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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 the body. The patient is imaged with a SPECT camera and the thyroid functionality is evident. Doctors can then identify what part of the thyroid gland is working properly, and areas that aren't. If you have ever known anyone who has battled thyroid cancer, they were likely diagnosed and treated successfully as a result of advancements made because of medical isotopes.
Generally speaking, medical isotopes come either from nuclear reactors or special particle accelerators known as cyclotrons. Tc-99m comes from the parent atom Molybdenum-99 or simply Mo-99. Mo-99 is produced in nuclear reactors (such as Canada's NRU reactor at Chalk River) by irradiating highly enriched "weapons-grade" uranium (U-235). The Mo-99 has a fairly long half-life (it takes on average 66 hours for half of a sample of Mo-99 to decay to Tc-99m). The Tc-99m is used at the hospitals all over North America to create the radiopharmaceuticals used in patients.
MDS Nordion Vancouver Operations (based at TRIUMF) currently operate three cyclotrons 24/7 365 days a year to produce non-Moly-99 medical isotopes primarily for export. Total production exceeds 2.5 million patient doses per year.
Together with MDS Nordion, TRIUMF is exploring a proof-of-principle demonstration for using the technology of "photo-fission" to produce Mo-99 without a reactor by using a new type of accelerator. The demonstration requires a new accelerator machine at TRIUMF and its associated capital infrastructure.
Canada first became a leader in the use of medical isotopes for healthcare diagnosis and treatment in 1951 (Canadians were involved in the world's first nuclear reactor built by Enrico Fermi in Chicago). Presently, medical imaging technology using medical isotopes plays an important role in the diagnosis and treatment of everything from neurological diseases to cancer. It drives a multi-billion dollar business per year worldwide with growth is predicted to be 1-4% per year for at least a decade.
Tc-99m is used in 85% of all nuclear medicine procedures, 20 million per year, around the world. Canada creates Moly-99 at the NRU reactor at Chalk River; the Mo-99 is extracted by AECL and then shipped to MDS Nordion where it is purified. The Mo-99 is then transported to two manufacturers in the U.S. who create the Tc-99m-generating device which is sold to hospitals.
Two categories of development are driving the future business of medical isotopes. Canada has a strong position in both.
SPECT technology stands for Single Photon Emission Computed Tomography and uses a particular set of medical isotopes (e.g., Tc-99m) and a particular type of camera. PET technology stands for Positron Emission Tomography and uses different medical isotopes (e.g., F-18) and a different camera. There are several medical isotopes, and the number is growing, that are imaged with a PET camera. At present, SPECT is lower resolution but cheaper than PET. As a result, nearly every hospital in North America has a SPECT system.
It turns out that PET isotopes are easier to work with than Tc-99m, and so more molecules are available for PET than SPECT with Tc-99m. As the number of these special molecules increases, hospitals are increasingly buying PET cameras rather than SPECT. Last year in the U.S., purchases of PET cameras were greater than SPECT for the first time. In Canada, the number of installed SPECT cameras is about 2,000; there are only 22 PET cameras. As the field advances, this gap will close and over the course of a decade, PET will dominate everywhere.
The half life of PET isotopes is usually quite short. The most widely used PET radiotracer is FDG (F-18-fluro-deoxyglucose), an isotope-labelled sugar, however there are several other "radiotracers" that are emerging for use in cancer diagnosis, staging, and therapy. For example, FES (fluoroestrodiol) is a PET radiotracer and determines whether a breast cancer tumour has estrogen receptors. If this is the case, then the doctor orders a particular therapy: hormone therapy, benefitting the patient and saving the cost of an incorrect and ineffective therapy.