This figure illustrates the effect of cross plane misalignment of beam spot on nominally opposed beams. Adapted from Ref. 12.
Setup for the corotational block measurement of the CPM method. Note that since the ion chamber is fixed to the collimator assembly the geometric relationship amongst the solid jaw, ion chamber, and CAOR are independent of collimator angle.
A graphical representation in the ion chamber (corotational) frame of reference for the CPM method showing the 50% isodose plane intercepting the IC as a function of collimator angle for an off axis electron beam spot.
Simulated IC signal in the CPM method as a function of collimator angle, for various combinations of beam spot ellipticity, beam spot distance off axis, and jaw distance off axis.
Graphic depicting how a triangle can be formed from the three images of the upper rod and similarly another can be formed from the lower rod images and the how an inscribed circle can be found for each of the triangles.
Graphic depicting how the beam spot position effects the projected x-ray shadow of rods at different distances from the image plane in the ICS method. In this figure the rods are seen at two collimator angles, 0° and 180°. When the beam spot is on the CAOR the distance between the shadows is independent of collimator angle. When the beam spot is not on the CAOR the distance between the two images is collimator angle dependent.
A composite RGB image of three exposures summed into one image. The colors of the rod images are assigned for clarity. In this composite RGB image each individual image is assigned a different color for clarity. The white lines are LSQ fit to the rod images. The sharper shadow being that of the lower rod (closest to the imager) and the broader inner shadow is for the upper rod.
Measured ion chamber signal for the CPM measurement as a function of collimator angle with the beam spot at two different distances from the CAOR.
Beam spot position relative to the CAOR. The first 4 columns of data show the measured distance from the beam spot to the CAOR when the beam spot is in its nominal clinical position, i.e., the bending magnet currents are set to their nominal clinical values. The second 4 columns of data show the measured distance from beam spot to CAOR when the steering currents are intentionally altered from their clinical values, and the final 4 columns of data show the difference or shift between the two cases.
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