Improving Health Through Medical Physics

AAPM Newsletter — Volume 42 No.6 — November|December 2017

ACR ACCREDITATION: FAQS FOR MEDICAL PHYSICISTS Priscilla F. Butler, MS, Senior Director and Medical Physicist, ACR Quality and Safety

Picture of Priscilla F. Butler

Does your facility need help on applying for accreditation? In each issue of this newsletter, I'll present frequently asked questions (FAQs) of particular importance for medical physicists. You may also check out the ACR's accreditation web site portal for more FAQs, accreditation application information, and QC forms.

The following questions are for the ACR Computed Tomography Accreditation Program's new CT quality control manual. Please feel free to contact us, if you have questions about the new manual or CT accreditation.

Q. The updated 2017 ACR Computed Tomography Quality Control Manual has recently been released. What has changed since the 2012 version?
A. While the required tests are the same, many of the test procedures, data interpretation and corrective action section descriptions have been updated to improve clarity within the manual. In addition, expectations and responsibilities have been further defined. These include:

  • Radiologist Section
    • Defined responsibilities of a CT protocol review and management team
    • Clarified qualified medical physicist responsibilities
  • Radiologic Technologist Section
    • Clarified expectation of action limits
    • Updated water CT number and standard deviation (noise) testing procedure and data interpretation and corrective action sections
    • Updated artifact evaluation test procedure section
    • Combined daily technologist quality control data form and weekly checklist into one form
    • Removed standard-artifact free image section
  • Qualified Medical Physicist Section
    • Revised review of clinical protocol section
    • Updated radiation beam width recommended performance criteria and corrective action section
    • Updated low contrast performance data interpretation and analysis section
    • Updated CT number accuracy recommended performance criteria and corrective action section
    • Updated artifact evaluation test procedure section
    • Updated CT uniformity data interpretation and analysis, timeframe and corrective action sections
    • Updated CT scanner display calibration data interpretation and analysis and corrective action sections
    • Removed the image thickness test

Q. When will current CT accredited facilities or new facilities applying for CT accreditation be required to comply with the test criteria in the updated 2017 ACR CT Quality Control Manual?
A. The required tests have not changed compared to the 2012 ACR CT Quality Control Manual so all facilities must be compliant with the tests listed in this manual.

Q. Is the ACR QC manual intended to replace the vendor vendor-recommended QC program?
A. The purpose of the ACR QC manual is to help guide facilities in establishing and maintaining an effective CT quality control program. The manual provides a minimum set of tests required to ensure that a scanner performs in a consistent manner and yields acceptable images. The vendor's specific quality control program should be followed and can be a substantial part of such a QC program. When these tests and phantoms are available, they can be used to fulfill the ACR CT Accreditation Program QC requirements. In some cases, using the manufacturer's recommended QC protocols, procedures, and frequencies may be a condition of maintaining warranties as well as purchase or service agreements. The ACR QC manual is certainly not intended to override or replace any such conditions.

Q. What if the ACR program describes a different set of testing conditions and/or specifications than the vendor-recommended QC program?
A. There is a great diversity in scanner technology, phantoms, testing procedures and tolerances. The primary intent of the ACR CT QC manual is to establish a QC program and a secondary goal was to provide a reasonably uniform approach to testing. However, there may be instances where the QC manual's described tests may not be appropriate for a specific test on a specific scanner. (For some existing examples, see the ACR CT Accreditation program FAQ) Use of the manufacturer's phantom, testing procedure and specifications, especially in these situations, is appropriate and encouraged.

Q. What if a scanner does not pass a specification recommended by the ACR in their QC manual or a specification that the medical physicist designed?
A. First, repeat the test to confirm the result. Next, consult the vendor's Technical Manual. If the same type of test is provided in the vendor's technical manual run the test as specified by the vendor and apply the vendor's specification. If the vendor's specification is passing and the clinical images do not have a clinically significant image quality issue, corrective action is likely not needed. If the vendor-provided test result is outside the vendor's specification, or there is believed to be a clinically significant degradation of image quality the images used for diagnosis, service should be contacted.

Communication is key in these instances. The QMP should not just perform a test and inform the site that a service call is required; the QMP has a responsibility to provide clear communication regarding the specific issue under discussion, what specific tests have been performed and what results have been observed and what specifications (e.g. manufacturer's specifications) are being used to determine if the system is in calibration.

If the vendor does not provide specifications for a particular test, then the ACR or medical physicist's test result should be benchmarked and monitored over time. Please note additional testing outside of the vendor specifications may not be supported by the manufacturer.

Q. My machine passes all the vendor specifications, but when I apply my clinical protocol to the ACR phantom I get a failing result. What could be the problem?
A. Some elements of a clinical protocol, such as the reconstruction algorithm, may make assumptions that apply to human anatomy but may cause distortion in phantom images. For example, beam hardening or scatter corrections may perform differently in phantoms than in patient and distort CT numbers or lead to phantom artifacts. The hardness of the x-ray beam can also vary among scanner models, shifting CT numbers relative to non-vendor specifications. In addition, specifications that involve adaptive aspects of clinical protocols, such as iterative reconstruction or size-specific calibration files, can depend on precise interaction of phantom, test object, and protocol. Therefore, the use of absolute limits may not be as important as recording the consistency (constancy) of values over time.

Q. Can you clarify the scope of "Acceptance Testing"?
A. Acceptance Testing encompasses the verification of the purchase specifications for a particular model 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, however, additional tests may be performed by the medical physicist. These tests are not used to determine if 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 additional testing outside of the vendor identified tests, methods, and acceptance criteria may not be supported by the manufacturer.

Q. Why was image thickness measurement removed from the QC Manual? What about single slice scanners?
A. The vast majority of clinical CT scanners now use multidetector technology. With such scanners, image thickness is largely independent of data collection, except for the minimum image thickness available. Since image thickness is a computed quantity, it is no longer a failure mode for modern scanners. As such, it was decided that it is no longer necessary to include it as a required test within the ACR CT QC Manual.

For the few single slice scanners still in use, it may be valuable for the QMP to verify both radiation beam width and image thickness, as the two are tied to each other and deficiencies may occur.

Standard Imaging 2017 Adv

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