On the effect of X-rays on the PET detectors in a PET/CT system

The combined PET and CT scanners (PET/CT) have added a new dimension to oncology PET, providing a precise co-registration of the two images. CT data, in addition, provide a low noise attenuation correction and more accurate scatter corrections. Current commercial PET/CT systems are based on a combination of a regular CT scanner and a regular PET scanner. During the CT scan scattered X-rays may affect the PET detectors. With the increasing interest for PET/CT systems, a more close integration may be attempted. However, this may increase the X-ray load on the PET detectors, especially for future detector systems with a large axial extension, where a proper side shielding of the PET detectors from scattered X-rays can be difficult to achieve. In this paper, the effect of scattered X-rays on a LSO scintillation detector has been investigated. Depending on the high voltage of the photo-multiplier tube (PMT) and the distance of the detector from the X-ray beam, PMT anode currents of the order of mA may be generated.