An astrophysicist in a biomedical universe | MIT News

For many of us, the pandemic has triggered momentous changes. And Magdelena S. Allen was no exception.

Growing up in Portland, Oregon, Allen wanted to know everything. She loved stargazing and the physical sciences, but she was also interested in law and writing. Her parents, who homeschooled her with her sister through high school, were extremely supportive of her various interests. “But what I always came back to was science,” she says.

An astrophysicist by training, Allen earned her undergraduate degree at the University of California, Berkeley. She then spent a year interning at physics research labs across the country, including Fermi National Accelerator Lab, Brookhaven National Lab, and NASA’s Marshall Space Flight Center.

In the fall of 2019, Allen began his PhD in the Nuclear and Particle Experiments Division of MIT’s Physics Department, studying cosmic rays with Professor Samuel C. C. Ting. She has spent her time analyzing data from the Alpha Magnetic Spectrometer, a cosmic ray detector on the International Space Station.

Around this time, Allen also joined MIT’s Emergency Medical Services (EMS), having heard about it from a friend involved a few years earlier. “It started out as what I thought was a few hours a week and quickly took over my life in the best way possible,” she says.

When the pandemic hit in the spring of 2020 and the MIT community rushed to disperse from campus, Allen was one of the few MIT paramedics left. She helped keep the ambulance on duty 24/7, serving MIT as well as the wider Cambridge and Boston community. “I ended up putting in over a thousand hours in service” that college year alone, she says.

Spending so much time in patient care has spurred her interest in patient outcomes. And it got her thinking about her long-term career goals. “I had my existential crisis during this time – as we all have,” she laughs. While she loved her research in fundamental physics, she wanted to have a more direct impact on people.

Allen began looking for research groups working on biomedical devices. In January 2021, she joined a new research project to build brain imaging hardware, which uses technology similar to some basic physics experiments. She now works at the intersection of physics and medical research, co-directed by Professor Ciprian Catana of Harvard University at the Massachusetts General Hospital AA Martinos Center for Biomedical Imaging MR-PET Integrated Laboratory and Professor MIT Gold Hen at the Nuclear Science Laboratory. . “It was a perfect fit,” she says.

Discover the secrets of the brain

Now, Allen and his collaborators are developing a next-generation brain positron emission tomography (PET) scanner that can be used simultaneously with 7 Tesla magnetic resonance imaging (MRI). The PET scanner will be a cylindrical insert that fits directly inside an MRI machine.

Each imaging technology offers a different perspective of the brain. While MRI captures anatomical images, PET captures biochemical processes, such as metabolism. By seeing two synchronized perspectives, scientists have valuable data to study brain tumors and neurological diseases, such as Alzheimer’s disease.

But, PET scans currently take a long time – typically 30 to 90 minutes – and patients must remain still for the entire duration to get clear images. In the next-generation PET scanner, Allen aims to make scans much faster, down to just a few minutes. It will also open doors for neuroscience research. With shorter imaging times, CT can work simultaneously with functional MRI (fMRI) to take rapid snapshots of dynamic biological processes. For example, PET can capture glucose metabolism in the brain while fMRI simultaneously captures blood flow. “It’s really exciting,” Allen said. “It’s never been done before.”

The reason why these scans currently take so long lies in how PET works. Before the scan, patients are injected with a radiotracer made up of biological compounds the body normally uses, such as glucose, which are slightly modified to be radioactive. When the body processes these tracer compounds, radioactive gamma rays are emitted. The PET scanner then acts as a camera to capture these gamma rays and form an image.

The problem, however, is that only small doses of radiotracer are injected into patients to limit the adverse effects of radiation. Thus, the gamma rays emitted are very weak, which makes it difficult to form images. “It’s like a long exposure photo with a regular camera where you just wait for the light to catch,” Allen explains.

To reduce the exposure time required, Allen’s group is redesigning the MR-compatible PET scanner to be 10 times more sensitive to gamma rays than the current state-of-the-art. And to achieve this, a new form of gamma ray detector will be used inside the scanner. While a typical scanner uses detector arrays arranged in a cylindrical tube surrounding the head, the new scanner’s detector arrays are configured more like a motorcycle helmet. “You can increase the [scanner’s] a lot of sensitivity just by getting more coverage,” she says.

Another key component to building a high-sensitivity PET scanner is the individual gamma ray detection modules inside the detector. The detector is made up of rings stacked from the neck upwards, and each ring contains a circle of detector modules. “The most interesting investigation to date is [figuring out] different geometries for the detector in order to derive specific information from it,” she says. One of the challenges is to find the interaction depth methodology and the optimal thickness for the detector. A thicker detector can trap more gamma rays for more imaging information. But too thick a detector gives blurred images. After some trial and error, however, Allen is “very close to nailing down a final design”.

Astrophysics at heart

While Allen ventured into the biomedical world, she did not completely abandon particle physics. It turns out that the PET scanner can also be used to study fundamental questions in physics.

One issue that interests Allen is symmetry violation. While the universe is made up of matter and anti-matter, it’s not a 50-50 split, giving us an asymmetrical universe. But, we do not know where this asymmetry comes from. “We are always looking for sources of asymmetry in the universe,” Allen says.

A potential clue could be found in the life cycle of positronium, an unstable atom composed of an electron and its antiparticle, a positron. Positronium lasts for a very short period of time – less than a millionth of a second – before the electron and positron annihilate, emitting gamma rays. Depending on the initial state of the positronium, different distributions of gamma rays are emitted.

“The PET scanner is basically just a gamma ray detector,” she says, which makes it “perfectly tuned” for observing gamma rays from positronium decays. To study symmetry violation, Allen plans to observe how the action on the initial states of positronium affects the orientation of the emitted gamma rays. If she sees asymmetries, it could provide insight into symmetry violation.

But first, Allen must finish building the PET scanner. After finalizing the design of the detector modules, it will begin assembling them into rings for the helmet this summer. In the meantime, she will continue to serve at MIT EMS, having completed her term as head of MIT EMS last year. “It’s a very addictive thing to do,” she says.


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