Quality control of CT systems by automated monitoring of key performance indicators: a two‐year study

Abstract

The purpose of this study was to develop a method of performing routine periodical quality controls (QC) of CT systems by automatically analyzing key performance indicators (KPIs), obtainable from images of manufacturers' quality assurance (QA) phantoms. A KPI pertains to a measurable or determinable QC parameter that is influenced by other underlying fundamental QC parameters. The established KPIs are based on relationships between existing QC parameters used in the annual testing program of CT scanners at the Karolinska University Hospital in Stockholm, Sweden. The KPIs include positioning, image noise, uniformity, homogeneity, the CT number of water, and the CT number of air. An application (MonitorCT) was developed to automatically evaluate phantom images in terms of the established KPIs. The developed methodology has been used for two years in clinical routine, where CT technologists perform daily scans of the manufacturer's QA phantom and automatically send the images to MonitorCT for KPI evaluation. In the cases where results were out of tolerance, actions could be initiated in less than 10 min. 900 QC scans from two CT scanners have been collected and analyzed over the two‐year period that MonitorCT has been active. Two types of errors have been registered in this period: a ring artifact was discovered with the image noise test, and a calibration error was detected multiple times with the CT number test. In both cases, results were outside the tolerances defined for MonitorCT, as well as by the vendor. Automated monitoring of KPIs is a powerful tool that can be used to supplement established QC methodologies. Medical physicists and other professionals concerned with the performance of a CT system will, using such methods, have access to comprehensive data on the current and historical (trend) status of the system such that swift actions can be taken in order to ensure the quality of the CT examinations, patient safety, and minimal disruption of servicePACS numbers: 87.57.C‐, 87.57.N‐, 87.57.Q‐

100 days with scans of the same Catphan phantom on the same CT scanner

“…Several QC phantoms are available for image quality testing of CT scanners, ranging from vendor specific phantoms to widely used commercial phantoms like Catphan (The Phantom Laboratory, Salem NY, USA) 3, 4, 5, 6, 7. Publications by international organizations like the International Electrotechnical Commission (IEC) and Institute of Physics and Engineering in Medicine (IPEM) provide specific recommendations related to test methods, test frequencies, and tolerance levels for different QC tests in CT 8, 9, 10.…”

“…Tolerance levels should be wide enough so that only serious drifting or sudden errors are detected and not clinically irrelevant daily fluctuations. The recommended test frequency should aim to balance between being frequent enough to ensure adequate image quality, while infrequent enough to reduce time spent by the staff and avoid unnecessary interference with clinical work 6, 7…”

“…To reveal beam hardening and cupping artifacts in CT images, uniformity is tested. Uniformity is a measure of the capability of the CT scanner to produce a uniform picture of a homogenous material 5, 7…”

“…Discussions on optimal testing methods, tolerance levels, and test frequencies are ongoing in the scientific environment 2, 3, 4, 5, 6, 7. More data from frequent measurements over time provide valuable insight into error rates and day to day fluctuations for the different tests.…”

“…Together with exploring different test methods, this will help establishing better QC routines for CT scanners. Information on day to day fluctuations of measured image quality parameters is available in the literature, but to our knowledge no study has been performed with Catphan which is widely used by medical physicists 6, 7. Studies using both axial and helical scan technique for QC are also available, but to our knowledge no systematic comparison of results from the different techniques has been carried out 4, 6.…”