The radio signals shouldn’t have been there, but they were. Having performed thousands of nuclear magnetic resonance (NMR) experiments, Princeton University physical chemist Warren S. Warren knew better than most scientists how to tune in radio echoes from atomic nuclei zapped by magnetic force. Those echoes provide the basis for one of the most powerful analytic tools in chemistry—NMR spectroscopy—and also for the magnetic resonance imaging (MRI) machines used widely in hospitals.
What baffled Warren and his team was that they could pick up some of the echoes at frequencies far higher than the band where they were expected. What’s more, even simple substances as well-understood as deionized water were giving those unexpected results. The anomalous signals, the researchers knew, just didn’t fit current magnetic resonance theory.
The situation was as outlandish as if a puffing tuba player were squeaking out the sounds of a piccolo, Warren says.
The anomalies had started popping up in the early 1990s in the Princeton group’s NMR experiments. At that time, magnetic resonance instruments had been working so well for so long that scientists were convinced the theory behind them was rock solid. “If theory and experiment didn’t agree, you went in with a screwdriver and soldering iron and fixed the spectrometer,” Warren says.
His team tried that. The scientists even switched to instruments from different manufacturers but couldn’t eliminate the seemingly impossible signals. When they finally began making conference presentations about the puzzling data—and their nagging suspicions about flaws in magnetic resonance theory itself—their colleagues would have none of it.
Leading researchers warned Warren that he would ruin his career by pursuing what they were convinced was a subtle error in his experiments. At one meeting, the organizers roasted the Princeton group with a mean-spirited, bogus presentation mocking its work, Warren says. In time, his grant proposals started to run into trouble: Three times, he faced funding cancellations.
Now, it seems that the Princeton team was right all along. What’s more, the anomalies that the scientists eventually explained are becoming recognized as useful data, complementary to the signals ordinarily detected.
In the area of chemical analysis, for example, Warren and his colleagues have recently examined molecular structures at higher magnetic fields than anyone has been able to employ with conventional NMR techniques. Use of higher fields may open the door to deciphering much larger protein structures than NMR can currently take on—a pressing challenge as human-genome studies push closer to identifying important proteins in the body.
And in medical imaging, the alternate signals could potentially aid in the diagnosis of cancer, stroke, and other diseases. They also could improve researchers’ ability to probe the causes of those maladies and the functioning of the brain.
Like off-kilter tops
First developed in the late 1940s, nuclear magnetic resonance instruments exploit the magnetic nature of atomic nuclei. Protons and neutrons have a quantum mechanical property, known as spin, that’s analogous to the angular momentum of a spinning top. Because of their spins, these particles and the atomic nuclei that contain them behave like minuscule bar magnets.
In an MRI machine or an NMR spectrometer (SN: 1/6/96, p. 4), a strong, uniform magnetic field forces all the nuclear spins in the subject or specimen to line up either in that field’s direction or opposite to its direction. Then, pulses of electric current in a coil of wire generate cross-wise magnetic fields that tip the spins of some nuclei away from the main field’s orientation. That sets the stage for the radio signals.
Like an off-kilter top, the axis of each tipped spin begins to rotate, or precess, swinging its own smaller magnetic field with it. The speed of this precession depends on both the identities of the nuclei and the strengths of fields.
Since, as magnetic fields change, they induce electric currents in a conductor, the coil that delivered the magnetic pulses also serves as a receiver, registering the tiny currents caused by the gyrating fields. Those electric oscillations, which are in the same frequency band as radio waves, are the NMR or MRI signals.
That’s the simple picture. In fact, there are interactions among spins that make things more complicated—and it is those interactions that scientists had to reexamine in order to understand what Warren and his colleagues had observed.
Magnetic resonance specialists have long known that instead of always responding independently to a magnetic kick, two or more spins within the same molecule may react as a team. Say that neighboring atoms, for instance, share a chemical bond. That could cause their nuclear spins to be linked via a quantum mechanical phenomenon known as spin coupling or coherence.
When spins team up, they precess faster than they do on their own. So, when scientists want to analyze a complex molecule’s structure, they routinely look for high-frequency signals from spin coherences. Those signals provide important structural clues about complicated molecules, such as the proximity of certain atoms and whether they are bonded.
