For the solutions of their inverse problems, Dunn (1976), Chiroiu et al. (2000), Kim and Kapania (2004), Trivailo et al.(2004), and Trivailo et al. (2004) used Neural Networks and Genetic Algorithms; Brakhage(1987), Hanke (1995), Rieder (1999), Burger and Muhlhuber (2002), Chock and Kapania
(2003), and Feijoo et al. (2004) used gradient based methods.