Warm detectors look at brain magnetism

The human brain generates magnetic fields 100 times weaker than those of the heart. Such fields can reveal subtle medical clues, different from those provided by electric fields, about such maladies as epilepsy and arrhythmias. Sensitive magnetometers, based on superconducting quantum interference devices (SQUIDs), are used to acquire detailed maps of the brain’s feeble fields, but those devices require liquid helium and all the associated cryogenic equipment. Michael Romalis (Princeton University) and his team have now used an atomic magnetometer to detect the brain’s faint magnetic fields without cryogenics. The core of the device is a heated cell filled with potassium atoms that have been polarized by a laser beam (see Physics Today, July 2003, page 21 ). At near-zero ambient fields and with a volunteer’s head resting on an insulating pad atop the device, the physicists monitored the atomic polarization. They could optically detect when and how much the K atoms precessed due to the brain’s magnetic-field activity when the subject was exposed to auditory stimuli. The device has already attained a sensitivity of 3.5 fT/Hz^1/2, comparable to that for SQUIDs, with the prospect of improving by another factor of 10. Romalis says that four cells can provide almost complete head coverage, which would create new options for functional brain imaging. (H. Xia et al., Appl. Phys. Lett. 89 , 211104, 2006 .)