Development of a low-sensitivity high resolution YAP(Ce) scintillation camera system toward the real-time imaging of an ¹⁹²Ir source during high-dose-rate brachytherapy

Measurement of the precise position of a high activity iridium-192 (192Ir) source during high-dose-rate (HDR) brachytherapy is desired to detect the mispositioning of the source and avoid incidents. Although real-time imaging of the source position using a high-energy gamma camera is a possible method for this purpose, electronics are saturated at small distances from the source because the 192Ir source activity is so high. To solve this problem, we developed a low-sensitivity high-resolution gamma camera. We used a 1-mm-thick cerium-doped yttrium aluminum perovskite (YA1O3: YAP(Ce)) scintillator plate to reduce sensitivity. The developed low-sensitivity gamma camera consists of this thin YAP(Ce) scintillator optically coupled to a flat panel photomultiplier tube (FP-PMT). It is encased in a 20-mm-thick tungsten shield with a 0.5-mm diameter pinhole collimator mounted on its head. The spatial resolution of the gamma camera at 100 mm from the 192Ir source was 3.3 mm FWHM and the sensitivity was 0.52 cps/MBq. The count rate of the camera was ∼180-k cps at 100 mm from the 212.4-GBq 192Ir source and real-time imaging of the 192Ir source was possible. With a 100-mm-thick water phantom positioned between the gamma camera and the 192Ir source, we decreased the count rate to half, but the 192Ir source images could be observed clearly. We conclude that the developed low-sensitivity high-resolution gamma camera system has the potential to be a new real-time imaging system for HDR brachytherapy.