A portal image alignment and patient setup verification procedure using moments and correlation techniques.

The objective of this study was to develop an integrated field shape and patient setup verification procedure for portal images. The proposed procedure used one of the approved portal images as the reference image for automated comparison with subsequent portal images. The first step of the procedure used a moments method to align treatment field boundaries. This step was necessary to compensate for the repositioning error of an image detector and to create a common frame of reference for comparing anatomical shifts relative to the field boundary. At the end of the moments alignment, a moments figure of merit was computed and compared with a pre-established threshold. This verified whether there was a potential shape change in the treatment field. To measure anatomical misalignment, the last step in the procedure was to use a grey-scale image correlation method to align translations and in-plane rotations relative to the anatomy of the reference image. The procedure was shown in phantom studies to err by less than 1 mm when detecting translational shifts and less than 1 degree when detection in-plane rotations. The moments verification method showed a sensitivity of detecting a placement error of 6 mm for a single leaf in a controlled experiment where a multileaf collimator was used for field shaping. The alignment procedure was fast and could be done in less than 12 s on an IBM-compatible 486 personal computer.

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