Researchers study radiation blockers while conducting nuclear imaging of Iodine uptake in mouse tissues

Apr 20 2005

Scientists have found that a dose five times higher than the FDA-recommended dosage of potassium iodide in the event of a nuclear accident is needed to protect small animals effectively from radioactive iodide in medical imaging procedures. The long-term animal nuclear imaging project is being conducted by a collaboration of biology and physics researchers from the Department of Energy's Jefferson Lab and The College of William & Mary (CWM).

The research was conducted at the CWM with a Jefferson Lab and CWM-built medical imaging system to perform investigational studies of mice. Bob Welsh, a JLab/CWM jointly appointed professor, is one researcher working on the project. The research demonstrates that scientists can learn about how the body uses certain substances of interest - such as insulin, the fat-regulating protein leptin and a wide range of other biological compounds - by tracking how these substances move through the body of a mouse.

"The way we follow those substances is to attach to them a radioactive isotope of iodine, Iodine-125. Iodine-125 emits a low-energy gamma ray," Welsh says, "It's not a tremendous amount of energy, but it's easy to track with these very precise detectors that have been designed and built by the Jefferson Lab Detector Group."

The thyroid needs iodine to regulate metabolism and is unable to distinguish between regular dietary iodine and ingested radioactive iodine. So the researchers weren't surprised when, in the course of the project, they noticed that the mice subjects' thyroids always absorbed a significant amount of radioactive iodine. In addition to being potentially bad for the mouse, the thyroid's absorption of radioactive iodine made the images difficult to interpret and could provide false-positive readings or possibly obscure substantial iodine uptake in nearby tissues.

The team decided to test what would happen if they gave the mice potassium iodide, the FDA-recommended drug for blocking radioactive iodine absorption by the thyroid in the event of a nuclear accident, before exposing the mice to a form of radioiodine used in imaging studies. CWM undergraduate William Hammond, who will be presenting the team's findings at the American Physical Society (APS) April Meeting, Session E12.0004, participated in this phase of the research for his senior thesis project.

The researchers started with the potassium iodide dose that's recommended for humans in the event of a nuclear incident, 130 mg (milligrams), and scaled that down to the mass of the mouse. They administered a liquid form of the drug to mice, injected the radioiodine for imaging an hour later, and then imaged the mouse.

"What we noticed was this: the dose that was exactly the scaled human dose did not completely block the uptake of radioiodine. But when we tried three times, five times, ten times the scaled human dose, we obtained results that indicate that ten times the human dose blocks 1.5-2 times better, though five times is just about as good as ten times," Welsh says.

The researchers recognized that the extra benefit gained by the largest potassium iodide dose administered could in some cases be outweighed by potential side effects. To protect their mice in future imaging studies, they're planning to use the potassium iodide dose that's five times the scaled-down human dose.

As for larger implications, the study should not simply be scaled-up and applied to humans. "It could say that a mouse's metabolism is so different from a human's that you can't just scale the human dose down for mice. But when it comes to small animals, I think the results should be taken into consideration," Welsh notes.

This research was made possible by a collaboration of Jefferson Lab and College of William researchers, including CWM physicists Robert Welsh, Julie Cella, Coleen McLoughlin, Kevin Smith and William Hammond; CWM biologists Eric Bradley and Margaret Saha; CWM applied science graduate student Jianguo Qian; and Jefferson Lab Detector Group scientists Stan Majewski, Vladimir Popov, Mark Smith and Drew Weisenberger.