Without electricity we would perish. We could learn to do without the flow of electrons that power VCRs and food processors, but the currents inside our bodies are vital. The brain needs electricity to issue its commands from neuron to neuron. When these signals reach a muscle, they set up a wave of electrical excitation in the fibers, which in turn triggers the chemical reactions that make the fibers contract or relax. The most important muscle is the heart; it shudders under a wave of electricity about once each second.
The heart’s electric field radiates out into the chest cavity, sending clues about the heart’s function toward the skin. Cardiologists can get a peek at the heart by taping electrodes to a person’s torso; each electrode produces a familiar squiggle on an electrocardiogram that shows how the voltage changes at that single point on the body. Cardiologists spend years learning to infer heart function from these signals and to recognize in EKG readings the telltale signs of dangerous heart conditions.
But looking at an EKG is like watching a hurricane from a porthole. The 6 to 12 electrodes on a patient’s torso and limbs provide only limited information about the heart, from which doctors draw inferences based on 80 years’ worth of experience with EKGs. So cardiologists also use isotope imaging, echocardiograms, and other methods to gather data. In rare cases, surgeons even have to open the chest just to find the cardiac cells that are misbehaving.
Chris Johnson wants to figure out how to see a heart without cracking open a rib cage. This University of Utah computer scientist has been working with physiologists, surgeons, and other researchers there to visualize the electric field created by a heartbeat. Ultimately Johnson hopes that he will help create an entirely new diagnostic tool--to let cardiologists, as it were, watch the hurricane from space.
Johnson has been developing his methods for making these calculations since 1985, when he was a graduate student. Last year, when he was ready to use some real heart measurements, he wanted to get them from a heart with clear electrical abnormalities. A condition known as Wolff- Parkinson-White syndrome was a perfect candidate. This birth defect gives its victims an extra patch of cardiac cells that sometimes sets off the contraction of the ventricles--the main pumping chambers--too quickly, throwing off the heart’s rhythm. While the condition is not difficult to diagnose, the offending patch of cells is hard to locate. Today’s treatment calls for cardiologists to put a catheter into a patient’s chest and grope around for four or more hours until they find the patch and then remove it. Two years ago, however, surgery was still common for this condition. To measure the electrical activity of the heart, surgeons would put a nylon sock covered with electrodes over the heart’s ventricles and measure the voltage on its surface.
Johnson got hold of a set of these measurements and put them into his computer. All he needed was a body to carry the heart, because the shape of the chest influences the way the heart’s electric field reaches the skin.
One of the professors’ sons happened to be around one day looking hungry, Johnson says, so we offered him a free lunch if he would lie in this magnetic resonance imaging chamber for several hours. Radiologists use MRI to measure the magnetic fields of different kinds of tissue and convert those measurements into images of slices of the body. Johnson made 200 scans of the man’s torso and turned them into a three- dimensional image in his computer. As the pictures in this photo-essay show, he was able to combine the heart and the torso to create remarkable views of both the interior and exterior of the body.
Johnson will be measuring more people’s heartbeats in the near future; a Utah colleague is now developing a nylon sock that will fit over the entire heart. Armed with the electrical information from the sock and his computer torso, he’ll be able to watch as the heart’s activity shows up as voltage changes on the skin. As he generates more pictures of body surfaces, he will learn how to reverse mathematically the entire process-- to infer the electric field on the heart from the way it appears on the skin, eliminating the need for the sock. With this information in hand, a cardiologist will be able not only to diagnose Wolff-Parkinson-White syndrome but to locate the errant cells as well. But it’s a tricky problem. The body acts as a kind of filter, smoothing things out, says Johnson. When there are two centers of positive voltage on the surface of the heart, for example, the skin only has one.
Johnson foresees a time when patients with heart trouble could put on an electrode jacket to measure the voltages on their skin. The ideal thing would be to measure the voltage on their body, he says, zip them through a magnetic resonance imager, and within the afternoon calculate the voltage on the heart itself, pop it up on the computer, and say, ‘Oh, well, here’s the abnormality. We can change this with drug therapy or use a catheter.’
For now, though, Johnson is trying to perfect the computer program, incorporating complicated features such as the large arteries around the heart. And since the size and shape of a patient affect the electric currents, he’s learning how to stretch his standard model to fit the infinite variety of human geometry. In four or five years we’re going to use a real patient, localize the abnormality, tell the surgeons where it is, and they’ll go and find it, says Johnson, and then they’ll tell us how well we did.