An orthogonal moment-based method for automatic verification of radiation field shape

The purpose of this paper is to develop a new method for automated on-line verification of the treatment field shape during radiotherapy. The treatmen t field boundary is extracted from the digital portal image and is then app roximated by a polygon. The proposed procedure used one of the approved fie ld shapes as the reference boundary for automated comparison with subsequen t portal field boundaries. The orthogonal moment-based method was applied t o align treatment field boundaries that include the translational shifts, s caling factor and rotation angle. Firstly, the moments of order up to one w ere used to adjust the magnification and translation of the rest field boun dary related to the reference one; this step created a common coordinate sy stem for the two images. Then a quadratic least-square objective function b ased on the orthogonal moments (e.g. Legendre moments) of the two field sha pes was employed to perform rotational correction. Since moment computation by a straightforward method required a large number of multiplication and addition operations, a fast method for computing Legendre moments was also developed to decrease the calculation time. Application of the method to so me simulated cases showed that our alignment procedure has an accuracy of 0 .5 mm in detecting translational shift, 0.004 in detecting magnification an d less than 0.3 degrees in detecting rotation angle between the test shape and the reference shape. The alignment procedure using the proposed method can be done within 2 s on a Pentium II. personal computer. Therefore, our m ethod is potentially useful for automated real-time treatment field shape v erification.