Positron Emission Tomography (PET)
What is PET?
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.
PET images demonstrate the chemistry of organs and other tissues such as tumors. A radiopharmaceutical, such as FDG (fluorodeoxyglucose), which includes both sugar (glucose) and a radionuclide (a radioactive element) that gives off signals, is injected into the patient and a PET scanner measures its emissions.
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.
Cancer & PET
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.
- Early Detection: Because PET images biochemical activity, it can accurately characterize a tumor as benign or malignant, thereby avoiding surgical biopsy when the PET scan is negative. Conversely, because a PET scan images the entire body, confirmation of distant metastasis can alter treatment plans in certain cases from surgical intervention to chemotherapy.
- Staging of Cancer: PET is extremely sensitive in determining the full extent of disease, especially in lymphoma, malignant melanoma, breast, lung, colon and cervical cancers. Confirmation of metastatic disease allows the physician and patient to more accurately decide how to proceed with the patient's management.
- Checking for recurrences: PET is currently considered to be the most accurate diagnostic procedure to differentiate tumor recurrences from radiation necrosis or post-surgical changes. Such an approach allows for the development of a more rational treatment plan for the patient.
- Assessing the Effectiveness of Chemotherapy: The level of tumor metabolism is compared on PET scans taken before and after a chemotherapy cycle. A successful response seen on a PET scan frequently precedes alterations in anatomy and would therefore be an earlier indicator of tumor response than that seen with other diagnostic modalities.
PET and CT or MRI
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.
- PET also is useful in differentiating Alzheimer's disease from other forms of dementia disorders, such as vascular dementia, Parkinson's disease, Huntington's disease, etc.
- Epilepsy: PET is one of the most accurate methods available to localize areas of the brain causing epileptic seizures and to determine if surgery is a treatment option.
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.