixed detector array and a detector probe that can be moved and inserted into the stomach, was proposed to identify lymph node metastases during stomach cancer surgery. This paper proposes a prototype detector for this system.
Methods: GAGG:Ce (Gd3Al2Ga3O12:Ce) scintillator crystals, one to one coupled with silicon photomultipliers (SiPMs), were used to identify 511 keV photons. An optical tracking system followed the position and orientation of the movable detector. SiPM outputs were read out by time-over-threshold (TOT)-based application-specific integrated circuits, which converted the electrical charges into digital pulses. To identify the arrival time, channel number, and pulse width, which indicate the energy information of each TOT output, a data acquisition system was developed based on a field-programmable gate array.
Results: The spatial resolution of the reconstructed images in lateral direction was better than 7 mm, but that of depth direction was limited, owing to inadequate projections. The prototype successfully reconstructed 44 kBq 18F-fluorodeoxyglucose sources during a 2-min scan.
Conclusion: We developed a prototype detector for an intraoperative laparoscopic coincidence imaging system. The prototype detector was evaluated successfully, and the produced images were similar to those obtained in the simulation results, thereby suggesting that this method offers a new possibility of imaging lymph node metastases intraoperatively.
Keywords: Image reconstruction; Intraoperative imaging; Lymph node metastases; Positron emission tomography; Stomach cancer.