PET — one of the most important tools in clinicians’ armamentarium for cancer diagnosis, staging and therapy evaluation — has a significant limitation: It can scan only a portion of the body at a time. That may change soon, with wide implications for research and clinical practice.
Image courtesy of Martin Judenhofer, UC Davis
Typically combined with CT, PET detects the photon signal from a radioactive tracer and its interaction with a target organ. The scanner uses the signal to create images of the organ and related processes, such as its cells’ absorption of glucose. How these cellular processes unfold provides key clues to a cancer’s presence and extent, as well as to its response to therapy. PET can distinguish between lesions in ways CT and MRI cannot, according to Paula Jacobs, PhD, Associate Director of the National Cancer Institute (NCI) Division of Cancer Treatment and Diagnosis Cancer Imaging Program.
“Neither [CT nor MRI] can tell if a lesion is metabolically active,” Jacobs says. “One specific example is lung lesions. A lesion may look much the same on CT if it’s benign, infectious or cancer, but it will appear quite different on ... PET.”
In spite of its utility as a clinical tool, PET is plagued by a fundamental problem, says Simon Cherry, PhD, a radiologist and Distinguished Professor of Biomedical Engineering at the University of California, Davis.
“The way we currently collect our data [using PET] is very inefficient because the scanner only sees part of the body at any one time, but the radiation is all throughout the body, emitting away,” he says.
To solve that problem, the patient needs to be surrounded by detectors that can collect as much of the radioactive tracer’s signal as possible, according to Cherry. He and his colleague, Ramsey Badawi, PhD, Chief of Nuclear Medicine at UC Davis Medical Center and Professor of Radiology and Biomedical Engineering at UC Davis, have developed a PET scanner that is intended to do just that.
A Surge in Signal Collection
After more than a decade of conceptualization, design and development, Cherry and Badawi have developed EXPLORER, a total-body PET scanner with a 6-foot gantry and 560,000 signal detectors — 10 times more than those featured in conventional machines, according to Cherry. Shanghai-based United Imaging Healthcare produced a small-scale version of EXPLORER in early 2017 that is being used in animal trials at UC Davis School of Veterinary Medicine. Cherry and Badawi expect to begin human research trials in 2018.
Conventional PET scanners require a patient to move through the scanner during an exam to image the entire body. With EXPLORER, the patient will enter the bore and remain stationary while the machine obtains a full-body image. Cherry and Badawi predict EXPLORER will improve radiotracer signal collection by a factor of 40 for a whole-body scan.
“You could choose to use that entire factor of 40 to speed up the scan; you could do a scan of the whole body in 1/40th of the time that we currently do the scan,” Cherry says. “A fairly typical 20-minute protocol could now be done in 30 seconds. You could use that factor-of-40 signal collection in other ways, as well, or at least part of it. You could use it to reduce the injected dose ... or to collect more signal and get better-quality scans. You have this massive improvement in signal collection, and you can use it in different ways depending on the task at hand.”
A Game Changer in Caring for Patients?
The impact of EXPLORER on the way clinicians detect and assess cancer, as well as evaluate the success of oncologic therapies, could be tremendous. The machine may be able to identify smaller, less active lesions and allow clinicians to be more definitive in their judgments of treatments’ efficacy, according to Badawi. He says EXPLORER may also eliminate a common problem of PET.
“Breathing motion is the single most important degrading factor in terms of spatial accuracy when scanning the thorax,” Badawi says. “If you [could scan a patient] in 30 seconds, you could do a breath-hold PET.”
A faster scan would be especially important for the pediatric population.
“You can reduce the need for anesthesia or even do away with it in some cases if you scan fast enough,” Badawi says. “[Children] are more susceptible to radiation dose [than adults], but anesthesia itself also carries significant risks — more so than radiation. If you could obviate the need for anesthesia in even a fraction of the pediatric population, that would be a big deal.”
Jacobs, with NCI, cautions that the device faces years of study before it enters clinical care.
“I’d expect the clinical efficacy trials to begin in about four to five years ... in parallel with engineering efforts to bring it to practical and cost-effective manufacturing, followed by gradual expansion of availability over the following years,” she says. “If the increase in throughput and quality are realized, both patient care and healthcare costs will benefit.”