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.”
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.”
Racing the clock
If a patient is scheduled for a PET scan at 10 a.m., chemists begin setup at 7:30 a.m., calibrating, sterilizing and cleaning all equipment. Access to the Molecular Imaging Core is limited; everyone who enters must wear lab coats, safety glasses and radioactivity detectors.
“Everything is coordinated to the minute. We put in a buffer of a few minutes, but if there is too much buffer time, then we have to start with a lot of extra radioactivity to end up with the amount the doctors need for a good PET scan,” Vavere says.
The beat goes on
The Molecular Imaging Core enables investigators to use radiotracers to see how drugs are delivered in preclinical models and to develop new radiotracers.
“We’re here to help St. Jude clinicians and researchers use this process to image various biochemical processes and target expression using PET imaging, which offers physicians a higher level of functionality when making treatment decisions,” Vavere says.
“We want to foster stronger and more collaborative work so our patients have access to the best this technology has to offer.”
Birth of a Radiotracer
Many meticulous steps must occur for a radiotracer to be used in a PET/CT scan:
- After cleaning is complete and components have been set up, the engineer fires up the cyclotron to begin its work.
- A nonradioactive material such as pressurized gas is placed into one of eight small chambers around the cyclotron’s perimeter.
- Hydrogen gas is sent through a high-voltage electrical current to create a high-energy beam of ionized gas.
- The hydrogen ions are accelerated by a strong electric field produced by an amplifier. Meanwhile, two large electromagnets steer the ions from the cyclotron’s center to the chambers on the outer edges.
- The beam of protons is aimed at the gas inside the target, converting it from one element to another. The material is radioactive, but not ready for use.
- The material is pumped into one of eight lead-shielded hot cells, where a chemist uses remote-controlled machines to convert the chemical into a drug.
- The resulting radiotracer is sterilized by filtering it into a glass vial.
- The dose vial is transferred into a cabinet called an isolator, where a robot fills syringes with a predetermined amount of the radiotracer specific to the patient.
- A small amount of that radiotracer is tested for quality and sterility in the lab’s Quality Control room.
- A pharmacist verifies the dose has passed required tests and approves its release.
- The dose is placed into a lead container and sent through a pneumatic tube to the Nuclear Medicine Clinic, where staff members conduct more safety checks.
- The patient receives the drug.
From Promise, Autumn 2018