An orthogonal moment-based method for automatic verification ofradiation field shape

Abstract

The purpose of this paper is to develop a new method for automated on-lineverification of the treatment field shape during radiotherapy. The treatmentfield boundary is extracted from the digital portal image and is thenapproximated by a polygon. The proposed procedure used one of the approvedfield shapes as the reference boundary for automated comparison withsubsequent portal field boundaries. The orthogonal moment-based method wasapplied to align treatment field boundaries that include the translationalshifts, scaling factor and rotation angle. Firstly, the moments of order up toone were used to adjust the magnification and translation of the test fieldboundary related to the reference one; this step created a common coordinatesystem for the two images. Then a quadratic least-square objective functionbased on the orthogonal moments (e.g. Legendre moments) of the two fieldshapes was employed to perform rotational correction. Since momentcomputation by a straightforward method required a large number ofmultiplication and addition operations, a fast method for computing Legendremoments was also developed to decrease the calculation time. Application ofthe method to some simulated cases showed that our alignment procedure has anaccuracy of 0.5 mm in detecting translational shift, 0.004 in detectingmagnification and less than 0.3° in detecting rotation angle betweenthe test shape and the reference shape. The alignment procedure using theproposed method can be done within 2 s on a Pentium II personal computer.Therefore, our method is potentially useful for automated real-time treatmentfield shape verification.