An instantaneous photomultiplier gain calibration method for PET or gamma camera detectors using an LED network

In current clinical positron emission tomography (PET) cameras, there are about 1000 photomultiplier tubes (PMTs) in the detector system. Even a less-complicated gamma camera has many dozens of PMTs. Image quality and resolution of a camera is dependent on the proper equalization of all the PMT gains. However, a PMT gain can change with many environmental factors, such as room temperature, patient load, short-term or long-term radiation exposure, and time. Hence, an instantaneous automated PMT gain calibration method is especially important for an ultrahigh-resolution PET camera. We have developed a new PMT gain auto-tuning method using a blue light-emitting diode (LED) network. Each LED shines directly into the center of a scintillation crystal block from the PMT side, and the light is collected by the surrounding PMTs. The effects of crystal optical transferring efficiency and PMT optical coupling efficiency have been considered. The calibration is done by changing the gains of these surrounding PMTs or their following amplifiers to have the same signal output. An LED has well known problems of large light-yield varieties and is very sensitive to temperature. To overcome these problems, the light outputs of two neighboring LEDs are aligned first by a shared PMT. Each LED flashes individually and is driven by a 250 KHz pulse generator. At such a high pulse rate, the data acquisition for the gain calibration can be finished within a very short time so the LED temperature effect can be ignored. The amount of LED light output is set as close as possible to the amount of scintillation light by programming the width or height of the pulses; therefore, the same system electronics can be used for both purposes. Our proposed high-resolution whole-body PET camera with 924 PMTs in a PMT-quadrant-sharing (PQS) design can be calibrated in 1 minutes or less.