CT Scan, MRI and PET Scans…What are the differences?

CT Scan, MRI and PET Scans

January 19, 2000

CT Scan, MRI and PET Scans…What are the differences?
CT, MRI and PET scans are all diagnostic tools to non-invasively (non-surgically) look inside the body. They are all based on the fact that certain things happen to atoms in our bodies when they absorb energy. Resonance refers to the level of absorption achieved by adjusting the frequency of the radiation and the strength of the magnetic field – like tuning a radio to a particular station.

CT (computerized tomography) uses a sophisticated X ray machine combined with a computer to create a detailed picture of the body’s tissues and structure. Usually a special dye called a contrast material will be injected prior to the scan. This makes it easier to see abnormal tissue due to specific absorption rates.

Nuclear magnetic resonance is produced by measuring the magnetism of spinning electrons and protons and their interactions with nearby atoms (usually protons) when they absorb energy. This provides information about the chemical structure of organic molecules. The use of the word “nuclear” has recently been avoided and Magnetic Resonance Imaging (MRI) is now preferred. MRI uses a magnetic field from super-cooled magnets and can often distinguish more accurately between healthy and diseased tissue. A contrast agent is usually used. MRI can provide pictures from various angles and construct a three dimensional image. Some patients who have received certain types of surgical clips, metallic fragments, cardiac monitors or pacemakers cannot receive this type of scan.

Positron Emission Tomography (PET) scans measure emissions from positron-emitting molecules. Because many useful, common elements have positron emitting forms (carbon, nitrogen, and oxygen), valuable functional information can be obtained. This is the main difference between the CT and MRI scans. The PET shows molecular function and activity not structure, and therefore can often differentiate between normal and abnormal (cancerous / tumor) or live versus dead tissue. Like SPECT (single photon emission tomography), PET also can product three dimensional images, and is usually used to compliment rather than replace the information obtained from CT or MRI scans.

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