Development of an intraoperative gamma camera based on a 256-pixel mercuric iodide detector array

A 256-element mercuric iodide (HgI2) detector array has been developed which is intended for use as an intraoperative gamma camera (IOGC). The camera is specifically designed for use in imaging gamma-emitting radiopharmaceuticals (such as 99m-Tc labeled Sestamibi) incorporated into brain tumors in the intraoperative surgical environment. The system is intended to improve the success of tumor removal surgeries by allowing more complete removal of subclinical tumor cells without removal of excessive normal tissue. The use of HgI2 detector arrays in this application facilitates construction of an imaging head that is very compact and has a high SNR. The detector is configured as a cross-grid array. Pixel dimensions are 1.25 mm squares separated by 0.25 mm. The overall dimension of the detector is 23.75 mm on a side. The detector thickness is 1 mm which corresponds to over 60% stopping at 140 keV. The array has good uniformity with average energy resolution of 5.2 +/- 2.9 % FWHM at 140 keV (best resolution was 1.9 % FWHM). Response uniformity (+/- sigma) was 7.9 %. A study utilizing realistic tumor phantoms (uptake ratio varied from 2:1 to 100:1) in background (1 mCi/l) was conducted. SNR's for the reasonably achievable uptake ratio of 50:1 were 5.61 sigma with 1 cm of background depth (''normal tissue'') and 2.74 sigma with 4 cm of background for a 6.3 mu l tumor phantom (similar to 270 nCi at the time of the measurement).