just to add to Martins answer: there are formulas that describe the relation between t-values as stored in spmT_* images and e.g. correlation coefficients. You can use them to compute a voxelwise correlation map. Some implementations of these conversions are publicly available. One of them can be found in the Volumes toolbox linked on the SPM extensions website. After installing the toolbox there should be a "Tools->Volume Handling Utilities->Stats Tools->Transform t-maps" menu entry in the SPM batch system to run the conversion.
Hope this helps,