The CT scan 64 slice is not the state-of-art for CAD detection. The CT scan 256 slice 3D images is very good as the images provide a view as if actually seeing the source. The 3D CCG differs from CT as it measures blood velocity to determine fault that may or may not be due to blockage but another cause. A cath angiogram has the advantage if there is blockage, the blockage can be stented at the time, and it is not painful at all!.
If the angiogram is not with a cath (invasive), what is going to be the procedure if the non-invasive test is positive?...it would require an invasive procedure. But if your mother's symptoms can be successfully treated with medication, there may not be a need for any procedure. I have blockages of coronary vessels (LAD completely blocked and another 72% blocked), I have been successfully treated for the past 6 years and no chest pain, etc.
Hope this helps, if you have any further questions please feel free to ask. Take care.