Imregister output volume and voxel intensities

I am using imregister to register VSPECT and CT images. My ultimate goal is to determine the volume of functioning lung as defined by VSPECT volume within CT volume. I registered the VSPECT (movingVolume) with the CT (fixedVolume) to produce the registered volume (movingRegisteredVolume) and the proceeded to determine the volume of SPECT outside of the CT volume (SPECTOutsideCT) with the following lines of code:

SPECTOutsideCT = (logical(movingRegisteredVolume) - logical(fixedVolume))>0;
SPECTOutsideCT = SPECTOutsideCT.*movingRegisteredVolume;
volumeViewer(SPECTOutsideCT);

Then, I subtracted this volume from the registered volume to determine the VSPECT volume inside the CT volume:

SPECTInsideCT = movingRegisteredVolume - SPECTOutsideCT;
volumeViewer(SPECTInsideCT);

Lastly, I tried subtracting the CT volume from the registered volume to determing the volume of functioning lungs, but was not able to do so successfully. I am confused as to what is the output volume of imregister. Is it just the transformed image of my moving volume (VSPCT)? or is it a combined image of both my moving volume and fixed volume? If so, then are the voxel intensities altered?

Any help would be really appreciated. Also, if you have any insight on how I could go about subtracting the CT from the registered volume that would also be super helpful.

Thank you in advance.