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In order to compare the treated field with prescribed field in radiation therapy, we comparatively
investigated the performance of alignment techniques of portal and simulator images using surface
landmarks. The registration algorithm of pairs of radiation fields consists of the following steps: (1)
alignment of pairs of radiation images by point matching (2) field displacement analysis by field
boundary matching. In the first step, one method of paired surface landmarks matching is to calculate the
transformation parameters (scale, rotation, shift) using the corresponding line pairs which are created by
connecting two landmarks of each image, the other comparative method is to find the scale factor using
least squares minimization approach and rotation angle solving the eigenvector problem. In the next step,
first we extract
portal field boundary based on morphological edge detection technique, and then
match the two field boundaries. The ρ- technique that converts from rectangular to polar coordinates of
field boundaries and then minimizes the difference of the area between two fields is applied to match two
fields. Also based on the method proposed by Jialing Cai et al.[Med, Phys.25,824-829], transformation
parameters for registering two field boundaries are determined by minimizing of the cost function which
is difference between simulator field boundary coordinates sets and portal field boundary coordinates sets
multiplied transformation matrix and we create simply.
Applying to image pairs of flat polystyrene
phantom and Alderson Rando phantom, detection errors are less than 1mm in translation and 1Û in
rotation.
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