Editing STIR FAQ (section)

=== Median Root Prior ===
Although there is not really a prior function for MRP, its gradient is independent of the image-scale, and the gradient of the log-likelihood is independent of counts as well (after enough iterations). Therefore, the effect of MRP is not count dependent like QP. However, it does depend on sensitivity/attenuation etc. You can use the "multiplicative form" of the
update (see the User's Guide and http://dx.doi.org/10.1109/NSSMIC.2001.1008688) to avoid this (in which case 1 is a high
penalty), although the "additive form" of MRP is in principle better as it has higher influence where the sensitivity is lowest (note that MRP with large penalty is hampered by convergence problems when using OSL).