What was striking–even shocking was that the signals the Princeton group found looked like spin coherences, but they couldn’t be spin coherences. Why not? Because it was well known that the simple molecules in those experiments, when kicked by the simple series of magnetic pulses that were used, couldn’t produce the required coupling behavior.
“These effects looked very strange, almost incomprehensible,” recalls Jean L. Jeener of the Free University of Brussels in Belgium. He, Warren, and other scientists struggled for several years to solve the mystery. Ultimately, they found an explanation. Spins were teaming up, they concluded, but they were spins on different, widely separated molecules, not within the same molecule.
Interacting magnetic spins
This actually was not a new idea. Even the founders of magnetic resonance theory had pondered whether interactions among spins on different molecules could contribute to the magnetic resonance signal. But they concluded that the magnetic forces between widely separated spins could be ignored.
For decades, that initial oversight proved to be only a benign theoretical error with no obvious consequences. In a nutshell, “it wasn’t very important when you had small magnets,” notes Warren. Most researchers attributed subtle signs of the intermolecular signals to instrument noise. “No one stood back and said, ‘Are these assumptions that we made in the ’40s still valid?'” Warren notes.
Today, the fields in magnetic resonance devices are 30 times as powerful as they were when the technology was invented. As a result, the limitations of that early error have become ever more apparent.
Warren and Jeener’s descriptions of the intermolecular spin-spin effect—in the twin languages of quantum mechanics and classical physics—eventually removed the long-standing doubt that these interactions show up in magnetic resonance data.
What the Princeton group members and their colleagues have contributed is “a deep insight into the fundamentals of the field,” comments MRI pioneer Paul Lauterbur of the University of Illinois at Urbana-Champaign. “I think it will loosen up thinking and inspire people to understand things they might not have thought about before and to invent things they might not have otherwise invented.”
The findings “will become part of the lore in the textbooks of NMR and MRI,” adds physicist Alexander Pines of the University of California, Berkeley.
Quality spectra
The emerging recognition of intermolecular spin interactions is far more than an advance in theory. As they push the frontiers of NMR, for instance, Warren and his colleagues are using one of the world’s most powerful magnets to stoke those interactions. Their aim is to help write the next big chapter in the understanding of human genetics.
Now that efforts to read the entire human genetic code are nearly complete, the pressure is on to understand what the proteins encoded by newly found genes do (SN: 7/1/00, p. 4). Biochemists already use NMR to deduce the structure of proteins that have masses up to about 30,000 times that of a hydrogen atom. However, many of the proteins and protein complexes that scientists will want to study are much bigger. Using NMR to extract details about those larger structures will require stronger magnets than those used today, but no one yet knows how much stronger.
Scientists suspect that a magnet at the National High Magnetic Field Laboratory (NHMFL) in Tallahassee, Fla., may be strong enough for these genetics studies. Officially called the Keck magnet but nicknamed Conqueror, it cranks out a field some 500,000 times that of Earth. Doing so takes a lot of power—nearly 10 percent as much as it takes to run the whole city of Tallahassee—and more than 7,000 liters of water per minute to keep the magnet from overheating. However, the gargantuan field produced by the magnet isn’t uniform enough for scientists to use with conventional NMR techniques.
This is where those intermolecular spin-spin interactions are coming in handy. Because those interactions produce sharp, undistorted signals despite magnetic field irregularities, Warren and his colleagues are beginning to work to discern structures of complex molecules in Conqueror’s field. In those experiments, they intend to learn whether it will be useful to build more-stable but extremely powerful magnets that might enable ordinary NMR to determine the structure of large proteins.
In the Oct. 23, 2000 Physical Review Letters, the researchers from Princeton, NHMFL, and the University of Florida in Gainesville described the first successful NMR study using the powerful magnet. As a proof of principle, they looked only at simple organic molecules, such as acetone and ethanol.
“It’s impressive that [Warren] can get the quality of spectrum that he’s getting,” comments Adriaan Bax, chief of biophysical NMR spectroscopy at the National Institutes of Health in Bethesda, Md.
Medical imaging
For medical imaging, tapping into intermolecular spin interactions offers several possible advantages over conventional MRI, even though the signals are only about 10 to 15 percent as strong, scientists say. The new approach doesn’t require additional hardware, only a reprogramming of the scanner’s magnetic-pulse sequences.
