Po‐Thur Eve General‐13: Commissioning a gated kV imaging system
The recent expansion in the use of breathing gating systems of various types necessitates the development of methods to efficiently evaluate them. An ideal commissioning procedure would not require extensive specialized equipment, would allow multiple measurements to be made quickly and easily, and be capable of detecting errors at levels which could conceivably be important in treatment situations. In this study, a simple method to evaluate timing accuracy and precision for gated kV imaging was developed, and used to evaluate the Brainlab ExactTracGating ™ kV imaging system. The phantom used has a moving platform and a set of IR markers moved in a sinusoidal vertical motion by a cam milled with 0.001 inch accuracy. A 1mm thick lead sheet with a 3 mm diameter hole was placed under the moving platform. Kodak PP‐L film was placed on the moving platform and kV images were obtained at 5%, 50% and 95% gating levels with each of the two tubes. Ten measurements for each tube could be easily made on a single 11×17 inch film. The measurement was repeated for periods between 1.5 and 5 seconds, and with a platform moving close to a sine function with an amplitude of 2.1cm. This commissioning system was capable of detecting systematic timing errors of less than 40 ms, but none were observed. The standard deviation in timing accuracy was measured under difficult physiological conditions (3 second period, 2 cm amplitude of motion) and found to be about 0.03 cm, which corresponds to 30 ms.