In December 2013, facilities that are currently ACR accredited in CT will have to maintain a CT QC program that follows the current ACR CT QC Manual. In addition, the frequency of specific checks will have to follow the periodicity stated in the ACR manual. One of these checks, measuring CT slice width, can be cumbersome and expensive to perform routinely.
Typically Gafchromic film is used to measure the beam slice width of a CT scanner. This film is one time use only, and is quite expensive. Checking several slice width configurations for each scanner in a facility with several CT scanners will become costly. An alternative technique for measuring the CT slice width utilizes a phosphorescent screen to temporarily record the slice width. A nicely designed and useful phosphorescent screen is made by RTI electronics, and is called the Visi-X radiation and light field analyzer. RTI also manufactures the popular Piranha x-ray measuring device used by many service engineers, technologists, and medical physics personnel.
RTI Visi-X Phosphorescent Screen Test Tool
The Visi-X tool is typically used for checking the light field and X-ray field alignment on a regular radiographic or fluoroscopic machine. However, it can also be used to measure the slice width of a CT scanner.
The cassette-shaped tool has a built-in scale and visible light screen to prevent phosphoresnce under normal room lighting. From experience, it’s best to make the exposure and read the screen while the room is as dark as possible. Even a small amount of ambient light will flood the phosphorescent screen. This will make reading the exposed area difficult, if not impossible. The tool can be used repeatedly, as the phosphorescent screen lifetime is not affected by the energy or number of exposures.
ACR Accreditation CT Beam Slice Width
The current ACR CT QC Manual specifies measurement of radiation beam width annually or after relevant service. The Visi-X can be used to measure the slice width by placing it so that the center of the flat test tool is at isocenter in the scanner. Through careful alignment the tool can be scanned at various beam widths and then immediately ‘read’ to verify slice width accuracy.
As ACR accreditation requirements for CT become more cumbersome, one must look for ways to efficiently and effectively meet the expectations of the ACR CT accreditation program. A copy of the ACR Accreditation CT QC Manual may be downloaded for $40 from the ACR website.