P Brunick*, J Wang , B Helbig , J Clements , Kaiser Permanente Southern California Permanente Medical Group, Los Angeles, CA

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  P Brunick

(Sunday, 7/14/2019)  

Room: ePoster Forums

Purpose: To illustrate the importance of medical physics personnel vetting their test equipment prior to actual use, a case study is presented pertaining to CT pencil ion chambers. A testing procedure was later developed by our organization to mitigate the issues experienced in this example.

Methods: CT pencil chambers from two different vendors (Vendor A & Vendor B) were initially tested with both a CT and radiographic x-ray system using various technique factors. The displayed measurements were then compared. By utilizing annual physics evaluation data from the preceding 12 months, system displayed CTDI_vol values were compared to those calculated by chamber measurements. Final testing, following Vendor A re-calibration, was performed using a portable x-ray system.

Results: Measurements indicated a difference of approximately 10% between Vendor A and Vendor B’s CT pencil chambers. Vendor A performed additional testing off-site. Calibration reports from Vendor A indicated deviations from reference values between -6.1% and -11.9% for all chambers tested. The average difference between CTDI_vol values calculated from Vendor A’s chamber measurements and those displayed by CT acquisition workstations differed by was -7.3%, where Vendor B only differed by 1.6%. Following re-calibration, average measurements from Vendor A chambers differed by -0.1 to +3.9% from a reference chamber manufactured by Vendor B that had previously been evaluated for accuracy.

Conclusion: In this case, all of Vendor A’s CT pencil ion chambers deviated by more than the manufacturer’s tolerance limit of 5%. It is important that physicists working in a clinical environment perform test equipment evaluations to reduce the potential for erroneous readings. Our institution later established a policy and QC procedure for new, repaired, or re-calibrated test equipment. It was validated using the re-calibrated CT pencil chambers from Vendor A.

Keywords

CT, Quality Control, Radiation Detectors

Taxonomy

IM- CT: Quality control

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