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Molecular Imaging

Read the following information to gain a general understanding of molecular imaging and related technology. Areas of interest include: PET Scan, PET-CT Scan, Cyclotron, FDG, SPECT, MRI Scan and CT Scan.

PET Scan

PET is a powerful diagnostic test that is having a major impact on the diagnosis and treatment of disease. Because disease is a biological process, and PET is a biological imaging examination, PET can detect and stage most cancers, often before they are evident through other tests. PET can also give physicians important early information about heart disease and many neurological disorders, like Alzheimer's.

A PET scan examines the body's chemistry. Most common medical tests, like CT and MR scans, only show details about the structure of the body. PET is different as it provides information about function. With a single PET procedure, physicians can collect images of function throughout the entire body, uncovering abnormalities that might otherwise go undetected.


A PET-CT is the simultaneous fusion of both PET and CT scans. PET provides the functional (or metabolic) image of cancer, and CT provides the anatomical information such as size and location of a tumour.


CT scanners detect and measure X-rays that are transmitted through the body. Using these measurements, a sophisticated computer algorithm then processes the data to produce images of the internal structures of the body.


Every PET scan begins with the injection of a glucose-based radiopharmaceutical called FDG, which travels through the bloodstream and collects more rapidly in cancer cells. As FDG begins to decay, it releases positrons. These positrons then crash into nearby electrons to emit gamma rays, which are then converted into 3-D digital images. The PET scan images highlight the metabolic “hot spots” where cancer grows. A PET-CT has the advantage of being a much faster examination that a regular PET. The entire PET-CT scan is usually completed in less than 30 minutes.


A medical cyclotron is necessary to manufacture the most common radiopharmaceutical used for PET scanning called Fluorodeoxyglucose (FDG). The cyclotron is a circular particle accelerator that makes use of magnetic force to propel atomic particles (negative hydrogen ions) in a circular pattern. These accelerated particles are stripped of their electrons resulting in positively charged particles (protons) that are then targeted onto a specially enriched form of water. This bombardment results in the formation of radioactive material, which is then transported to the radiopharmacy or “hot lab”, where it is purified and finally incorporated into an FDG molecule.

It is important to note that the cyclotron does not single-handedly manufacture the complete radiopharmaceutical. More specifically, the cyclotron produces the organic building blocks or core elements of the radiopharmaceutical such as Fluorine 18, Carbon 11, Oxygen and Nitrogen. After the cyclotron process ends, these organic building blocks are moved through tubes into the “hot lab” where these elements are synthesized into more common compounds such as fluorine-18-based FDG for use in PET and PET/CT.

Cyclotrons are complex pieces of equipment that produce ionizing radiation when they operate. As such, they are operated in specially constructed, shielded facilities, and require a license from the Canadian Nuclear Safety Commission (CNSC). The radiopharmacy is also designed for the handling of the radioisotopes and the production of the radiopharmaceuticals. Canada is an abnormality in that there are only two commercial cyclotrons currently in operation, one in Montreal and another in Sherbrooke, Quebec. In comparison, there are 131 cyclotron facilities in the U.S that supply the necessary radiopharmaceuticals for over 1,400,000 PET scans annually.

If you consider that Canada is approximately 1/10th the population of the U.S., it should have 13 commercial cyclotrons and have performed 100,000 PET scans in the previous year. Canada is behind the USA and the rest of the developed world in completing PET scans for patients with critical disease.

FDG - Fluorodeoxyglucose

FDG is the most common Positron-Emitting Radiopharmaceutical (PER) used in 95% of PET scan procedures. Simply stated, FDG is radioactive glucose and is useful in determining abnormal metabolic processes within the body. FDG is of tremendous value in the early detection of critical disease using PET technology. In the future, radio tracer drugs based on core elements other then fluorine-18 will be developed in order to more efficiently detect disease states beyond the capability of current technology.


SPECT is a medical diagnostic imaging technique similar to PET. However, SPECT uses different radioactive substances (Xenon-133, Technetium-99, Iodine-123) that have longer decay times than those used in PET, and emit single instead of double gamma rays.

SPECT can provide information about blood flow and the distribution of radioactive substances in the body. Its images have less sensitivity and are less detailed than PET images, but the SPECT technique is less expensive than PET. Also, SPECT centers are more accessible than PET centers because they do not have to be located near a particle accelerator.

MRI Scan - Magnetic Resonance Imaging

An MRI scan is a versatile, powerful, and sensitive tool that can generate thin-section images of any part of the body—including the heart, arteries, and veins—from any angle and direction, without surgical invasion and in a relatively short period of time. An MRI scan also creates “maps” of biochemical compounds within any cross section of the human body.

In current medical practice, an MRI scan is preferred for diagnosing most diseases of the brain and central nervous system. MRI scanners provide equivalent anatomical resolution and superior contrast resolution to that of X-ray and/or CT scan. They produce functional information similar to that of PET scan, but with superior anatomical detail. MRI scanners also provide imaging complementary to X-ray images because an MRI scan can distinguish soft tissue in both normal and diseased states. Although an MRI scan is relatively expensive, it may actually reduce costs to patients and hospitals by providing diagnostic evaluation to outpatients and thereby frequently limiting more expensive hospitalization.

CT Scan - Computed Tomography

CT (Computer Tomography) scanning is a valuable diagnostic medical exam that combines x-rays and computerized digital imaging. A CT exam gives a radiologist a non-invasive way to see inside the human body. It has the ability to rapidly acquire two dimensional pictures of a person’s anatomy. Using a computer with specialized software, 3-D images are reconstructed for in-depth clinical evaluations. State-of-the-art technology allows doctors to acquire faster and more precise images, which is crucial in detecting disease in its earliest stages.

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