Particle physics can be a tough sell to those who think all science should have some life-enhancing benefit or real-world application, or else it’s a waste of money. Those big accelerators don’t come cheap, and our culture increasingly doesn’t value curiosity-driven research. Inevitably someone who doesn’t know any better will remark that it’s a waste of time and resources when scientists could be off, say, curing cancer.
The irony is that past high energy machines that have outlived their usefulness to cutting-edge physics continue to give back to society in surprising and unexpected ways. Synchrotron radiation (used in archaeology, biology, and to monitor metal fatigue in bridges, among other areas) is one example; proton therapy is another. Yep, that’s right: proton beam facilities once used for physics research are now helping treat (if not cure) cancer.
And now another tool from particle physics is emerging as a boon to cancer imaging and treatment. At the American Physical Society conference in Anaheim, California, Nicole Ackerman of Stanford University explained how she is collaborating with colleagues in Stanford’s Small Imaging Core Facility to develop a new imaging technology using Cerenkov radiation — the faint blue glow commonly emitted from radioactive materials when they’re immersed in water.
We’ve known about this unusual effect since 1934, when Soviet physicist Pavel Cerenkov first observed a faint blue light shining from a bottle of water that had been bombarded with radiation. From then on, he usually spent an hour or so in a completely darkened room before beginning his daily routine to increase his eyes’ sensitivity to the light. Ilya Frank and Igor Tamm followed up with a 1937 paper explaining why the “Cerenkov effect” occurs, and the three shared the 1958 Nobel Prize in Physics for their work.
The underlying principle behind Cerenkov radiation is similar to that of a sonic boom. If an aircraft is traveling faster than the speed of sound, the air flowing around its wings can’t move out of the way fast enough, creating a sudden pressure drop moving away from the wing at the speed of sound. And this pressure front, or shock wave, creates that loud boom you hear after an aircraft flies overhead.
With Cerenkov radiation, the same kind of shock wave is created, only with light instead of sound. Think of it as a “photonic boom.” It occurs when a charged particle moves through a medium faster than the speed of light and generates a similar shock wave.
“But wait!” Someone out there who remembers their high school physics class objects. “Light is the cosmic speed limit, and nothing can move faster than light!” This is true when applied to the speed of light in a vacuum, which is constant (c). But the speed of light as it moves through water is considerably slower, and in that medium, it is very possible for a charged particle to be traveling faster than the speed of light. So you get the same kind of shock wave effect, with a flash of blue light instead of a sonic boom.
Those flashes of light can be detected, which makes them very useful for all manner of scientific experiments. IceCube, for instance, which searches for neutrinos deep under the ice at the South Pole, relies on Cerenkov radiation as an indicator of a neutrino colliding with an atom in the ice. You also see Cerenkov radiation in some types of nuclear reactor, as captured by engineers at the Nuclear Engineering Teaching Lab (NETL) at University of Texas, Austin:
Ackerman came into the biomedical realm from particle physics, so she knew all about Cerenkov radiation and how it’s used to detect neutrinos and other particles. But it’s only now beginning to be used by biologists who work with radioactive isotopes to image cells in living organisms using such techniques as Positron Emission Tomography (PET).
Ackerman is developing a technique called Cerenkov Light Imaging (CLI), and it employs the standard charge-coupled devices (CCDs) typically used for fluorescence of bioluminescence imaging, except here the light comes from Cerenkov radiation. (Biological tissue is mostly water, which has the right refractive index to produce the effect.)
It’s common practice to “tag” molecules with nanoparticles containing just the right antibody to “stick” to cancer cells. Ackerman’s approach essentially creates “radioactive glucose”: cancer cells are highly metabolic — these are cells that are growing wildly out of control — so they absorb more glucose than normal cells. Thus, these specially designed biomolecules will eventually clump together in cancerous tumors.
For an isotope, Ackerman settled on 225 Actinum: its decay chain produces three alpha particles very quickly, then a bunch of beta “daughter” particles that produce quite a bit of light, which can be used for bioimaging. The whole decay process lasts about 15 minutes.
Ackerman and her cohorts in the Small Animal Imaging Core Facility have done tests on mice as proof of principle, with Ackerman comparing her computer simulations to data and tissue samples collected from the mice.
They injected human cancer cells into mice and then injected the probe to see how well it targeted the cancer cells by measuring the emitted Cerenkov radiation. Ackerman reported that it’s possible to image four or five mice at a time in an optical scanner within three minutes using this technique.
That short time frame is a significant advantage, given the speed of the radioactive decay process. Typically, the patient undergoes surgery, is closed up, and then is sent to a separate imaging facility to determine if the surgeon removed all the cancer cells. By then, any radioactive emissions would be gone. If 225 Actinum could be used as an isotope in conjunction with CLI, it might be possible to do the imaging right there in the operating room.
There is now a group at Memorial Sloan-Kettering in New York conducting clinical trials to further explore CLI’s potential. Ackerman hopes that her research will lead eventually to using CLI for actual treatment — specifically, a modified kind of targeted drug delivery exploiting the heavier alpha particles that are produced initially in the decay process for 225 Actinum. Those Alpha particles don’t emit light, nor do they penetrate very far into tissue, so they are not useful for imaging. But they can zap cancer cells with a high dose of radiation without damaging surrounding tissues.
Who knows? There may be other innovative uses for CLI — and Cerenkov radiation more generally — that scientists haven’t considered yet. Says Ackerman, “I’m the person with the hammer looking for the right kind of nail.” She seems to have found those first couple of nails.
Image (top): Argonne National Laboratory’s (ANL) Advanced Test Reactor core exhibiting Cerenkov radiation. Credit: ANL