Minisensor Can Detect the Signature of a Human Heartbeat

Researchers at the National Institute of Standards and Technology (NIST; Gaithersburg, MD) and the National Metrology Institute of Germany (PTB; Braunschweig) have used NIST's miniature atom-based magnetic sensor to successfully track a human heartbeat, confirming that the sensor has potential biomedical applications.

Described in Applied Physics Letters, the study focusing on this sensor is the first to be performed under conditions resembling a clinical setting. The sensor--a tiny enclosure containing about 100 billion rubidium atoms in gas form, a low-power infrared laser, and optics--measured the heart's magnetic signature in picoteslas (trillionths of a tesla). The tesla is the unit that defines magnetic field strength. For comparison, the Earth's magnetic field is a million times stronger than a heartbeat, and an MRI machine uses fields several million times stronger still.

Placed 5 mm above the left chest of a person lying face up on a bed during the tests at PTB, the sensor was able to detect the weak but regular magnetic pattern of the heartbeat. The same signals were recorded using a superconducting quantum interference device (SQUID), the "gold standard" for magnetic measurements. A comparison of the signals confirmed that the NIST minisensor correctly measured the heartbeat and identified many typical signal features. While it generates more noise in the signal than SQUIDs, the sensor has the advantage of operating at room temperature, whereas SQUIDs work best at -269°C and require a more-complicated and more-expensive supporting apparatus.

A spin-off of NIST's miniature atomic clocks, these minisensors were first developed in 2004. Recently, they were packaged with fiber optics for detecting the light signals that register magnetic field strength. In addition, their control system has been reduced in size, so that the entire apparatus can be transported easily to other laboratories.

The new results suggest that the sensors could be used to make magnetocardiograms, a supplement or alternative to electrocardiograms. The study also demonstrated for the first time that atomic magnetometers can offer sensing stability lasting tens of seconds, the amount of time required for an emerging technique called magnetorelaxometry (MRX), which measures the magnetization decay of magnetic nanoparticles. MRX is used to localize, quantify, and image magnetic nanoparticles inserted into biological tissue for such medical applications as targeted drug delivery.