Hidden Specialties
Meet the doctors of sleep, images, and microscopes
The radiology department at the Hospital of the University of Pennsylvania, one of the largest and most diverse anywhere, is a case in point. There are hands-on types like Conant. Then there's Mitchell Schnall, an MRI expert and head of Penn radiology's research division. (Schnall does have a darkish basement office, but like most radiologists these days, he reads his images on a computer screen.)
"I'm about as close to a researcher as someone who still practices medicine can be," Schnall says. Like anesthesiology and pathology, radiology is a cerebral field. It hooks doctors as interested in the science of medicine as they are in the art and practice of it. Schnall says he might well have become a physicist had he not come from a medical family. As an M.D. with a Ph.D. in biophysics, he's a critical link in the "translational medicine" chain of collaborations that take discoveries from the lab bench to a patient's bedside. Radiologists like Schnall work with physicists and engineers to develop more powerful, safer, and less expensive imaging techniques. "Radiology is moving beyond showing the fact that something is there to finding out what's happening to tissues at the molecular level," he says.
Using special radioactive probes targeted at specific cellular activities, for example, radiologists can use a PET scan to judge how fast cancer cells are growing after chemotherapy. The scanner tracks the decay of subatomic particles called positrons--the "P" in PET, or positron emission tomography--as they fly out of a small amount of radioactive dye injected into the patient. "You can see if metabolism is shutting down after chemotherapy," says Schnall, "so you'll know early on if it's working." That can give an oncologist early warning if a treatment isn't working, and it also helps evaluate the effectiveness of experimental drugs without waiting to see if a tumor in a patient enrolled in a clinical trial continues to grow.
Drawing a line. In another form of this "molecular profiling," radiologists are starting to use an MRI-based technique called MR spectroscopy to detect and characterize prostate cancer. By manipulating the MR machine's magnetic field, says Schnall, he can tease out the faint "resonance" signals of specific biological molecules. Coupled with a knowledge of cell chemistry, that can tell radiologists what sorts of cells are present. Normal prostate cells, for example, contain high levels of a substance called citrate, while rapidly growing cancer cells are loaded with choline, an important ingredient in cell membranes. The two compounds, says Schnall, mark a precise line between healthy tissue and cancer in an MR spectroscopy image.
While radiology is a specialty given to fondness for ever more esoteric imaging beams and force fields, Schnall is working with a team of physicists on a new imaging technology that uses plain old light. Near-infrared light rays--those at the low end of the visible light spectrum--can pass through up to several inches of tissue without harm (think of the red glow through your cheeks if you put a flashlight in your mouth in a dark room). Schnall hopes they can capture images of breast lumps and other abnormalities. During a recent visit, he was using a green dye (which absorbs reddish light) injected into blood vessels that supply a volunteer's breast to see if a prototype optical imaging system could detect specific patterns of blood flow that indicate the presence of a tumor.
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