(a) Medical linear accelerator used for acquisition of volumetric images. The kV imaging system is integrated into the gantry and allows a patient to be imaged in the treatment position immediately prior to treatment delivery. (b) Patient setup for imaging/treatment. (c) Centered detector geometry compared with offset detector geometry. The panel is offset from center, effectively expanding the physical detector (solid line) to a “virtual” detector (dotted line), which corresponds to a -diam transverse FOV at iso-center.
Schematic of the kV-MV calibration procedure. (a) Relative position of “iso-centers” and ball-bearing (BB) prior to adjustment in BB placement based upon MV portal images. The portal imaging procedure provides an estimate of the BB location with respect to the MV radiation iso-center of the treatment unit. (b) Following adjustment in BB location to the MV radiation iso-center, the BB position is taken as an accurate estimate of the MV radiation iso-center. A calibration table is formed from a series of kV radiographs over 360° which capture the BB location. The kV cone-beam CT reconstruction system is designed to place the reconstruction center at this location in the world coordinate system (i.e. MV radiation iso-center is located at the center of all subsequent reconstructions).
Schematic of the unambiguous object used for targeting. Nine acrylic spheres of varying diameters were embedded within a cube of polystyrene foam. A -diam sphere was embedded at the center of the cube and used as the surrogate target for the treatment planning and image guidance procedure. Two sets of four smaller spheres of varying diameter were embedded at the four corners of the cube to form a unique pattern that could verify the orientation of the phantom while not obscuring the target sphere.
Long-term stability of XVI imaging system as demonstrated by the deviation of the x-ray piercing point from the center of the kV imager as a function of gantry angle. Results are presented for the centered and offset panel positions. The solid lines show the average value while the dashed line shows the 95% confidence interval over an eight-month window.
The residual error in targeting from all sources following XVI image fusion and correction, as determined by centroid analysis of orthogonal MV portal images. The three-dimensional points (closed circles) in (a) are projected onto each axis (open circles) to display L/R-A/P error, the L/R-S/I error, and the A/P-S/I error. The ellipsoid and its projections (dashed line) show the principal components of the 95% confidence interval. (b), (c), and (d) the same data in histogram form for the L/R, A/P, and S/I directions, respectively. The average and standard deviation of the error in each direction is , , , respectively.
The difference between the final position of the sphere measured under cone-beam CT guidance and the final position determined centroid analysis of orthogonal MV portal images. As in Fig. 5, the three-dimensional points (closed circles) are projected onto each axis (open circles) to demonstrate L/R-A/P error, the L/R-S/I error, and the A/P-S/I error. The ellipsoid and its projections (dashed line) show the principal components of the 95% confidence interval. The average and standard deviation of the error in each direction is , , , respectively.
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