I am not familiar with the viewer you are using, do you see different values in MRIcroGL and SPM? I wonder if your viewer is not taking the rescale slope and intercept into account. I suspect that the issue here is that your data exceeds what can be stored in a signed 16-bit integer (e.g. the DICOM data is UNSIGNED 16-bit, but this is not a common NIfTI format). In this case, there are different strategies to preserve precision. Feel free to send me a sample dataset by personal email (e.g. send link to dropbox). A full report from fslhd would also be useful. By the way, there may also be differences in PAR/REC conversion as my software tries to convert the float value to NIfTI rather than the display value (as described here http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3998685/).