POSITRON EMISSION TOMOGRAPHY

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Positron emission tomography, also called PET imaging or a PET scan, is a type of nuclear medicine imaging. Positron emission tomography (PET) is a nuclear medicine imaging technique which produces a three-dimensional image or map of functional processes in the body. The system detects pairs of gamma rays emitted indirectly by a positron-emitting radioisotope, which is introduced into the body on a metabolically active molecule. Images of metabolic activity in space are then reconstructed by computer analysis, often in modern scanners aided by results from a CT X-ray scan performed on the patient at the same time, in the same machine.

Nuclear medicine is a sub specialty within the field of radiology that uses very small amounts of radioactive material to diagnose or treat disease and other abnormalities within the body. Nuclear medicine imaging procedures are noninvasive and usually painless medical tests that help physicians diagnose medical conditions. These imaging scans use radioactive materials called a radio-pharmaceutical or radio-tracer.

Depending on the type of nuclear medicine exam you are undergoing, the radio-tracer is injected into a vein, swallowed by mouth or inhaled as a gas and eventually collects in the area of your body being examined, where it gives off energy in the form of gamma rays. This energy is detected by a device called a gamma camera, a (positron emission tomography) PET scanner and/or probe. These devices work together with a computer to measure the amount of radio tracer absorbed by your body and to produce special pictures offering details on both the structure and function of organs and other internal body parts.

In some centers, nuclear medicine images can be superimposed with computed tomography (CT) or magnetic resonance imaging (MRI) to produce special views, a practice known as image fusion or co-registration. These views allow the information from two different studies to be correlated and interpreted on one image, leading to more precise information and accurate diagnoses.

A PET scan measures important body functions, such as blood flow, oxygen use, and sugar (glucose) metabolism, to help doctors evaluate how well organs and tissues are functioning.

PET is actually a combination of nuclear medicine and biochemical analysis. Used mostly in patients with brain or heart conditions and cancer, PET helps to visualize the biochemical changes taking place in the body, such as the metabolism (the process by which cells change food into energy after food is digested and absorbed into the blood) of the heart muscle.

PET differs from other nuclear medicine examinations in that PET detects metabolism within body tissues, whereas other types of nuclear medicine examinations detect the amount of a radioactive substance collected in body tissue in a certain location to examine the tissue’s function.

Since PET is a type of nuclear medicine procedure, this means that a tiny amount of a radioactive substance, called a radio pharmaceutical (radionuclide or radioactive tracer), is used during the procedure to assist in the examination of the tissue under study.

Specifically, PET studies evaluate the metabolism of a particular organ or tissue, so that information about the physiology (functionality) and anatomy (structure) of the organ or tissue is evaluated, as well as its biochemical properties. Thus, PET may detect biochemical changes in an organ or tissue that can identify the onset of a disease process before anatomical changes related to the disease can be seen with other imaging processes such as computed tomography (CT) or magnetic resonance imaging (MRI).

PET is most often used by oncologists (physicians specializing in cancer treatment), neurologists and neurosurgeons (physicians specializing in treatment and surgery of the brain and nervous system), and cardiologists (physicians specializing in the treatment of the heart). However, as advances in PET technologies continue, this procedure is beginning to be used more widely in other areas.

PET may also be used in conjunction with other diagnostic tests such as computed tomography (CT) or magnetic resonance imaging (MRI) to provide more definitive information about malignant (cancerous) tumors and other lesions. Newer technology combines PET and CT into one scanner, known as PET/CT. PET/CT shows particular promise in the diagnosis and treatment of lung cancer, evaluating epilepsy, Alzheimer's disease and coronary artery disease.

Originally, PET procedures were performed in dedicated PET centers, because the equipment to make the radio pharmaceuticals, including a cyclotron and a radio chemistry lab, had to be available, in addition to the PET scanner. Now, the radio pharmaceuticals are produced in many areas and are sent to PET centers, so that only the scanner is required to perform a PET scan.

Further increasing the availability of PET imaging is a technology called gamma camera systems (devices used to scan patients who have been injected with small amounts of radionuclides and currently in use with other nuclear medicine procedures). These systems have been adapted for use in PET scan procedures. The gamma camera system can complete a scan more quickly, and at less cost, than a traditional PET scan.

PET works by using a scanning device (a machine with a large hole at its center) to detect positrons (subatomic particles) emitted by a radionuclide in the organ or tissue being examined.

