Electronic portal imaging devices are valuable for analyzing patient treatment data for geometric errors. The analysis can be more robust if an absolute reference position such as the isocentre (collimator axis of rotation) is used, allowing discrimination between setup errors and Jaw/MLC calibration errors. Conventionally, investigators have extracted field edges through global thresholding, and compared the treated to the planned field placement using techniques such as the method of moments and normalized cross‐correlation. The accuracy of these methods depends on the accuracy and consistency of the MLC and Jaw calibration as well as on EPID performance such as intensity uniformity. In this work we describe how the MLC inter‐leaf radiation leakage, hidden in the background of portal images, can be extracted and analyzed to find the field isocentre. The peak locations of inter‐leaf radiation leakage in a portal image are extracted providing a very precise and accurate determination of the isocentre location in the direction perpendicular to the MLC leaf travel. Clearly orthogonal portal images can provide a very accurate determination of beam isocentre in both directions. The orientation of the interleaf leakage can also be used to determine mechanical inconsistencies in the EPID structure as a function of gantry angle. The image analysis of the inter‐leaf radiation leakage for determining imager translation and orientation is described.

Additional Metadata
Persistent URL dx.doi.org/10.1118/1.2031041
Journal Medical Physics
Gao, Z., Gerig, L., & Szanto, J. (2005). Sci‐PM Fri ‐ 09: Image analysis of inter‐leaf radiation leakage, a new approach to the correction of EPID mechanic inconsistencies. Medical Physics, 32(7). doi:10.1118/1.2031041