Calculate the mean of region with nonzero pixels

Yes, your approach is correct for calculating the mean of the regions labeled with 1, 2, and 3 separately in your 3D volume. Each line of code correctly identifies the voxels corresponding to the specified label in your mask and calculates the mean of those voxels in the volume.

Here's a summary and the code:

1. For region labeled with 1:

   matlab

   m1 = mean(volume(mask == 1));
   

2. For region labeled with 2:

   matlab

   m2 = mean(volume(mask == 2));
   

3. For region labeled with 3:

   matlab

   m3 = mean(volume(mask == 3));
   

This will give you the mean values of the pixels in the 3D volume corresponding to regions labeled 1, 2, and 3, respectively. Ensure that volume and mask are appropriately defined and that they are the same size.