Physicists at the National Institute of Standards and Technology, along with researchers at the German national metrology institute, say they've successfully tracked a human heartbeat with NIST's mini atom-based magnetic sensor, a development that might one day lead to readings more precise than currently possible using electrocardiograms.
The key advantage to using the magnetic signals of the heartbeat, says principal investigator John Kitching, is that they "are not affected at all while propagating from the source--the heart, say--to outside the body because the body is essentially transparent to magnetic fields." Electrical signals, on the other hand, "can be affected very strongly by fluids in the body."
But while the findings, published in Applied Physics Letters, indicate that this sensor could be used to make magnetocardiograms (MCGs), they would require powerful shielding from the Earth's magnetic field to take measurements of the comparatively weak signals of the human heart. (The study itself was conducted in a lab in Berlin that is described as having the world's best magnetic shielding to block the Earth's magnetic field from interfering with the high-precision measurements.)
The body is essentially transparent to magnetic fields
--John Kitching, principal investigator
Roughly the size of a sugar cube, the sensor--developed in 2004 as a spinoff of NIST's miniature atomic clocks--contains an inner square cell with rubidium atoms in gas form; red, black, and white electrical wires that power the cell's heaters; and a clear optical fiber that connects to a control box.
Placed just 5 millimeters above the left side of a person's chest, it detected the regular magnetic pattern of the heartbeat. When compared to readings by a superconducting quantum interference device (SQUID)--a type of supersensitive magnetometer associated with magnetic resonance imaging, neurology, oil prospecting, and other activities--it was not only just as accurate, but also showed nearly identical signal features. (SQUIDs, however, require more expensive materials and are most accurate at minus 269 degrees Celsius.)
I asked principal investigator John Kitching if (and why) an MCG would ever be used in place of, or even alongside, an electrocardiogram. He says that because the magnetic signalas detected by MCGs aren't affected by fluids in the body (as they are in ECGs), and because MCGs do not require contact with the body (so no probes attached to the chest), they will some day be the preferred device.
"With reduced system cost, we expect MCG to be used more widely. And so the possibility for reduced cost is one important feature of the technology we are developing."
Kitching admits that it may be a long way off before MCGs are developed, and that at least for the foreseeable future they will require shielded rooms. But given the success of the initial readings, further testing of the NIST sensor is already being planned at the Berlin lab.