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Magnetic Particle Imaging: New Imaging Modality Could Advance Stroke Assessment, Other Applications

In This Article

  • Magnetic particle imaging (MPI) works by directly detecting the magnetization of nanoparticles injected into the body, rather than relying on secondary effects of magnetic resonance relaxation times, as with MRI
  • Researchers in the Martinos Center for Biomedical Imaging have been developing the technology since 2014, when they were awarded a grant from the National Institutes of Health to assess its potential for neuroimaging in humans
  • They are now nearing completion of the first-ever human-scale MPI scanner
  • Once validated, the technology could contribute to a host of applications, including both basic neuroscience studies and a range of clinical applications

An emerging technology could give functional magnetic resonance imaging (fMRI) a run for its money. With dramatically higher sensitivity than is currently available with fMRI, it could boost a range of clinical applications.

In the Martinos Center for Biomedical Imaging at Massachusetts General Hospital, Larry Wald, PhD, a principal investigator in the Magnetic Resonance Physics & Instrumentation Group, and colleagues are nearing completion of a magnetic particle imaging (MPI) scanner for imaging of the human brain. Once validated, the scanner will provide an exciting new means to study activity in the brain.

Introduced in the late 2000s, MPI shares many of the same technologies as MRI, but it differs in one crucial way: It directly detects the magnetization of nanoparticles injected into the body, rather than relying on secondary effects of magnetic resonance relaxation times. Directly imaging the source of contrast like this allows for the vastly improved sensitivity.

Dr. Wald started working with MPI in 2014. With the support of one of the first BRAIN Initiative grants awarded by the National Institutes of Health (NIH), he and his group set to work assessing the potential of the technology, examining the barriers to its use for neuroimaging in humans and, through simulations, testing the performance of MPI scanner designs.

The results of the work were encouraging. The researchers concluded that a first-generation human-scale MPI scanner could offer tenfold higher sensitivity than conventional functional MRI with similar spatial resolution, with the possibility of improvement with further development.

The opportunity to construct the scanner came in 2017, when Dr. Wald applied for and was awarded his second BRAIN Initiative grant. This grant would support the construction and validation of an MPI device for use in humans, as outlined during the earlier research. Wald knew there would be challenges to face.

"There is currently no human MPI scanner," he said at the time. "And there is a reason for this: It's hard."

But he had no doubt that the team he had assembled—which also included Clarissa Cooley, PhD, Ken Kwong, PhD, Emiri Mandeville, PhD, Joe Mandeville, PhD, Erica Mason, PhD, and Wim Vanduffel, PhD—would be able to address them. (The team quickly expanded to include Eli Mattingly, who focused on the project for his PhD thesis, Monika Sliwiak, and Jorge Chacon-Caldera, PhD. All have played critical roles in the equipment design and construction.)

The greatest difficulties lay in translating the technology for human-size imaging—scaling up the field generation and detection, for example. Wald also anticipated a variety of "industrial type" challenges in designing and building the scanner. Not least: figuring out how to keep a two-ton, water-cooled electromagnet spinning around the subject's head.

The researchers have now tackled all of these challenges and are putting the finishing touches on the first-ever human-scale functional MPI scanner. The scanner can continuously image objects the size of the human head with a 5-second temporal resolution and a spatial resolution comparable to conventional fMRI (about 5 mm). Its sensitivity to changes in cerebral blood volume accompanying brain activation is expected to significantly exceed that of fMRI.

Once validated, the new scanner could advance a range of applications, both in the lab and otherwise. "We are betting that it will be a valuable tool for clinical neuroscience, where the sensitivity must be sufficient to see functional differences in individuals rather than averaging over a cohort of individuals with a disorder," Dr. Wald says today. "But also, as a diagnostic imaging modality, it could monitor for hemorrhage, assess stroke volume or, with targeted agents, provide a sensitive new molecular imaging modality to detect cancer or other disease processes."

Breast Conserving Surgery Could Also Benefit

In 2015, Dr. Wald received an MGH Research Scholar Award for his project, "Magnetic Particle Imaging for Breast Cancer Screening and Monitoring." The project aimed to demonstrate the first use of magnetic particle imaging (MPI) in humans. He and his team chose breast cancer as a target both because of the societal importance of the disease and because the breast, as a small, localized, externally accessible body part, was well suited for a proof-of-principle study with MPI.

The MPI team completed the five-year project in 2020. In 2021, they reported in Scientific Reports a prototype device they had developed and demonstrated its potential for breast conserving surgery, a widely used treatment for early-stage breast cancers but with relatively high re-excision rates. (See also here.) They showed that MPI could provide a fast, specific, sensitive, easy-to-use tool for assessing tumor margins during the procedure itself and thus could reduce the need for additional surgeries.

Learn more about the Martinos Center for Biomedical Imaging

Learn more about the Magnetic Resonance - Physics & Instrumentation Group


Erica E. Mason, PhD, and Lawrence L. Wald, PhD, of the Martinos Center for Biomedical Imaging, and colleagues are exploring magnetic particle imaging as a fast way to assess margin status during breast-conserving surgery, with the aim of reducing the need for a second surgery.


Matthew Rosen, PhD, and Susie Huang, MD, PhD, both researchers in the Martinos Center for Biomedical Imaging at Massachusetts General Hospital, discuss the possibilities—and a few of the challenges—of translational research in radiology.