About Positron Emission Tomography, Article

What is PET?
Positron Emission Tomography (P.E.T.) is a powerful imaging technique that holds great promise in the diagnosis and treatment of many diseases, particularly cancer. A non-invasive test, P.E.T. scans accurately image the cellular function of the human body. In a single P.E.T. scan a physician can examine the entire body. P.E.T. scanning provides a more complete picture, making it easier for physicians to diagnose problems, determine the extent of disease, prescribe treatment, and track progress.

Clinical Applications

Clinical Uses of PET and PET/CT in Oncology:

How does PET work?
A small amount radioactive substance is administered to the subject by injection. In clinical oncology we use radioactive glucose. This compound accumulates in the region of the body under study. Most cancer cells are highly metabolic and overexpress the glucose transport mechanism, thereby ensuring that they receive more glucose than their neighboring cells. As the radioactive atoms decay, they release positrons. The positively charged electrons, or “positrons”, collide with electrons which have a negative charge. Gamma rays are formed by the collision and annihilation of positrons and electrons. An image is constructed from the detection of these annihilation events or gamma ray “emissions”. Gamma rays consist of photons which are light particles. These photons move in opposite directions and the detector ring of the PET scanner, through which a body is conveyed, detect this activity. Finally, a computer is used to create three-dimensional, cross-sectional images or “tomographs” of this information.

What Is PET Used For?
P.E.T. is used for studying the brain, heart, other organs, their functions and biochemical processes which include glucose metabolism, oxygen uptake, and blood flow. In clinical medicine, this data is used to diagnose disease.

P.E.T. is used to study brain activity and diagnose neurological diseases. PET has proven to be one of the most accurate methods of diagnosing, staging, and restaging various forms of cancer. Since PET is looking for metabolic, rather than anatomic abnormalities, we can often discover disease processes and changes long before they would show up on conventional imaging, allowing the physician to ensure that the right treatment is being provided to the patient. P.E.T. scans can also provide very accurate and detailed information in diagnosing heart disease, such a cardiac viability, which can tell the cardiac surgeon whether a revascularization procedure would be effective.

What are the Benefits of PET?
Positron Emission Tomography (P.E.T.) has many advantages over other conventional imaging techniques. It is painless and non-invasive, and is a whole body exam. P.E.T. images biological disorders at the molecular level, often before anatomical changes are even visible.

In diseases like cancer for example, we frequently see changes in glucose metabolism before a physical change or symptoms occur. This enables physicians to detect disease or recurrence at much earlier stages. Other applications include: reduction of biopsy sampling errors, improvement of therapy planning, and assessment of response to chemotherapy and radiation therapy.

Early detection prevents unnecessary tests and treatments, and and helps physicians to deliver the right treatment and provide better surgical outcomes for their patients. In a disease like cancer, the earlier it is detected, the greater is the chance for survival and complete remission, or cure. P.E.T. is a non-invasive test, and allows the surgeon to sort his/her patients to allow for the greatest chance of curative surgery, and avoid the need for unnecessary surgical probes. It is helpful to think of P.E.T. as a “molecular probe”.

The information that P.E.T. provides about cellular metabolism is not available from other conventional imaging techniques such as ultrasound, x-rays, CT scans, or MRI. While these imaging modalities are often only able to identify an indeterminate mass, P.E.T. can non-invasively distinguish between benign and malignant tumors, live or necrotic tissue. When used in combination with these conventional imaging tools, P.E.T. becomes an even more robust diagnostic tool. The information that PET/CT provides, for example, is frequently even more sensitive and specific than a P.E.T. scan alone.

What is PET/CT?
When we combine the molecular imaging of PET with the anatomical imaging of CT, we call it PET/CT. Alone, each imaging technique has particular benefits and limitations, but when “fused” together the combined image provides complete information on cancer location and metabolism. Essentially, small lesions or tumors are detected with PET and then precisely located with CT.

These combined technologies enable physicians to determine the exact size, shape and location of diseased tissue or tumors all at once. Highly metabolic tumors, which may not yet show anatomical changes in size or shape, can be detected with PET; the exact location of the tumor can be found with CT. Combining metabolic and anatomic images, as in PET/CT, results in even greater sensitivity and specificity than either imaging technique alone.

How PET/CT Works
A computer tomography scan provides detail at the anatomical level, such as the size and location of tumors, whereas a PET scan provides detail at the metabolic level, i.e., the cellular activity or glucose metabolism of tumors. Combined, the results of both of these technologies provide more information than either one alone.

Anatomical: CT scanners send x-rays through the body, which are then measured by detectors in the CT scanner. A computer algorithm then processes those measurements to produce pictures of the body's internal structures.

Metabolic: PET images begin with an injection of a solution of glucose (sugar) that has been "tagged" with a radioactive chemical isotope (generally fluorine 18, or FDG). Metabolically active organs or tumors consume sugar at high rates, and as the tagged sugar starts to decay, it emits positrons. These positrons then collide with electrons, giving off gamma rays, and a computer converts the gamma rays into images. These images indicate metabolic "hot spots," often indicating rapidly growing tumors (because cancerous cells generally consume more sugar/energy than other organs or tumors).

The advantage of PET/CT is that this scanning device combines both PET and CT scanners into one device, which utilizes both technologies in a single scan. This allows for the detection and precise location of a wide variety of cancers, and other diseases.