Algorithm workflow for fiducial detection in cine MV images acquired during VMAT treatments.
Varian TrueBeam linear accelerator with electronic portal imaging system and reference systems employed in this work. The linac reference system is centered at the isocenter and fixed with respect to the linac room. The EPID reference system rotated with the gantry. Its origin was on the upper left corner of the imager. The reference system employed for comparing the marker with the MLC leaves positions (collimator reference system), as well as the gantry reference system are not represented. The gantry reference system was centered at the isocenter and rotates with the gantry, around the Z≡Z ′ axis.
Template with rotation employed for the detection. The templates T1–T4 represent the fiducial by a 2.4 × 0.8 mm2 rectangle, the templates T5–T8 by a 3.4 × 0.8 mm2 rectangle. These templates were used to localize the fiducials, whose projected length onto the image depended on its 3D orientation. The template T9 represented the case in which the fiducial was orthogonal to the image. The fiducials T10–T17 were originated from T1–T8 by cropping their area of 13 × 13 pixels (5.1 × 5.1 mm2) to an extension of 7 × 7 pixels (2.74 × 2.74 mm2), to represent the situation in which a fiducial was partially occluded by the leaves. The size of templates T10–T17 relative to that of T1–T9 keeps their proportion. Note that the fiducial dimensions were scaled when projected to the EPID and measured in pixels.
MV image of a control point projection acquired with the EPID, without (left) and with (center) Laplacian of Gaussian (LoG) filtering, which enhanced the fiducials, and after the removal of field edges with the mask (right).
Intensity image (left) with frame artifact and (right) after applying the intensity correction to remove it. Clearly the correction enhanced the image quality, prior to performing a minimum intensity search.
Detection of fiducials in the EPID imager for patients 1 and 2. The 2D displacements as measured from the good fiducial detections were projected to the isocenter and displayed in mm. The directions in which the displacements are presented were the fixed Z axes and the rotating (with the gantry) X ′ axes.
Localization of the three implanted fiducial markers on a set of cine MV images for patient 1. The 3D fiducial positions were calculated from the 2D data obtained by measurement on the EPID imager (displayed in Fig. 6 ). The bad localizations have been removed. This data correspond to a fraction on a patient where clinically relevant displacement of the fiducials with respect to their planned position was detected. The maximum absolute distance measured for a fiducial from its planned position was 5.7 mm (fiducial 1). Fiducial 1 had a large variation in AP and LAT directions starting in image 165, after being occluded by the leaves. Since the other two fiducials did not experienced that change, part of that difference could be real movement and part could be due to noise in the reconstruction, having had few 2D projections close to that gantry angle. Tracking with at least one fiducial was achieved in 86% of the images.
Localization of the three implanted fiducial markers on a set of cine MV images for patient 2. This data corresponds to a fraction on a patient where good positioning during most part of the treatment was achieved (the fiducials were at a position closer than 3 mm from its planned position in most of the treatment). The corresponding 2D detections are presented in Fig. 6 . The 3D estimation was robust (especially for the LAT and SI directions). For the AP projection, the estimation was less robust at the starting gantry angles (180° and above), since the direction of estimation was parallel to the gantry angle and some more projections were necessary to get the noise smaller than 1 mm. Tracking with at least one fiducial was achieved in 64% of the images. The relative movement of the three fiducials was quite correlated.
Algorithm performance. For all five patients, the percentage of images in which each fiducial was detected, the percentage of images in which at least one fiducial was detected, the percentage of valid detections for each fiducial, the average or valid detections for each patient, and the average detection error are represented in mm. The mean detection error in the 2D images was calculated assuming an error of 0.392 mm (pixel size) for the valid detections, and the measured error with respect to the ground truth as determined by manual introduction for the bad detections.
Motion assessment. The results of the absolute value of the displacement, averaged over all the images, are displayed. The 3D estimation was performed while the detection algorithm was running. After the identification of the bad 2D localizations, the motion assessment data corresponding to those images were removed. The mean displacement between the localized positions and the CT planned position of the centroid of the fiducials is displayed in mm, together with the standard deviation (STD).
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