You can import the data using dicomreadVolume (https://www.mathworks.com/help/images/ref/dicomreadvolume.html)
There is an introduction on how to register 2D multimodal image data using imregister (https://www.mathworks.com/help/images/registering-multimodal-mri-images.html). imregister also works with 3D volumes so you would probably need to adapt the code a little bit.
You might also be able to use the spatial information that you get from dicomreadVolume
[V, spatial] = dicomreadVolume(source)
and use interp3 to interpolate the data from the SPECT image onto the grid of the CT data. However the second approach only works if both images are taken in the same coordinate system and your patient/object did not move between the exams.
