DEVELOPMENT OF A MERCURIC IODIDE DETECTOR ARRAY FOR MEDICAL IMAGING APPLICATIONS

A nineteen element mercuric iodide (HgI2) detector array has been deve loped as a prototype for a larger (169 element) array, which is intend ed for use as an intra-operative gamma camera (IOGC). This work is mot ivated by the need for identifying and removing residual tumor cells a fter the removal of bulk tumor, while sparing normal tissue. Prior to surgery, a tumor seeking radiopharmaceutical is injected into the pati ent, and the IOGC is used to locate and map out the radioactivity. The IOGC can be used with commercially available radioisotopes such as Tl -201, (TC)-T-99m, and I-123 which have low energy X- and gamma-rays. T he use of HgI2 detector arrays in this application facilitates constru ction of an imaging head that is very compact and has a high signal-to -noise ratio. The prototype detectors were configured as discrete pixe l elements joined by fine wires into novel pseudo crossed-grid arrays to promote improved electric held distribution compared with previous designs, and to maximize the fill factor for the expected circular pro be shape. Pixel dimensions are hexagonal with 1.5 mm and 1.9 mm diamet ers separated by 0.2 mm thick lead septa. The overall detectors are he xagonal with a diameter of similar to 1 cm. The sensitive detector thi ckness is 1.2 mm, which corresponds to >99% efficiency at 59 keV and 6 7% efficiency at 140 keV. Row, column, and pixel spectra have been mea sured on the prototypical detector array. Energy resolution was found to vary with the width of the row/column coincidence window that was a pplied. With the low edge of the coincidence window at 30% below the p hotopeak, pixel energy resolutions of 2.98% and 3.88% FWKM were obtain ed on the best individual pixels at 59 keV (Am-241) and 140 keV (Tc-99 m), respectively. To characterize this array as an imaging device, the spatial response of the pixels was measured with stepped point source s. The spatial response corresponded well with the pixel geometry, ind icating that the spatial resolution was determined by the pixel geomet ry.