Nếu một vị phóng xạ gửi trong bộ phận nguồn có nhiều khí thải, liều lượng cân bằng hấp thụ liên tục nên được tính cho mỗi phát thải và summated. | R. Accorsi et al. 99 Figure . Pictorial representation of A a true coincidence B a scatter event and C a random coincidence. In B and C the dashed line represents the line of response to which the event is assigned. It coincides with the path of the photons solid line only in case A. type of event is called a random coincidence. The likelihood of random coincidences depends on the level and spatial distribution of activity and the temporal width of the coincidence window as well as the geometry of the object. To appreciate this it is helpful to consider that after the detection of a first photon above the LLD the scanner waits up to a time t for the photon emitted simultaneously to arrive. The time interval 2t is called the time coincidence window. If the rate at which the first event is recoded is N events per unit time then the rate at which random coincidences occur in the scanner is tN2. Time coincidence windows are usually set between 6 and 12 ns billionths of a second . 10-9 s depending on the scintillator. PET Instrumentation It is desirable to detect photons with high spatial energy and time resolution with high sensitivity and count rate capabilities all at reasonable cost. Different classes of detectors of high-energy photons have long been under development no single class however offers the best performance in all respects. For example solid-state detectors offer the best energy resolution but their sensitivity is usually low especially when their cost and availability over a large area are considered. Commercial clinical PET cameras are based on scintillation detectors. In these systems the photon interacts with a scintillating material which converts the photon energy into visible or near-visible light. These low-energy photons are then conveyed to PMTs for conversion into an electrical signal. The front end of the PMT is the photocathode which is a thin element of material capable of absorbing light photons and emitting electrons called .
