MONITOR
Quantum leap for medicine
Jun 20th 2002
From The Economist print edition
Quantum effects used in devices called SQUIDs are providing a powerful new tool for diagnosing ailments from the faint magnetic signatures they create
PHYSICISTS and physicians used to do quite different things. Physicists dealt with esoteric formulae and equations; physicians poked and listened to the wheezes of sick people. Today, however, medics must be physicists as well, to understand the workings of medical devices such as magnetic resonance imaging (MRI) machines, which rely on manipulations of individual hydrogen atoms to produce images of the body. Now, a new breed of diagnostic instrument that relies on superconductivity and quantum mechanics is being added to the doctor's bag.
The new high-tech medical tool, which goes by the name of “superconducting quantum interference device” (SQUID), relies on loops of superconducting wire chilled to the temperature of liquid nitrogen (-196°C) or liquid helium (-270°C). In the ordinary world, a current in such a loop can flow in only one direction at once, either clockwise or anti-clockwise. But in the world of quantum mechanics, current can flow in both directions simultaneously. This makes the SQUID sensitive to extremely weak magnetic fields, as small as those produced by the body—ie, a millionth to a billionth of the strength of the earth's own magnetic field.
While physicists have been using SQUIDs for some time to perform laboratory experiments, doctors have only just begun to get their hands on them. From what they have seen, they are keen for more. That is because, just as heart attacks or epilepsy carry electrical signatures, other ailments have magnetic fingerprints. But until the advent of the SQUID, it was impossible to detect minute magnetic signals that can provide clues to a patient's complaint.
The first patients to benefit from medical SQUIDs were those suffering from haemochromotosis, an ailment caused by excess iron in the blood. The extra iron accumulates in the liver, and can reach such a toxic level that it causes a heart attack or permanent liver damage. The best way to diagnose this disorder now is to extract a bit of tissue from the liver and check it for excess iron—a procedure that is both painful and potentially dangerous. Another way is painstakingly to analyse a large volume of the patient's blood. A SQUID-based sensor can provide a rapid and non-invasive (ie, painless) alternative to both methods.
Tristan Technologies of San Diego, California, was the first to build prototypes of SQUID-based liver sensors. More than 1,000 patients have already had the concentration of iron in their livers measured by Tristan machines in Italy and Germany. Tristan is now seeking approval to market the device in America.
Meanwhile, engineers at Hitachi in Japan have developed a device that uses SQUIDs to detect fluctuations in the magnetic fields produced by the heart. A heartbeat is produced by electrical current generated by differences in the concentration of various ions. As with all currents, there is an associated magnetic field. Hitachi's “magnetocardiogram” detects this minute magnetic signal and uses it to generate an image of the corresponding electrical waveform.
The Hitachi prototype gathers 64 channels of data and can monitor the heart, one beat at a time. Because of its sensitivity, the new magnetocardiogram is expected to help doctors make faster diagnoses of dangerous conditions such as arrhythmia and ischemic heart disease. It could also help obstetricians to monitor the beating hearts of fetuses in the mother's womb.
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