CT equipment specifications and performance shall meet state and federal requirements and applicable ACR Practice Parameters and Technical Standards.
A quality control (QC) program must be established and implemented under the supervision of a qualified medical physicist (QMP). Initial performance testing (acceptance testing) is required upon installation. All ACR-accredited CT facilities and those applying for accreditation must maintain a documented QC program and must comply with the minimum frequencies of testing outlined in the 2012 ACR CT QC manual, revised edition (2017). CT sites applying for the first time or renewing will receive an electronic copy of the manual via email. Accreditation requires documentation of a continuous quality control program as well as completion of the Annual Medical Physicist Survey.
Acceptance Testing
Acceptance testing is intended to measure quantifiable system parameters, which may then be compared to the manufacturer’s specifications. A complete evaluation of system performance should be performed by a QMP after completion of installation and prior to patient imaging.
Acceptance testing encompasses the verification of the purchase specifications for a particular CT system. This testing should be conducted using the phantom, procedures, and specifications specified in the purchase contract. Discrepancies should be reported to the Service Engineer.
At the time of acceptance testing, additional tests may be performed by the medical physicist. These tests are not used to determine whether the CT scanner meets the manufacturer’s specification, but they may serve as the initial set of baseline results needed for future QC testing or they may be used to further characterize and optimize use of the CT scanner. Please note that additional testing outside of the vendor identified tests, methods, and acceptance criteria may not be supported by the manufacturer.
Annual Medical Physicist Survey
All facilities applying for accreditation or renewal must demonstrate compliance with ACR QC requirements by including a copy of the facility’s most recent CT Annual System Performance Evaluation Summary Form (to include evaluation of the technologist QC) signed by a QMP (see attachment below). Your medical physicist must use the summary form provided by the ACR or one similar that itemizes pass-fail results of all the same tests using the same names and order as outlined on the ACR form.
The QMP must evaluate the performance of each CT unit at least annually. The ACR realizes that surveys cannot usually be scheduled exactly on the anniversary date of the previous survey. Therefore, a period of up to 14 months between surveys is acceptable. This evaluation must include, but not be limited to:
Participation in Review of Clinical Protocols with the CT Protocol Review & Management Team | Scout Prescription and Alignment Light Accuracy |
Table Travel Accuracy | Radiation Beam Width* |
Low-Contrast Performance | Spatial Resolution |
CT Number Accuracy** | Artifact Evaluation |
CT Number Uniformity | Dosimetry |
CT Scanner Display Calibration | Other Tests as Required by State or Local Regulations |
*Radiation beam with is one of the fundamental factors that determine patient dose. It is vital to ensure that the radiation beam is no wider than necessary for the NxT combination selected. Members of the Physics Subcommittee have observed on many occasions beam widths significantly larger than the nominal values, particularly for thin beams. In most cases, this is easily corrected by field service calibration. The specific method used to determine beam width is up to the QMP. An example of one method is provided in the CT QC Manual, but it is not intended that this be a prescriptive requirement. Any method that allows the QMP to measure the beam width within the tolerances in the QC manual is acceptable. |
** At a minimum, the annual physicist CT number accuracy testing should include the adult head, adult abdomen, pediatric head and pediatric abdomen protocols unless the unit is not used to scan pediatric patients. Note that, for protocols using kVps other than 120, the CT numbers for phantom inserts other than water may not fall within the ACR ranges. However, these CT numbers should still be recorded in the annual physicist survey as a validation of consistency from year to year. |
If recommended in the Annual System Performance Evaluation, documentation of any corrective action taken (i.e. test failures or data outside of action limits) should also be submitted.
In the case of dual source scanners, the primary and secondary tubes should both be evaluated in the annual system performance evaluations as the goal of the annual evaluations is to ensure that the scanner is functioning as designed in all respects and that it is being used optimally. However, the secondary tube is not typically used in the ACR Accreditation physics testing as it is not normally used for diagnostic protocols (adult head, adult abdomen, pediatric head and pediatric abdomen).
Continuous Quality Control
A continuous QC program must be established for all CT units with the assistance of a QMP. An on-site radiologic technologist should be identified to be responsible for conducting routine QC. The continuous QC program must include, but not be limited to, the following:
Task | Frequency |
|---|
Water CT Number and Standard Deviation* | Daily |
Artifact Evaluation | Daily |
Wet Laser Printer Quality Control | Weekly (if applicable) |
Visual Checklist | Monthly |
Dry Laser Printer Quality Control | Monthly (if applicable) |
Display Monitor Quality Control | Monthly |
*It is recommended that this be performed in both the axial and helical scan mode. The QC manual suggests performing these scans on alternate days to speed up the process. This is a recommendation and not a requirement. |
The Laser Printer QC Form and the CT Equipment Quality Control Data Form are attached below.