The radionuclides used in PET scans are chemical substances such as glucose, carbon, or oxygen used naturally by the particular organ or tissue during its metabolic process. A radioactive substance is attached to the chemical required for the specific tests. For example, in PET scans of the brain, a radioactive substance is applied to glucose (blood sugar) to create a radionuclide called fluorodeoxyglucose (FDG), because the brain uses glucose for its metabolism. FDG is widely used in PET scanning.

Other substances may be used for PET scanning, depending on the purpose of the scan. If blood flow and perfusion of an organ or tissue is of interest, the radionuclide may be a type of radioactive oxygen, carbon, nitrogen, or gallium.

The radionuclide is administered either into a vein through an intravenous (IV) line or inhaled as a gas. Next, the PET scanner slowly moves over the part of the body being examined. Positrons are emitted by the breakdown of the radionuclide. Gamma rays are created during the emission of positrons, and the scanner then detects the gamma rays. A computer analyzes the gamma rays and uses the information to create an image map of the organ or tissue being studied. The amount of the radionuclide collected in the tissue affects how brightly the tissue appears on the image, and indicates the level of organ or tissue function.

In general, PET scans may be used to evaluate organs and/or tissues for the presence of disease or other conditions. PET may also be used to evaluate the function of organs such as the heart or brain. The most common use of PET is in the detection of cancer and the evaluation of cancer treatment.

More specific reasons for PET scans include, but are not limited to, the following:

  • To diagnose dementias (conditions that involve deterioration of mental function) such as Alzheimer's disease, as well as other neurological conditions such as
    • Parkinson's disease - a progressive disease of the nervous system in which a fine tremor, muscle weakness, and a peculiar type of gait are seen
    • Huntington’s disease - a hereditary disease of the nervous system which causes increasing dementia, bizarre involuntary movements, and abnormal posture
    • Epilepsy - a brain disorder involving recurrent seizures
    • Cerebrovascular accident (stroke)
  • To locate the specific surgical site prior to surgical procedures of the brain.
  • To evaluate the brain after trauma to detect hematoma (blood clot), bleeding, and/or perfusion (blood and oxygen flow) of the brain tissue.
  • To detect the spread of cancer to other parts of the body from the original cancer site.
  • To evaluate the effectiveness of cancer treatment
  • To evaluate the perfusion (blood flow) to the myocardium (heart muscle) as an aid in determining the usefulness of a therapeutic procedure to improve blood flow to the myocardium.
  • To further identify lung lesions or masses detected on chest x-ray and/or chest CT.
  • To assist in the management and treatment of lung cancer by staging lesions and following the progress of lesions after treatment.
  • To detect recurrence of tumors earlier than with other diagnostic modalities.

Over the past 30 years, PET research at Brookhaven has focused in the integration of basic research in radiotracer chemistry with the tools of neuroscience to develop new scientific tools for applications in human health. Major areas of medical research include: drug and alcohol addiction; the development of a new strategy for addiction treatment; obesity and eating disorders; attention deficit hyperactivity disorder (ADHD); aging and neurodegenerative disorders. There is a special commitment to the development of radiotracers for imaging specific neurotransmitter systems in the brain including the dopamine (see figure), norepinephrine and nicotine systems. There are also new efforts in integrating micro PET and micro dialysis as a new tool for basic research in the neurosciences; in integrating PET and MRI to study brain development and in biomedical engineering targeted to the design and development of a new PET camera to image the awake animal brain.

Dopamine is a natural brain chemical which is involved in movement, in motivation and in the sense of well-being and pleasure. This a simplified cartoon of a dopamine cell showing radiotracers and PET images of specific elements of the dopamine cell including receptors to transmit the signal and transporters and enzymes to terminate the signal. Brain function is imaged with FDG.

The PET programme at TRIUMF and UBC is in a unique position among world medical research centres in having many of the expensive major facilities required to mount a powerful PET programme. These include the cyclotrons and radiochemistry laboratories at the TRIUMF project that are the source of the PET scanning agents. The laboratories are linked to the UBC Health Sciences Centre Hospital by the world’s longest "pea shooter", a 2.4 km pneumatic pipeline used for the delivery of PET scanning agents in the shortest possible time. These facilities have a capacity that has not been equaled by any other university/health sciences center in the world.

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