Getting seleted voxel mask from MRSI data

Hi,recently I am working on 2D 31P MRSI based on Philips sdat data. I want to get seleted voxel mask from 2D MRSI data for tissue calibration。How can I get the mask?I try to get the mask from Oryx-MRSI toolbox. But seemed there is a mismatch between big fov and setted voxel, and also there is a overlap between two adjacent voxels. is there some matlab code can show what happens? Thanks a lot