I have performed a VBM study using a liberal threshold (p<0.001 with minimum cluster 100 voxels) and found several clusters. 

Now I would like to do a calculation to examine how many subjects I would need to find these effects with a more stringent correction for multiple comparisons. So basically a sample size calculation, based on the effect size found in this first study. 

Please let me know if you have any thoughts on how to do this. I could not find much on the SPM list.

Thanks in advance for any input. 

Regards, 

Mara