The Design Of PMT Cathode Uniformity Detecting System

The paper is from the medical PET/CT research and production project, in order to meet the growing medical diagnostic requirements, this R&D project is expected to achieve higher image quality by integration and fusion of two kinds of image reconstruction methods which are Positron Emission Tomography(PET) and X-ray tomography(CT).PET is one of the most popular medical equipment, it’s widely used in neurological diseases, cardiovascular disease and cancer in clinical diagnosis and therapeutic monitoring with its proven technology. It has become the most important medical equipment in clinical diagnosis and treatment through its continuous technical performance improvement and integration with other imaging modalities.PET belongs to the field of nuclear medicine, one of its core technology is detector, which is a kind of photoelectric equipment coupled with scintillation crystal and photomultiplier tube(PMT), its function is to capture gamma photons produced by positron annihilation which produced by radioactive tracer and then turn this signal into practicable electrical signal by electro-optical conversion and signal amplification and provides a reliable and stable signal source for the latter part of signal and image reconstruction. According to the principles of PET, in order to capture a pair of gamma photos with opposite direction, detectors are always arranged in a ring, so lots of PMTs are required.PMT is most important part of a detector used in PET, as the signal source of PET image reconstruction, its performance is extremely important. Among all the features of PMT, cathode uniformity is very important, and the purpose of this paper is to test the cathode uniformity of PMT in the production process of PET by designing a set of inspecting equipment.Firstly, this paper describes the basics of PET and the fusion of PET/CT, lists some former methods of PMT testing, and explains the source and purpose of this paper.Secondly, this paper introduces the structure of PET detector, including radioactive tracer and scintillation crystal, then makes an full introduction about structure and principle of PMT, including five most important features.Thirdly, this paper introduces the testing equipment. Due to the special characteristics of PMT, a project including dark box, translation stage, light source, data acquisition and software has been settled. In this paper, a precision-adjustable two-dimensional translation stage assembled with basic parts is controlled by upper computer written by myself. LED is used in the light source, with help of flare stop, a light beam with diameter of 1mm is satisfactory for the experiment. The light source carried by the translation stage can provides point-by-point scanning. In order to meet the requirements of data acquisition, I used ORTEC nuclear electronic instruments as DAQ part which achieves two-stage amplification, data accumulation and data analysis.Finally, I tested CR260 with this testing equipment and got the data of PMT cathode uniformity. The detailed test procedures are introduced in this paper. A data fitting has been programmed in MATLAB and gave the image of PMT cathode uniformity, and also a calculation of the tested PMT photocathode’s uniformity. Through experiments, system availability and reliability has been verified.