A CAT Scan or Computed Axial Tomography, also known as Computed Tomography (CT Scan), is another painless method used to scan your body when trying to determine if you have developed any metastases. CT scanning uses x-rays to rapidly obtain multiple images taken in “slices,” which are then compiled by computer into an image. The CT scan produces a picture of a cross-section of soft tissue, blood vessels and bones.
Although considered safe, CT Scans employ x-ray radiation that may increase your risk for the development of another primary cancer. Be sure to tell your doctor if you have had any prior CT scans. Be sure to ask if the CT is really necessary when perhaps an MRI could accomplish the same task without the radiation exposure.
Like MRIs, CT Scans may also use contrast agents to highlight an organ or specific tissue. Follow the same procedure mentioned in the MRI discussion above by letting your doctor and the technician know about any allergic issues you might have with contrast materials.
Like MRIs a number of newly developed CT contrast materials have been developed. However none of these new contrast agents are FDA-approved and may not be covered by your insurance. The most common contrast agent is:

[18F] fluorocholine- based PET/CT.
What are the Significant Differences Between an MRI and a CT Scan?
An MRI allows the doctor to create images from any angle, while a CT Scan allows only a cross-sectional (transverse) view of the body. MRI systems do not employ x-ray radiation and they yield more detailed images.