What goes into making a better brain scanner?
Gobs of power, for sure. As much as a commercial jet releases when landing, according to one expert. But it also takes detailed knowledge of the human body, a deep understanding of some highly complicated mathematics, and lots of careful calibrations involving both. That’s the challenge two university consortia are now taking on.
The key piece of muscle in a magnetic-resonance imaging machine, or MRI scanner, is its huge central magnet. That magnet lines up the protons in atoms that make up the molecules of water that fill each person’s body. Those protons are key to the detailed image produced by the machine.
Once the magnet gets the protons pointing in the same direction, the MRI machine knocks them all loose for an instant. Like a tuning fork that’s been pinged, they vibrate for a split second as they get realigned by the force of the big magnet. The MRI uses those vibrations to tell what kind of atoms are where, then turns that information into pictures of the structures—like bones or brains—made up of those atoms.
Current MRI capabilities allow brain researchers to see objects as tiny as about two to four millimeters across. That’s small, but not small enough. “A millimeter of human brain contains hundreds of thousands of neurons and perhaps a billion or so individual synapses,” says David C. Van Essen, head of the Human Connectome Project’s operations at Washington University in St. Louis.
A nine-institution team, led by Washington University and the University of Minnesota-Twin Cities, is taking on the problem by increasing the power of the main magnet. And a team at Harvard’s Massachusetts General Hospital and the University of California at Los Angeles is increasing the speed and capabilities of the sensing equipment. Both approaches have advantages and drawbacks.
Magnetic-field strength is measured in teslas; the typical commercial MRI in a hospital has a main magnet rated at three teslas. Kamil Ugurbil, a professor of radiology, neurosciences, and medicine at Minnesota, helped Siemens, a German engineering conglomerate, to build a seven-tesla model. Now he is leading his Connectome colleagues on a 10.5-tesla machine.
One problem they face is that bigger magnets require a lot more energy. The challenges of reaching 10.5 teslas, Mr. Ugurbil says, include keeping the overall unit sufficiently cooled and the wires precisely aligned so that the magnetic field’s strength doesn’t vary across the patient’s body.
The Harvard team, meanwhile, is attempting to drastically improve performance at the three-tesla level. A magnet of that strength, large enough to fit around a human body, is still powerful, storing as much energy as a jumbo jet releases upon landing, says Lawrence L. Wald, an associate professor of radiology at Harvard Medical School. The goal for Mr. Wald, who is also director of the Nuclear Magnetic Resonance Core Facility at Massachusetts General Hospital, is to strengthen the machine’s gradient, or the changes in the power of its magnetic field.
A stronger gradient, meaning wider and quicker variations in the magnetic strength, can produce sharper images, both by allowing a clearer identification of the affected protons and by seeing them better before they snap back into the alignment created by the big magnet. Yet a stronger gradient also poses a variety of challenges, primarily related to the noise and vibration caused by rapid repeated shifts in the magnetic fields, he explains.
“It’s not just the loudness,” Mr. Wald says. “It’s the mechanical forces involved, potentially tearing the thing apart.”
