Physics Contributes to New Medical Imaging Technique

By Calla Cofield

Stanford University physics graduate student Nicole Ackerman spent the first three years of her graduate career studying neutrinos – subatomic particles that carry no electrical charge. Now, she’s working in the field of radiation oncology, investigating the use of Cherenkov radiation in medical imaging. Cherenkov radiation is produced by a particle passing through a medium at a speed greater than that of light through the medium.

“I’m still simulating particles interacting with matter,” said Ackerman. “They are just in mice now instead of in a detector.”

During the 2011 APS April Meeting in Anaheim, Calif., Ackerman delivered a general session talk and spoke to reporters about her work.

One of the biggest goals of modern cancer research is to develop better imaging techniques. Imaging is key to early diagnosis, effective treatment and finding cancer cells that have metastasized. Many medical imaging techniques rely on nuclear and particle physics principles, and yet, says Ackerman, many of the biologists working with those techniques don’t understand the physics behind them.

In positron emission tomography, or PET, positron-emitting radioactive isotopes are attached to molecules designed to bind to specific types of cancer cells. When the isotopes decay, they produce gamma rays that signal the presence and location of those cancer cells.

In 2009, scientists in Cambridge, Mass., published a paper demonstrating that radioactive isotopes used in medical imaging will cause water-dense tissue to emit optical Cherenkov radiation. In materials, the speed of light is lower than in a vacuum, and high energy particles may emit Cherenkov radiation when they travel faster than the photons. Radioisotopes have been used to treat cancer for more than 50 years, and while some biologists and doctors had noted the optical glow before, no one, it seems, had thought to use it.

In a preclinical and research setting, the technique offers some significant benefits over PET scans, including the fact that optical scans only take 3 minutes, whereas PET scans take 30, and optical scanners are less expensive and used more frequently by research staff.

In the future, Cherenkov light may offer imaging where there is currently none. There are presently no direct ways to image alpha and electron emitters in the body. Cherenkov radiation, however, can be used with positron, gamma, electron and alpha emitters, at short intervals. Rather than delivering one dose of radioactive isotopes to image a tumor and a second to treat it, doctors could watch the treatment dose directly.

Because Cherenkov light is optical, it scatters quickly when traveling through tissue and would likely be used to image shallow tumors such as skin cancer or some breast cancers, or cancer of the esophagus, viewed via an endoscopy. A recent paper proposed using Cherenkov light immediately following tumor removal surgery, to see if any cancerous cells are left behind. Another group has proposed using a molecular component called a fluorophore that would lengthen the wavelength of the Cherenkov light at its source and allow it to travel further through tissue to a detector.

Ackerman’s work is focused on modeling the path of the Cherenkov photons as they travel through tissue. She uses a software program called Geant4, which was designed to model particle tracks in high-energy physics experiments. She says she isn’t sure yet how exactly the models will be used, but she wants to understand the mechanisms behind the observations her group is making.

“My goal is to find the places where the physics details are important and then take the equations and simulations and turn them into something useful for the other researchers in the field,” she said.

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