A CT (also known as a CAT Scan or Computerized Axial Tomography) is usually used for bone injuries, lung imaging, chest imaging, and cancer detection. They’re used a lot for emergency procedures because it’s fast (that’s why they always ask for CTs on Grey’s Anatomy).
An MRI (or Magnetic Resonance Imaging) is used for soft tissue (ligaments, tendons, spinal cord, brain tumors, etc.). It can take up to a half hour, and is usually more expensive, but it also doesn’t use any radiation.
MRI machines use a magnet to send radio waves to read the energy produced by the water molecules as they re-align themselves after each pulse of radio waves. That data actually creates the 2D image of the axis of the body. Since there’s essentially no water in bones, they don’t have an image, which is where the black space comes in.
CT machines use x-ray waves, which rotate around the patient, sending waves towards the “x-ray detector” on the other side of the patient. The beam goes through the patient, and the detector receives the image, measuring the strength of the beam about 1000 times per second. The comparisons of the beams create the image.
You can see a full comparison chart here, at Diffen.com or you can call us with any questions!