One way of treating the partial volume effect in PET image reconstruction is by using anatomical information from other imaging modalities (MRI or CT). The a priori information of a maximum a posteriori reconstruction algorithm is defined from the anatomical images. In this paper the ordered subsets modified median root prior one step late (OS-MMRP-OSL) algorithm [1], which uses information derived from MR images, is evaluated in a computationally simulated PET FDG myocardium study. The algorithm was implemented in STIR (Software for Tomographic Image Reconstruction) [2], (http://stir.sourceforge.net). Realistic PET data have been used, to compare the standard ordered subsets median root prior one step late (OS-MRP-OSL) algorithm with the OS-MMRP-OSL algorithm using well-aligned segmented and non-segmented MR images. In some cases the quantitative results indicate lower bias (by 6.5%) for OS-MMRP-OSL using segmented MR images and decreased root mean square error (RMSE) by 3%. Moreover, we have improvement in edge preservation.