All QC testing must be carried out in accordance with written procedures and methods. The results of the QC program must be monitored annually by the QMP. If the results of a QC test fall outside the control limits, corrective action should be taken. A QMP should be available to assist in prescribing corrective actions for unresolved problems. All deficiencies must be documented and service records maintained by the CT facility.
Preventive Maintenance
Preventive maintenance must be scheduled, performed, and documented by a qualified service engineer on a regular basis. Service performed to correct system deficiencies must also be documented and service records maintained by the CT site.
QC Procedures Following Repairs
The ACR 2017 CT QC manual indicates that QC procedures should be performed at acceptance testing, during an ongoing QC program following parameter specific frequencies and following major repairs. A major repair includes replacement or repair of components such as an x-ray tube or detector assembly. The evaluation should be determined by the qualified medical physicist based on the type of component repaired or replaced. A matrix of response is provided below for guidance for several major repairs or replacements:
Item | Major Repair? | QMP Response | Timeframe |
X-Ray Tube Replacement | Yes | QMP conducts evaluation in person | Technologist QC must be performed before clinical use. If results pass, then QMP should complete appropriate evaluation as soon as possible within 30 days. |
CT Service Calibrations | No | *QMP oversight | Follow normal QC guidelines |
HV Generator Replacement | Yes | QMP conducts evaluation in person | Technologist QC must be performed before clinical use. If results pass, then QMP should complete appropriate evaluation as soon as possible within 30 days. |
HV Generator Service Calibration | No | *QMP oversight | Follow normal QC guidelines |
Control Console Replacement | No | *QMP oversight | Follow normal QC guidelines |
Detector Assembly Replacement | Yes | QMP conducts evaluation in person | Technologist QC must be performed before clinical use. If results pass, then QMP should complete appropriate evaluation as soon as possible within 30 days. |
Collimator Adjustments | No | *QMP oversight | Follow normal QC guidelines |
Software Upgrade | Maybe | QMP must discuss with vendor for affected parameters |
|
mA/kV Modulation Install | Yes | QMP conducts evaluation in person | Technologist QC must be performed before clinical use. If results pass, then QMP should complete appropriate evaluation as soon as possible within 30 days. |
Protocol Change | No | *QMP oversight with radiologist and technologist | Follow normal QC guidelines |
*QMP oversight means that the medical physicist directs the site to follow established QC procedures and to share these results with the QMP in order to determine that the equipment is functioning properly. The qualified medical physicist does not need to conduct this on site. |
Beam Width Measurement Options
As stated in the QC Manual, beam width may be measured in a number of different ways. The methodologies listed each have their strengths and weaknesses, and some are currently only appropriate for measurement in axial mode, while others are only designed for helical mode. Note that specific CT scanners may have beam widths that are only available in one mode or the other. Some manufacturers of probes designed for helical data acquisition are currently developing devices to move the probe through the beam during an axial scan. It is imperative to verify that these "probe movers" are properly calibrated to yield accurate results. The table below lists various measurement devices and the scan modes in which they can be used.
Measurement Devices | Axial | Helical |
CR | Yes | No |
Film | Yes | No |
OSL | Yes | No |
Small Ion Chamber | *Only with fully-calibrated probe-moving device | Yes, when mounted to scanner table |
Solid State Probe | *Only with fully-calibrated probe-moving device | Yes, when mounted to scanner table |
*If probe moving device does not yield accurate velocity values, then beam width estimates will be off and will most likely differ from specifications and/or values obtained with other methods. |
Artifact Evaluation Absent Large Poly Disk
Should your facility not have a large poly disk available to perform artifact evaluation on the scan field of view (SFOV) as required, this approach provides a method to qualitatively evaluate the level of non-uniformities in the SFOV outside of the phantom FOV.
Taking care to center the phantom to the SFOV, perform the usual daily QC phantom scan using the smallest T allowed by the maximum NxT
Perform artifact evaluation using ACR CT QC manual method to determine if there are potentially clinically significant artifacts over the phantom FOV (if artifacts are seen, use clinical image significance evaluation method below)
After verifying the absence of such artifacts over the phantom FOV, at least monthly perform an air scan using a modified version of the daily QC protocol that uses the largest clinical SFOV available
To evaluate the L SFOV images for potentially clinically significant artifacts outside of the phantom FOV, set a window width of approximately 100 HU and a window level of approximately -1000 HU
Draw a 20 cm diameter ROI in the center of the image
Review all images qualitatively comparing the level of visualized non-uniformities outside the ROI to that inside the ROI
Consider a similar level of non-uniformities to be acceptable
If there is an increased level of non-uniformities seen outside the ROI, review clinical scans to determine if artifacts are clinically significant
Clinical scans reviewed should be of the large scan field of view using all typical reconstruction window and level settings
Revision History for this Article |
Date | Section | Description of Revision(s) |
12-12-19 | All | Article created; FAQs incorporated; No criteria changes |
11-9-2022 |
| Added forms as an attachment to the article |
