Scott D. Booth Jeffrey A. Fessler Combined diagonal/Fourier preconditioning methods for image reconstruction in emission tomography. 441-444 1995 conf/icip/1995 ICIP http://doi.ieeecomputersociety.org/10.1109/ICIP.1995.537510 db/conf/icip/icip1995-2.html#BoothF95