Kindly refer to the following MATLAB Answers post, in which a similar question on calculating regional gray matter volume has been answered: https://www.mathworks.com/matlabcentral/answers/2080931-how-to-get-grey-matter-volume-of-specific-brain-regions-in-spm-matlab
The post also suggests to use the CAT12 toolbox, which provides tools for brain related morphometric analysis. More information can be found in the CAT12 manual: https://neuro-jena.github.io/cat12-help/
Hope this helps!
