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Verification of multileaf collimator leaf positions using an electronic portal imaging device
Author(s) -
Samant Sanjiv S.,
Zheng Wei,
Parra Nestor Andres,
Chandler Jason,
Gopal Arun,
Wu Jian,
Jain Jinesh,
Zhu Yunping,
Sontag Marc
Publication year - 2002
Publication title -
medical physics
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.473
H-Index - 180
eISSN - 2473-4209
pISSN - 0094-2405
DOI - 10.1118/1.1515760
Subject(s) - pixel , multileaf collimator , image guided radiation therapy , collimator , optics , computer vision , image resolution , artificial intelligence , physics , dosimetry , distortion (music) , mathematics , medical imaging , computer science , nuclear medicine , linear particle accelerator , beam (structure) , medicine , amplifier , optoelectronics , cmos
An automated method is presented for determining individual leaf positions of the Siemens dual focus multileaf collimator (MLC) using the Siemens BEAMVIEW(PLUS) electronic portal imaging device (EPID). Leaf positions are computed with an error of 0.6 mm at one standard deviation (σ) using separate computations of pixel dimensions, image distortion, and radiation center. The pixel dimensions are calculated by superimposing the film image of a graticule with the corresponding EPID image. A spatial correction is used to compensate for the optical distortions of the EPID, reducing the mean distortion from 3.5 pixels (uncorrected) per localized x‐ray marker to 2 pixels (1 mm) for a rigid rotation and 1 pixel for a third degree polynomial warp. A correction for a nonuniform dosimetric response across the field of view of the EPID images is not necessary due to the sharp intensity gradients across leaf edges. The radiation center, calculated from the average of the geometric centers of a square field at 0° and 180° collimator angles, is independent of graticule placement error. Its measured location on the EPID image was stable to within 1 pixel based on 3 weeks of repeated extensions/retractions of the EPID. The MLC leaf positions determined from the EPID images agreed to within a pixel of the corresponding values measured using film and ionization chamber. Several edge detection algorithms were tested: contour, Sobel, Roberts, Prewitt, Laplace, morphological, and Canny. These agreed with each other to within ⩽1.2 pixels for the in‐air EPID images. Using a test pattern, individual MLC leaves were found to be typically within 1 mm of the corresponding record‐and‐verify values, with a maximum difference of 1.8 mm, and standard deviations of <0.3 mm in the daily reproducibility. This method presents a fast, automatic, and accurate alternative to using film or a light field for the verification and calibration of the MLC.