Ringing to a single proton's magnetic nudge.

A proton behaves as if it were a miniature bar magnet twirling like a top. This combination of spin and magnetism has permitted researchers to develop a variety of sophisticated techniques--based on a phenomenon known as nuclear magnetic resonance--for determining the composition and structure of molecules in pure samples and for picturing complex processes such as blood flow through the heart.

A medical physicist has now suggested an alternative, potentially more sensitive means of extracting information from a nuclear magnetic resonance experiment. His calculations show that, under the proper conditions, a single proton's interaction with a magnetic field may be strong enough to set a nearby, microscopic sliver of quartz quivering in much the same way that a tuning fork begins to ring when bathed in sound waves of just the right frequency. By monitoring these induced vibrations, researchers could, in principle, detect and locate single protons deposited on a surface.

"It turns out that the predicted signal levels are well above quantum and [thermal] noise limits," says John A. Sidles of the orthopedics department at the University of Washington School of Medicine in Seattle. Although little is known about fabricating mechanical oscillators small enough to work in such an experiment, the technique may eventually allow the imaging of individual biological molecules--a level of resolution not possible with conventional magnetic resonance imaging.

"Sidles' idea is kind of revolutionary in the field [of nuclear magnetic resonance imaging]," says physicist Myer Bloom of the University of British Columbia in Vancouver, who studied a related effect in the 1960s. "I don't see why the idea shouldn't be right, but I'm not completely sure that the claimed sensitivity can be achieved."

Sidles presents the theoretical basis of his proposed technique in the Feb. 24 Physical Review Letters. Through a quirk of the review process, a subsequent paper describing possible designs for such a detector appeared last summer in the June 17 Applied Physics Letters.

Sidles' scheme ingeniously combines nuclear magnetic resonance techniques with the kind of technology that made possible both the scanning tunneling microscope and the atomic force microscope (SN: 4/1/89, p.200; 2/29/92, p.135). Already used to measure tiny variations in magnetic force across a surface, these scanning methods--when further refined and developed--could form the basis for detecting single-proton magnetic resonance.

In such an experiment, a miniature mechanical oscillator would ride just a few angstroms above a surface dotted with protons. When the distance between a proton and the oscillator reached a certain critical value, the oscillator would begin to vibrate, generating a detectable signal. Successive scans could produce enough information to reconstruct the three-dimensional structure of a protein or some other complicated molecule.

"I hope my work will establish ... molecular imaging [via nuclear magnetic resonance] as a legitimate, publishable area of research," Sidles says. "Even if the approaches I have described prove impractical, perhaps other, more ingenious scientists will be encouraged to do better."

"It may be hard to make the thing work properly, but it has the potential of being very important," Bloom says. "It gives you a different way of thinking about fundamental measurements."