We have performed single-and two-channel high transition temperature (high-T c ) superconducting quantum interference device (SQUID) magnetoencephalography (MEG) recordings of spontaneous brain activity in two healthy human subjects. We demonstrate modulation of two well-known brain rhythms: the occipital alpha rhythm and the mu rhythm found in the motor cortex. We further show that despite higher noise-levels compared to their low-T c counterparts, high-T c SQUIDs can be used to detect and record physiologically relevant brain rhythms with comparable signal-to-noise ratios. These results indicate the utility of high-T c technology in MEG recordings of a broader range of brain activity. V C 2012 American Institute of Physics. [http://dx.doi.org/10.1063/1.3698152]The first magnetic recordings of human brain activity were made with an induction coil 1 and led to a significant leap in neuroscience research. A few years later, the invention of the low transition temperature (low-T c ) superconducting quantum interference device (SQUID) revolutionized the field by improving the sensitivity of magnetic recordings by orders of magnitude. 2 In modern magnetoencephalography (MEG) systems, hundreds of low-T c SQUID sensors are enclosed in a helmet that surrounds the subject's head and map the magnetic field emanating from the brain. Low-T c SQUIDs are preferred because of their high fabrication yield and superior noise performance. A typical noise figure for such a SQUID is 1-5 fT/HHz at 10 Hz, 3,4 roughly one order of magnitude better than a similar high-T c device. However, in order to keep the low-T c SQUIDs operating at 4 K, thermal insulation limits the separation between the cold sensors and the room temperature environment to 18 mm at best (Elekta, Neuromag V R ). The possibility to operate high-T c SQUIDs at 77 K has enabled some researchers to reduce this distance to just a few hundred microns. [5][6][7] The first MEG recordings with high-T c SQUIDs were accomplished some years after the discovery of high-T c superconductivity when Zhang et al. 8 recorded the brain's response to auditory stimuli in 1993. Similar studies have proven high-T c technology is sensitive enough to record such well-understood evoked MEG sources by averaging hundreds or thousands of stimulus-response signals. 9-11 However, recordings of spontaneous brain activity have yet to be demonstrated, perhaps because many spontaneous rhythms present themselves at frequencies below 20 Hz, where 1/f noise can be problematic for high-T c SQUID technology. Furthermore, it is not possible to average spontaneous brain activity in the time-domain due to its inherently spontaneous nature.Herein, we present MEG recordings of spontaneous brain activity in humans with single-and two-channel high-T c SQUID magnetometer systems. We demonstrate time resolved modulation of the occipital alpha rhythm via visual stimulation as well as modulation of the mu rhythm in the motor cortex via muscle activation, both of which are wellcharacterized phenomena and are present ...

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Eddy currents induced by applied magnetic-field pulses have been a common issue in ultra-low-field magnetic resonance imaging. In particular, a relatively large prepolarizing field-applied before each signal acquisition sequence to increase the signal-induces currents in the walls of the surrounding conductive shielded room. The magnetic-field transient generated by the eddy currents may cause severe image distortions and signal loss, especially with the large prepolarizing coils designed for in vivo imaging. We derive a theory of eddy currents in thin conducting structures and enclosures to provide intuitive understanding and efficient computations. We present detailed measurements of the eddy-current patterns and their time evolution in a previous-generation shielded room. The analysis led to the design and construction of a new shielded room with symmetrically placed 1.6-mm-thick aluminum sheets that were weakly coupled electrically. The currents flowing around the entire room were heavily damped, resulting in a decay time constant of about 6 ms for both the measured and computed field transients. The measured eddy-current vector maps were in excellent agreement with predictions based on the theory, suggesting that both the experimental methods and the theory were successful and could be applied to a wide variety of thin conducting structures. V C 2014 AIP Publishing LLC.