Purpose: Conformance between alternate methods that evaluate the positive beam limitation (PBL) of X-ray systems is determined using Bland-Altman analysis.
Methods: The PBL of GE Precision, Fuji Velocity and Philips Easy-Diagnost systems was evaluated using the gold-standard and two alternate methods, light-field-width and DR-coin. First, the tube was set at a detent position where the PBL engaged. The gold-standard method was carried by placing a CR imaging plate at a known distance from the image receptor and exposing it. For the light-field-width method, the light field projected on the Bucky’s surface was measured with a measuring tape. For the DR-coin method, coins were placed on the Bucky’s surface at the edges of the light field and the DR detector inside the Bucky was exposed. All field sizes were measured in PACS and magnification corrected. Measurement cohorts were collected for both wall and table Buckies for the aforementioned units. The two methods were compared for conformance with the gold-standard using Bland-Altman technique. This method relies on plotting difference of results obtained using different methods against the average of results obtained by the two methods.
Results: The mean difference between the gold-standard and DR-coin method for PBL assessment is -0.50 and the standard deviation of differences is 0.76. The mean difference between the gold-standard and light-field-width method for PBL assessment is 0.0 and the standard deviation of differences is 0.81.
Conclusion: Preliminary results suggest that the DR-coin to gold-standard methods demonstrate slightly lesser variation between the observations as compared to the light-field-width method. This analysis along with our practical experience in the field favors the use of the DR-coin based method if the CR-based method is not available. Therefore, the DR-coin method is a more promising alternative to the conventional CR-based PBL assessment, given the shrinking availability of CR-based imaging in clinics.