Treatment field shape verification using elliptic Fourier transform

An automated field shape correlation technique based on elliptic Fourier tr ansform (EFT) is developed to verify the radiation treatment held in digita l portal images. In this method, the edge of the treatment field is initial ly extracted from the portal image and is then approximated by a polygon. T he polygon is further represented with elliptic Fourier coefficients. The i nvariants to shift, rotation, and scale are computed from the elliptic Four ier coefficients to characterize the genuine shape feature and are used to match the reference treatment field. Invariants calculated from both test a nd reference field shapes are compared to determine the similarity between two treatment fields. The proposed procedure uses the first approved held s hape as the reference for automated comparison with subsequent portal image s. This technique not only verifies the shape of each portal field but also provides information about relative shift, rotation, and scale. A set of g eneric shapes is simulated to test the robustness of the algorithm and to d etermine the parameters used in the decision procedure. Experimental result s on the simulated shapes show that this method can detect shape distortion s of 2% in area and the standard deviations are 0 for shifting, 0.24 degree s for rotation, and 0.0031 for scaling. Preliminary tests on clinical porta l images indicated that this technique is potentially useful for automated real-time portal verification. (C) 1999 American Association of Physicists in Medicine. [S0094-2405(99)00811-1].