A PMT converts a light pulse into an electrical signal of measurable magnitude. An array of these tubes is situated behind the sodium iodide crystal and may be placed directly on the crystal, connected to the crystal by light pipes, or optically coupled to the crystal with a silicone-like material. A scintillation event occurring in the crystal is recorded by one or more PMTs. Localization of the event in the final image depends on the amount of light sensed by each PMT and thus on the pattern of PMT voltage output. The summation signal for each scintillation event is then formed by weighing the output of each tube. This signal has three
components: spatial coordinates on x- and y-axes as well as a signal (z) related to intensity (energy). The x- and y-coordinates may go directly for real-time display on a cathode ray tube (CRT) or may be recorded in the computer. The signal intensity is processed by the pulse height analyzer (PHA).
The light interaction caused by a gamma ray generally occurs near the collimator face of the crystal. Thus, although a thicker crystal is theoretically more efficient, the PMT is farther away from the scintillation point with a thick crystal and is unable to determine the coordinates as accurately. Therefore spatial resolution is degraded. The number of PMTs is also important for the accurate localization of scintillation events; thus for spatial resolution, the greater the number of PMTs, the greater the resolution. Most gamma cameras use about 40 to 100 hexagonal, square, or round PMTs.
Some newer scintillation imaging systems have used position-sensitive PMTs (PS-PMTs) and avalanche photodiodes (APD). PS-PMTs are usually used with small FOV devices that have pixilated detectors, rather than a large single crystal. APDs are solid-state photon converters that can be thought of as a light-sensitive diode and are being used in positron emission tomography/magnetic resonance imaging (PET/MRI) applications because they are less sensitive to magnetic fields.
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