There are a few specific quality control tests for dedicated PET systems. These procedures are intended to monitor system stability and maintain consistency and accuracy of performance.
Ambient Temperature
The scanning room temperature should be checked daily because the sensitivity of the system changes with temperature. As the temperature rises, fewer visible photons are produced by the crystals. The PHA spectrum in BGO crystals also changes with temperature, with the energy range varying inversely with room temperature (e.g., appearing lower as the temperature rises and vice versa).
Normalization Scan
Because a state-of-the-art PET camera may have thousands of crystal elements coupled to hundreds of PMTs, there are inevitable small variations in axial sensitivity among the detector units. To produce uniform images, these discrepancies must be corrected. A normalization scan is accomplished by scanning a uniform calibrated positronemitting source placed in the FOV. This data set measures
the response of each detector pair and is used to obtain a calibration factor to normalize the lines of response that pass through the source. These stored calibration factors can be applied to patient data sets to correct for differences in detector response so that accurate images of tracer distribution are produced. Normalization scans should be performed at least monthly, but they may be obtained weekly or more frequently as needed.
Blank Scan
This is accomplished by performing a scan by using the system transmission radiation sources with nothing in the FOV. This usually takes an hour or less. The data acquired are used with the patient transmission data to compute attenuation correction factors. Blank scans should be performed daily and, as such, are also an excellent method to monitor system stability, including significant discrepancies in individual detector sensitivities. Some PET instruments will perform this function automatically at a specified time during the night and even compare the results to previous blank scans.
Image Plane Calibration
Calibration of each image plane by using a radioactive source is also required on multi-ring detectors. This can be done with a uniform cylinder filled with a positron-emitter and may be done weekly or monthly. This procedure is essential for the production of accurate whole-body scans.
CT Scanner
Daily calibration begins with manufacturer's warm-up and automatic monitoring program. This checks a number of parameters, including tube coolant temperature, kVp and mA settings, and detector response. A phantom is then used to check that water measures 0 Hounsfield units and air measures minus 1000 units with a standard deviation of 2 to 3 units. The water image is evaluated for standard deviation to assess for image noise. The image quality is usually assessed by assuring that the Hounsfield units have a standard deviation of 1 to 5 across the phantom image. Many of these procedures are automated, but, if the images are evaluated visually, they should be inspected to see that there are no arc or ring artifacts.
责任编辑：admin
上一篇：没有了
下一篇：医学文章阅读——NEW THERAPEUTICS
微信公众号搜索“译员”关注我们，每天为您推送翻译理论和技巧，外语学习及翻译招聘信息。