These types of imaging studies use different technologies. X-ray studies use what is called “ionizing radiation” to reveal certain body parts for viewing either on film or computer screen. Don’t be afraid of the word “radiation” here as the amounts used are generally very small.
X-rays are the best way to look at bones in your body. The way an x-ray study is done for any part of your body uses pretty much the same equipment but the differences are where it is focused and how you are positioned. For example, in an x-ray of the knee, depending on which direction the x-ray is pointed, the physician can look for fluid in the joint, look for arthritis, and of course look for fractures. In a typical 3 view study of the knee, any one of these things may show up on only one of the views.
CT also uses “ionizing radiation” to look at internal body parts. The main difference from x-ray is that it is more sensitive and gives a better view of the non-bony parts of the body. It also lets the radiologist look at the images in three-dimensions instead of two.
MRI uses magnetic fields to look at body parts. The magnetic fields look at the water molecules in your cells and the water molecule will respond differently depending on what cell it is in and what other molecules are around it. The computer looks for the slight differences in the way the water molecules respond to paint a picture of your internal body parts. Interestingly, MRI is not nearly as good at looking at bone in general as CT or X-ray.