Before you take that walk or ride a bike, think again—especially if you will be undergoing a positron emission tomography (PET) scan any time soon.

"Any type of physical activity -- from tapping your feet while in the waiting room to jogging the neighborhood the day before—can affect the results of a PET scan and lead to false-positive results," said Medhat M. Osman, MD, ScM, PhD, assistant professor in the department of internal medicine's division of nuclear medicine and director of PET at St. Louis University Hospital, St. Louis, Mo.

"Knowing whether a patient has participated in physical activity is relevant in interpreting a PET imaging study," added the co-author of "Prevalence and Patterns of Physiologic Muscle Uptake Detected With Whole-Body 18F-FDG PET," which appears in the March issue of the Journal of Nuclear Medicine Technology.

"Individuals undergoing PET scans should take it easy -- and if you do any kind of physical activity outside of the ordinary -- you should let your nuclear medicine technologist know so it can be noted," said Thomas C. Schlarman, MBA, CNMT, educational coordinator with the department of nuclear medicine technology at St. Louis University.

Physical activity may trigger a potential source of false-positive results on a PET scan in about one out of eight patients, he noted. In order to obtain the most accurate reading of PET scans, patients should not exercise excessively 48 hours prior to having a PET scan and should remain quiet prior to scanning, Schlarman said.

PET is a powerful medical imaging procedure that noninvasively demonstrates the function of organs and other tissues. It is used primarily as a diagnostic tool in oncology, cardiology, neurology and many other medical specialties.

To image cancer, a radiopharmaceutical such as fluorodeoxyglucose (FDG), which includes both sugar (metabolized at a higher rate by cancer cells) and a radionuclide, is injected into the patient. Because cancer cells metabolize sugar at higher rates than normal cells, the radiopharmaceutical is drawn in higher concentrations (called "uptake") to cancerous areas. The PET scan shows where the radiopharmaceutical is by tracking the gamma ray signals given off by the radionuclides.

PET scans are very sensitive, and increased physical activity can create problems in differentiating between normal and pathologic uptake, explained Osman. Normal muscles accumulate little FDG, but muscles exercised just before or around the time of the scanning can exhibit intense uptake into those muscles—for example, possibly mimicking cancer, he added.

The study advises technologists to instruct the patients to minimize muscle activity during the uptake phase and to telephone patients ahead of their appointments to advise them to refrain from any excessive muscle activity at least 48 hours before a PET scan. The study's findings emphasize that the accuracy of studies for patients can be improved through educating technologists of the relationship between muscle activity and uptake and in coordinating a team approach with physicians, said Schlarman. "Technologists play a major role in informing physicians of patients' activity" that could skew the reading of their PET scans, Osman said. "Any unexpected or unexplained excessive muscle uptake should initiate communications between technologists and reading physicians to avoid false-positive PET scan interpretations," he added.

The team of technologist and physician researchers in St. Louis—all SNM or SNM Technologist Section members—studied the whole-body PET scans of more than 1,100 cancer patients over a one-year period. Of those patients, nearly 13 percent had excessively increased muscle uptake on the PET scan that matched technologists' notes of muscle activity during the uptake phase or prior to scanning. Increased uptake was seen in individuals' head and neck, thorax and upper extremities, indicating the influence of activities such as raising one's head while on a stretcher, writing or turning the pages of a book, shoveling snow or pushing a wheelchair.

In addition to Osman and Schlarman, authors of "Prevalence and Patterns of Physiologic Muscle Uptake Detected with Whole-Body 18F-FDG PET" include Ryan S. Jackson, B.S., division of nuclear medicine, department of internal medicine, School of Medicine, St. Louis University, St. Louis, Mo., and William L. Hubble, MHA, department of nuclear medicine technology, College of Health Sciences, St. Louis University, St. Louis, Mo.