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.