Turn Up the Radiochemistry: The Molecular Imaging Core

It’s a car-sized, state-of-the- art machine with a name that sounds out of this world. Although the cyclotron at St. Jude Children’s Research Hospital doesn’t blast off to distant planets, it spins possibilities to rival any science fiction tale.

Resting under 7 feet of concrete in an underground bunker, the cyclotron serves as the foundation for work done in the hospital’s Molecular Imaging Core.

The cyclotron creates radioactive chemicals that are used to make drugs, known as radiotracers, for a procedure known as positron emission tomography, or PET imaging.

Amy Vavere, PhD, directs the Molecular Imaging Core, which launched as a resource within the Department of Diagnostic Imaging earlier this year.

The Molecular Imaging Core enables St. Jude researchers to use nuclear medicine and nuclear chemistry in their studies. Although the most common PET radiotracer, FDG, is available from a local vendor, other radiotracers are made onsite at the Molecular Imaging Core, an advantage for researchers and clinicians.

“We support research across the institution, and our work crosses over into the clinic,” says Vavere, who joined St. Jude in 2007, shortly after the cyclotron arrived on the hospital’s campus. “We can measure all kinds of biological processes using radiotracers.”

PET project

Radiotracers can diagnose illness, measure blood flow, monitor tumor growth and track response to therapy. For instance, a patient with a brain tumor may undergo frequent PET scans to track treatment response. As part of the procedure, the child lies flat on the scanner while an imaging technologist injects the radiotracer.

Barry Shulkin, MD, chief of Nuclear Medicine, monitors the radiotracer as it travels through a child’s body.

“A PET scan shows what is going on inside the body in real time,” he says. “Not only does it capture a snapshot of what is happening, but it also provides details into the functions of what you’re seeing. It is functional imaging that you don’t get from an MRI or CT scan.”

PET imaging is an important asset at St. Jude. When a combination of MRI or CT scans yields conflicting readings, PET imaging often provides the information clinicians need to make treatment decisions. Physicians can use PET to decide what levels of radiation or chemotherapy are needed as well as examine radiation’s effect on a tumor.

Dancing to the radio

The challenge with PET scans is radiotracers decay quickly — some last up to a few hours while others dwindle much more rapidly. This decay rate, known as a half-life, is crucial to planning. It’s why Molecular Imaging Core staff coordinate their efforts like a well-choreographed dance when a patient requires a specialty PET scan.

Half-life refers to the amount of time a radioactive element takes to decay in half. The most used tracer produced by the Molecular Imaging Core has a half-life of 20 minutes. Every 20 minutes, half of the usable radioactivity has decayed, meaning the team must act quickly and make enough radioactivity to prepare the radiotracer to leave time for quality testing before sending it for injection. Vavere compares this process to a melting block of ice.

“Imagine that you have a block of ice the size of a loaf of bread,” she says. “The half-life tells you that in that amount of time, half of the ice, or loaf, will be gone. What remains is only half as much, and it’s still potent and efficient at cooling. Even though only half of the radiotracer is remaining, it is still just as effective at doing its job, emitting positrons that we can detect.”