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Modern healthcare routinely requires examining a patient with more than the unaided eye. Molecular imaging-the imaging of molecules, biochemical processes, and physiological activity within the human body-is rapidly becoming one of the most powerful tools for diagnosis and staging of disease. The main tools for molecular imaging are the SPECT and PET scans that tag (or "label") specific biologically active molecules (biomolecules) with medical isotopes. A medical isotope is an unstable (i.e., radioactive) atom derived from a stable one. When the unstable atom decays, it emits a particle that can be detected and used to pinpoint its location. By chemically connecting the medical isotope to a biomolecule and injecting the compound into the human body, one can then "see" where the body is using the biomolecule.
Watch a short movie to learn more about nuclear medicine and TRIUMF (click on "Life Sciences").
A CAT scan cannot, for instance, tell if a patient is dead or alive because it only shows anatomy and structure. A PET or SPECT scan indicates what biochemistry is happening inside the body. An MRI has some capability to see activity but is primarily used for anatomical study.
PET and SPECT scans differ by the type of decay of the isotope and therefore use different "cameras" to image or "scan" the patient. SPECT is the better established modality and is prevalent in every hospital and is presently cheaper than PET. PET is the emerging technology and offers higher resolution scans and access to more sophisticated biology in the body.
If an incoming patient is thought to have had a heart attack, a doctor will often inject the patient with a medical isotope called Tc-99m attached to biomolecule called teboroxime (the combination is called a "radiotracer"). The patient will then typically perform a rest-and-stress treadmill test. The Tc-99m goes to the heart because the teboroxime molecule is designed to accumulate there. When the heart is imaged with a SPECT camera, the picture will tell the doctor if the heart muscle has been damaged.
Iodine isotopes (such as I-123 for imaging and I-131 for therapy), accumulate in the thyroid when injected into