An epileptic seizure is the outward sign of an electrical storm in the brain, a sudden surge of uncontrolled electric currents. If neurosurgeons can pinpoint the damaged brain tissue that sparks the storm, they can remove it, potentially sparing a patient a lifetime of debilitating attacks and antiseizure medications. But zeroing in on the precise bits of defective gray matter using the scalp electrodes of a standard electroencephalograph (EEG) machine is difficult, because electrical fields generated in the brain “get spread out and distorted” as they pass through the skull, says William Sutherling, a neuroimaging expert at the nonprofit Huntington Medical Research Institutes (HMRI) in Pasadena, CA. So Sutherling looks as well to the magnetic fields generated by each electrical impulse in the brain; those pass through the skull virtually unaffected. Using one of only a few dozen magnetoencephalography (MEG) machines in the world, Sutherling is measuring the vanishingly faint magnetic fluctuations generated by epilepsy sufferers’ brains, and combining that data with 3-D information from magnetic resonance imaging (MRI). His hope is to prove that the method is a reliable, practical way to narrow and delimit the sources of seizures, so that surgeons can remove the offending tissue without damaging the healthy, functioning cells around it. This fall, Sutherling gave TR senior editor Wade Roush a tour of HMRI’s $2.5 million MEG facility and demonstrated how his team gathers data from the hopeful patients who venture into his chamber.
1. A Room with No View. Sutherling strides into the MEG chamber, a magnetically shielded room-within-a-room with an interior floorspace of about 10 square meters. The room houses HMRI’s whole-head MEG unit – so named because its array of internal sensors fits snugly over a patient’s head like a giant hair dryer. The sensors detect even the tiniest changes in any magnetic fields threading through them. Such fields are normally all around us, so tracing fluctuations to specific areas of the brain with millimeter-scale accuracy would be impossible unless the sea of ambient magnetic waves generated by fluorescent lights, computers, power lines, and the earth itself – not to mention the nearby MRI machine, which is essentially a giant magnet – were kept out.
2. Magic Metal. The secret to shielding the MEG unit, Sutherling explains, is a thin layer of an alloy called “mu metal,” visible between the blue exterior pane and the layer of white foam on the edge of the room’s bank-vault-like door. Once the door is closed, exterior magnetic fields flow around the mu metal cage, leaving the interior magnetically silent.
3-4. Head Cold. Superconductivity is the key to the MEG unit’s exquisite sensitivity. At the core of HMRI’s unit, built by VSM MedTech of Coquitlam, British Columbia, is an array of small metal rings called superconducting quantum interference devices, or SQUIDs, which look much like the suckers of an actual squid’s tentacles (3). When a ring is cooled to temperatures just above absolute zero, it becomes a superconductor, meaning that an electrical current traveling around it encounters virtually no resistance and could, in principle, keep circling forever. That current, in turn, produces a magnetic field. “If a magnetic field spreads through from the brain, it opposes the magnetic field already in that ring,” says Sutherling. “And the ring reacts by trying to keep the total current going through that ring the same.” Any new current induced in the ring causes a change in voltage that can be amplified thousands of times over and precisely measured by electronics. To stay superconducting, the SQUIDs must reside inside a huge flask of liquid helium, Sutherling says, touching the ungainly – and frosty – apparatus that fits around the patient’s head (4).