Room: ePoster Forums
Purpose: Medical linear accelerators with online magnetic resonance imaging system (MRL) offer a promising platform for high-precision, image-guided adaptive radiation therapy. Such systems are capable of producing images of soft tissue with excellent contrast resolution. A calibration phantom and an algorithm previously developed for cone-beam imaging systems as well as megavoltage (MV) treatment beam delivery system were employed to characterize the MRL system geometry.
Methods: The calibration phantom consists of 24 tungsten ball bearings (BBs) with 4 mm in diameter precisely located in two ring arrangement in a plastic cylinder. Diameter and separation of two rings are 180 mm and 70 mm, respectively. In each ring, twelve BBs are evenly spaced at 30Â°. A single pose of the phantom is sufficient to determine the source position, electronic portal imaging device (EPID) position and tilt angles with respect to the world coordinate system located at the center of the phantom.
Results: Geometric parameters such as source position, EPID panel position and tilt angle, and gantry angle were calculated at each gantry angle using MV radiation. The MV source position and piercing point were well within 1.0 mm from ideal circular motion for all gantry position in 3 dimensional spaces. Average source to isocenter distance (SAD) was 1438 mm and source to detector distance was 2680 mm. Scale factor was 1.859 with uncertainty of 0.24%. Gantry angle shows good linearity with a uncertainty of 0.58o. Phantom and couch motion were detectable with uncertainty of 0.07 mm (1 std) at imaging coordinate and 0.15 mm at phantom coordinate.
Conclusion: The double ring phantom was found capable to validate the stability of the MV source, EPID, isocenter, gantry angle and couch position of MRLs. Stability of the MRLs is essential to obtain full benefit of the image quality of a magnetic resonance imaging system.