David M. Gauntt PhD, Presenter: Co-owner, X-Ray Imaging Innovations, LLC Stockholder, General Electric Company

Rani Al-Senan PhD, Abstract Co-Author: Nothing to Disclose

It is practical to use dosimetric techniques to routinely measure CT beam widths. However, care must be taken to ensure that the beam width is measured not only for each collimation width, but for each collimation/focal spot combination.

The ACR now requires annual measurement of the CT beam width for all clinically used collimations in accredited scanners. We have developed a technique for purely dosimetric measurement of CT beam width. We are presenting the results of one year of clinical experience with this technique.  

We determine the dose-length product (DLP) per millimeter of beam width on a given scanner by measuring the DLP in air for a wide beam, both with and without a 10mm tungsten mask on the pencil chamber. The difference between these measurements is the DLP that would be measured for a beam exactly 10mm wide. The beam width for all collimations on this scanner can be determined by measuring the DLP in air, and dividing by the DLP per millimeter. Over the past year this measurement technique has been used on three GE CT scanners and seven Philips scanners at our facility. We have compared the results to film width measurements on some scanners, and compared the results of each scanner to other scanners of the same model, and to the ACR recommendations and vendor specifications.

The excess beam width varies significantly from one scanner model to another, but remains fairly consistent between scanners of the same model. For example, the Philips Brilliance 40 scanners consistently fail to meet the recommendations of the ACR for beam width, while the Philips Brilliance 64 scanners consistently meet the recommendations. The CT beam width of General Electric scanners varies between low tube currents and high tube currents. This appears to be due to the use of two different focal spot sizes; at high currents the large focal spot is used, and so the collimation width is increased to keep the beam penumbra from overlapping the detector. In all cases, the beam width was within specifications set by the manufacturers, which are consistently looser than the ACR recommendations.  

Gauntt, D, Al-Senan, R, Clinical Experience with Dosimetric Measurement of CT Beam Width.  Radiological Society of North America 2014 Scientific Assembly and Annual Meeting, - ,Chicago IL. http://archive.rsna.org/2014/14008611.html