Yes, it is possible to create a mask from what is displayed by volshow in MATLAB.
You can use the vol3d function instead of volshow, which will return a handle to the vol3d object. You can then use the vol3d object's cdata property to access the voxel data displayed by the vol3d object. The cdata property is a three-dimensional matrix that represents the voxel data. You can use the imfill function to fill in any holes in the voxel data, and then use a thresholding function like imbinarize to create a binary mask from the voxel data.