A couple of years ago, Warren and his colleagues reported MRI studies on rats, pigs, and people that indicated that the spin interactions provide information on physiological characteristics different from those addressed by conventional MRI. For example, the strength of signals from intermolecular spins appears to directly reflect the amount of oxygen in the tissue being scanned, Warren says.
In a recent experiment, researchers at the University of Pennsylvania Medical School in Philidelphia, working with Warren’s group, have seen “huge contrast differences” between test tubes filled with oxygenated and unoxygenated blood, says Rahim Rizi, a Penn radiologist. In other investigations on pigs, he adds, the group is also seeing “promising, preliminary results” indicating that signal intensity directly reflects oxygen concentration.
If confirmed, these unpublished results suggest that MRI specialists might be able to use intermolecular spin interactions to create detailed maps of oxygen concentrations of tissues. Since the amount of oxygen in tissue and its location indicate what processes are going on there, such maps could be a boon to research and diagnosis of cancer, stroke, and other diseases, Rahim says.
In other studies, researchers are exploring a type of fine-tuning made possible by intermolecular spin interactions. Typically, an MRI machine scanning through a person’s body picks up radio signals at any one instant from a block of tissue a few cubic millimeters in volume. With the new approach, however, MRI specialists can tune their machines to detect signals from spins separated by as little as 100 micrometers.
That small-scale sensitivity may help physicians detect early breast cancer, when tumors are too small to be seen on a conventional MRI image, says Penn radiologist and physicist Mitchell D. Schnall. He estimates that it will take his group, which is testing the approach on women, about 2 years to evaluate its potential.
Brain as well as breast studies may benefit from the fine-tuning. In so-called functional MRI, researchers determine what various parts of the brain do by taking a fast series of MRI scans while subjects perform cognitive tasks (SN: 3/5/94, p. 159).
In the June 29, 2000 Magnetic Resonance Imaging, researchers from the National Research Council of Canada, the University of Minnesota, and Princeton presented evidence from tests on seven people that suggests that functional MRI based on intermolecular spin interactions may pinpoint active brain areas more precisely and sensitively than conventional methods.
Studies of many other organs may also benefit from tuning in to intermolecular spin interactions, Warren speculates. “We need to entice good radiologists to try out the method on their tough problems,” he says.
Magnetic chorus plunges into chaos
Heart rhythms, the interplay of oscillating electrical signals, and even the drifting of a leaf may fall into so-called chaotic patterns that appear random but follow strict rules. Because a slight change at the start of a chaotic process can lead to a radically different outcome, the specific course of chaotic events defies prediction (SN: 1/25/97, p. 52; 10/31/98, p. 285).
Now, to their surprise, researchers have found that radio signals elicited from molecules by magnetic field pulses can also turn chaotic. Magnetic resonance imaging (MRI) machines and related scientific devices rely on those signals to characterize both tissues and nonliving materials.
Yung-Ya Lin, Warren S. Warren, and their colleagues at Princeton University stumbled upon the unpredictable behavior during experiments with a nuclear magnetic resonance (NMR) spectrometer, an instrument commonly used for determining the configurations of molecules. Warren says that the chaotic patterns, which the scientists reported in the Oct. 6, 2000 Science, probably arose because of subtle magnetic interactions between molecules. The role of those interactions in magnetic resonance signals was not recognized until the Princeton team focused on them in the early 1990s (See main story).
The newly unveiled chaos has probably been at the root of some widely noticed, puzzling anomalies in NMR studies of biomolecular structures, the Princeton researchers speculate. Now scientists are beefing up NMR devices to serve as more powerful molecular microscopes for the next step in analyzing proteins identified by the huge human-genome-sequencing projects. As they continue, problems related to chaos could increase, NMR researchers say. The new findings are “a very useful warning that very nasty things may happen,” says Jean L. Jeener of the Free University of Brussels in Belgium.
Although Jeener rightfully worries that chaotic behavior may amplify the unwanted signals in NMR experiments, swamping the meaningful signals, the presence of chaos could also prove to be a benefit, Warren notes. Researchers who learn to control the chaotic amplification of tiny fluctuations in the magnetic environment may find themselves able to sense extremely small magnetic variations in biological tissues and other samples. That enhanced sensitivity might raise NMR techniques to new and even more useful heights.
