Positron Emission Tomography (PET) is rapidly becoming a major diagnostic imaging modality used predominantly in determining the presence and severity of cancers, neurological conditions, and cardiovascular disease. It is currently the most effective way to check for cancer recurrences. Studies demonstrate that PET offers significant advantages over other forms of imaging such as CT or MRI scans in diagnosing disease. Last year more than 200,000 PET scans were performed at more than 700 sites around the country.
A PET scanner consists of an array of detectors that surround the patient. Using the gamma ray signals given off by the injected radionuclide, PET measures the amount of metabolic activity at a site in the body and a computer reassembles the signals into images. Cancer cells have higher metabolic rates than normal cells, and show up as denser areas on a PET scan. PET is useful in diagnosing certain cardiovascular and neurological diseases because it highlights areas with increased, diminished or no metabolic activity, thereby pinpointing problems.
The imaging in PET is all indirect. Like CT, MRI, and SPECT, PET relies on computerized reconstruction procedures to produce tomographic images. It is performed by means of detecting positron-emission by use of tomography. Two ways in which radionuclides decay that will reduce excess positive charge on the nucleus include the neutralization of a positive charge with the negative charge of an electron or the emission of a positron from the nucleus. The positron will then combine with an electron from the surroundings and annihilate. Upon annihilation both the positron and the electron are then converted to electromagnetic radiation. This electromagnetic radiation is in the form of two high-energy photons which are emitted 180 degrees away from each other. It is this annihilation radiation that can be detected externally and is used to measure both the quantity and the location of the positron emitter.
Simultaneous detection of two of these photons by detectors on opposite sides of an object places the site of the annihilation on or about a line connecting the centers of the two detectors. At this point mapping the distribution of annihilations by computer is allowed. If the annihilation originates outside the volume between the two detectors, only one of the photons can be detected, and since the detection of a single photon does not satisfy the coincidence condition, the event is rejected. Simultaneous detection provides a precise field of view with uniform sensitivity. This occurs because wherever the disintegration takes place between the two detectors, the photons must in sum have traveled the full interdetector distance in order that the event be recorded.
PET is considered particularly effective in identifying whether cancer is present or not, if it has spread, if it is responding to treatment, and if a person is cancer free after treatment. Cancers for which PET is considered particularly effective include lung, head and neck, colorectal, esophageal, lymphoma, melanoma, breast, thyroid, cervical, pancreatic, and brain as well as other less-frequently-occurring cancers.
Because PET measures metabolism, as opposed to MRI or CT, which "see" structure, it can be superior to these modalities, particularly in separating tumor from benign lesions, and in differentiating malignant from non-malignant masses such as scar tissue formed from treatments like radiation therapy. PET is often used in conjunction with an MRI or CT scan through "fusion" to give a full three-dimensional view of an organ and the location of cancer within that organ. Newer PET scanners are being made that are a combination of PET/CT devices.
PET's ability to measure metabolism also has significant implications in diagnosing Alzheimer's disease, Parkinson's disease, epilepsy and other neurological conditions, because it can vividly illustrate areas where brain activity differs from the norm.
Alzheimer's Diagnosis: Until recently, autopsy has been considered the only definitive test for Alzheimer's disease (AD). Recent studies indicate that PET can supply important diagnostic information and confirm an Alzheimer's diagnosis (Journal of Nuclear Medicine, November 2000). When comparing a normal brain versus an AD-affected brain on a PET scan, a distinctive image appears in the area of the AD-affected brain. This pattern is seen very early in the AD course. Conventionally, the confirmation of AD is a long process of elimination that averages between two and three years of diagnostic and cognitive testing. Early diagnosis can provide the patient access to therapies, which are more effective earlier in the disease.
By measuring both blood flow (perfusion) and metabolic rate within the heart, physicians using PET scans can pinpoint areas of decreased blood flow such as that caused by blockages, and differentiate muscle damage from living muscle, which has inadequate blood flow (myocardial viability). This information is particularly important in patients who have had previous myocardial infarction and who are being considered for a revascularization procedure.
PET scan charges range from $1200-$3500, depending on the type of scan. Insurance companies will cover the cost of many PET scans. Medicare reimburses for PET scans for the following cancers: colorectal, lung, lymphoma, and melanoma, head and neck and esophageal cancers, and also for refractory seizures (epilepsy). Medicare will begin PET reimbursement to initially stage, to determine recurrence and to measure effectiveness of treatment of breast cancer as well as for myocardial viability. These new reimbursement categories become effective October 1, 2002. Medicare is constantly updating reimbursements, so visit the SNM website (www.snm.org) to find the latest information.
In the 1970's PET scanning was formally introduced to the medical community. At that time it was seen as an exciting new research modality that opened doors through which medical researchers could watch, study, and understand the biology of human disease.
In 1976, the radiopharmaceutical fluorine-18-2-fluoro-2-deoxyglucose (FDG), a marker of sugar metabolism with a half-life of 110 minutes, enabled tracer doses to be administered safely to the patient with low radiation exposure. The development of radiopharmaceuticals like FDG made it easier to study living beings, and set the groundwork for more in-depth research into using PET to diagnose and evaluate the effect of treatment on human disease. To perform PET studies in the late 1970's, a large staff was needed: physicists to run the cyclotron that produces the F-18 and to oversee the scanner, chemists to make the tracers such as FDG, and dedicated, specialist physicians.
During the 1980's the technology that underlies PET advanced greatly. Commercial PET scanners were developed with more precise resolution and images. As a result, many of the steps required for producing a PET scan became automated, and able to be performed by a trained technician and experienced physician, thereby reducing the cost and complexity of the procedure. Smaller, self-shielded cyclotrons were developed, making it possible to install cyclotrons at more locations.
Until recently a PET center required a cyclotron and a radiochemistry laboratory on site to produce the FDG. As a result there was a scarcity of centers. However, there are now multiple sites that make FDG and distribute it to the centers that only need to have a PET scanner to perform the imaging study.