How MRI Works

MRI Images and How They're Made

doctors examine mri scan
Ron Levine/The Image Bank/Getty Images
Doctors examine the contrasts on an MRI scan.

The MRI scanner can pick out a very small point inside the patient's body and ask it, essentially, "What type of tissue are you?" The system goes through the patient's body point by point, building up a map of tissue types. It then integrates all of this information to create 2-D images or 3-D models with a mathematical formula known as the Fourier transform. The computer receives the signal from the spinning protons as mathematical data; the data is converted into a picture. That’s the "imaging" part of MRI.

The MRI system uses injectable contrast, or dyes, to alter the local magnetic field in the tissue being examined. Normal and abnormal tissue respond differently to this slight alteration, giving us differing signals. These signals are transferred to the images; an MRI system can display more 250 shades of gray to depict the varying tissue [source: Coyne]. The images allow doctors to visualize different types of tissue abnormalities better than they could without the contrast. We know that when we do "A," normal tissue will look like "B" -- if it doesn't, there might be an abnormality.

An X-ray is very effective for showing doctors a broken bone, but if they want a look at a patient's soft tissue, including organs, ligaments and the circulatory system, then they'll likely want an MRI. And, as we mentioned on the last page, another major advantage of MRI is its ability to image in any plane. Computer tomography (CT), for example, is limited to one plane, the axial plane (in the loaf-of-bread analogy, the axial plane would be how a loaf of bread is normally sliced). An MRI system can create axial images as well as sagitall (slicing the bread side-to-side lengthwise) and coronal (think of the layers in a layer cake) images, or any degree in between, without the patient ever moving.

But for these high-quality images, the patient can't move very much at all. MRI scans require patients to hold still for 20 to 90 minutes or more. Even very slight movement of the part being scanned can cause distorted images that will have to be repeated. And there's a high cost to this kind of quality; MRI systems are very expensive to purchase, and therefore the exams are also very expensive.

But are there any other costs? What about the patient's safety?