MRI or Magnetic Resonance Imaging uses magnetic field to present the images. However, the setback is that you will be given headphones with music
because when the scan begins, there will be some constant banging noises.

CT Scan or Computed Tomography Scan uses radiation to present the images but there will be not banging noises and you don't need to have the headphones. These days, with technology CT Scan have advanced tremendously where a whole body scan can be completed in one breath hold. However, this is only possible with thye 64 slice and above CT scan.

By the way, both scans require the patient to go through a gantry (doughnut shape) to allow the scan to take place. Depending on your medical condition, the doctor will advise the best scan for the patient. The doctor usually will perform a prognosis and then decides on the type of scan that the patient require and request for it.