Laurie Wang

A recent study by the Yale School of Medicine could help lower radiation exposure during PET/CT scans, a medical imaging technique that produces functional images of the body.

Published last month, the study sought to determine a threshold for the minimum radiation exposure necessary for PET/CT imaging. Since evidence from the past decade has indicated that radiation exposure during medical imaging may carry a risk of cancer, scientists have worked to minimize radiation exposure during medical imaging involving x-rays and radiation, lead study author and medical school professor Ming-Kai Chen said. The researchers found that dosage for the radioactive isotope injected during PET/CT scans, fluorine-18, could be decreased to threshold levels lower than the current recommendations of the European Association of Nuclear Medicine without compromising the quantitative accuracy of the image.

“We are trying to provide the best imaging quality while trying to reduce the radiation exposure to patients, so certainly you have to trade off between the clinical information you need to get and also the radiation exposure a patient could receive,” Chen said.

The researchers injected small spheres called “phantoms,” which acted as substitutes for humans, with varying levels of fluorine-18, which is used to detect functional features and abnormalities during a PET/CT scan. They then compared the varying concentrations of fluorine-18 with the scan image’s quality in order to quantify the degradation of scan images using lower radiation doses.

Lower doses of radioactivity correspond to higher amounts of image “noise” which can compromise the clarity of the imaged area. The researchers pursued “as low as reasonably achievable” doses while keeping image noise to a minimum.

The risk of negative long-term side effects from radiation exposure must be weighed against the more immediate need for accurate imaging from PET/CT scans, as patients who undergo these exams often have an urgent clinical need for medical evaluation, Chen said.

Research in the field of PET imaging seeks to not only lower radiation exposure during imaging but also quantify the amount of radiation detected in a scan, he added. This interest stems from the increase in repeated use of PET/CT scans to track progression of long-term disease in a single patient over time, Chen explained.

“There’s a recent interest in quantification [of radioactivity] using PET because PET has been traditionally used more for detection, while it’s also being increasingly used in evaluation for therapy response now,” medical school professor Chi Liu said.

Chen also noted that human trials are required to validate whether current doses of radioactivity should be lowered in PET/CT scans, as it is still uncertain how the results from phantom imaging can be applied to patients. Even with a determined imaging threshold for PET/CT scans, different individuals may require adjusted doses based on factors such as size and weight, he added.

Future steps for the quantitative PET imaging include determining whether results can be applied to effectively lower radiation exposure during scans, as well as finding ways to standardize scanning settings and harmonize image quality on different PET/CT imaging devices.

“With the current nature of scanners, if each vendor has his own hardware and reconstruction software, even though all PET scans are done the same way, they may not give you the same number [for quantification],” Liu said.

The first prototype PET/CT scanner became operational in 1998.