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A simple method to quantify the coincidence between portal image graticules and radiation field centers or radiation isocenter
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Image of FIG. 1.
FIG. 1.

Experimental setup of a BB phantom. source, detector, source, and detector. defines the orthogonal axes of the detector panel with being parallel to the gantry rotation axis.

Image of FIG. 2.
FIG. 2.

Detection of the edges (horizontal and vertical lines) and the center (cross) of a rectangular radiation field in an anterior-posterior MV EPID image.

Image of FIG. 3.
FIG. 3.

Localization of the mechanical graticule in an MV EPID image. (a) The original MV image in Fig. 2 was convolved with a Gaussian template to enhance the graticule dots. A mask (high-intensity cross-shaped region containing the mechanical graticule) was used to limit the computation of the Hough transform. (b) A localized mechanical graticule (orthogonal lines) was overlaid on the original image.

Image of FIG. 4.
FIG. 4.

Localization of the digital graticule in an MV EPID image. (a) Screen snapshot of a magnified MV image displayed in Varian Trilogy OBI software. The orthogonal lines represent Varian’s original MV digital graticule. (b) The same EPID image displayed in MATLAB with manually inserted graticule.

Image of FIG. 5.
FIG. 5.

Modeling of the BB in a BB-only image (a) and the central dot of mechanical graticule in a graticule-only image (b). The image intensity profiles (right) along the white lines in the images were better fitted with a modified Gaussian model [Eq. (2)] than with a standard Gaussian function.

Image of FIG. 6.
FIG. 6.

Localization of the BB (large circle) in the presence of mechanical graticule (small circle) in a real MV EPID image (a) and a simulated image (b). The EPID image is zoomed in to show the central region of interest. Notice that the peak intensity does not necessarily correspond to the center of graticule dot (see Sec. ???).

Image of FIG. 7.
FIG. 7.

Localization of the BB in a kV portal image. (a) Original kV portal image of the BB at a source angle of 0°. (b) Close-up view of the BB showing the detected edge (circle) and the center of BB (cross). Nonuniform background across the image (a) was due to the half-fan filter in the beam path.

Image of FIG. 8.
FIG. 8.

Misalignments of MV mechanical graticule (left), MV digital graticule (middle), and kV digital graticule (right) from the radiation field centers at four source angles. Shown are the results from three measurements made in 1 month interval. Notice that a source angle of 0° for the kV graticule corresponds to the gantry angle of 90° on Varian linacs.

Image of FIG. 9.
FIG. 9.

Reproducibility and room-for-play of the MV mechanical graticule. Locations of the graticule center (GratCenter) were compared to those of the radiation field center (RadCenter), while the mechanical graticule was in either the innermost (In) or the outermost (Out) position.


Generic image for table

Source angle-specific misalignments (in mm) of MV mechanical graticule, MV digital graticule, kV digital graticule, and radiation field center from the radiation isocenter measured in April 2009.


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752b84549af89a08dbdd7fdb8b9568b5 journal.articlezxybnytfddd
Scitation: A simple method to quantify the coincidence between portal image graticules and radiation field centers or radiation isocenter