Antti Ahonen

122-channel squid instrument for investigating the magnetic signals from the human brain

A 122-channel d.c. SQUID magnetometer with a helmet-shaped detector array covering the subjects head has been operational in the Low Temperature Laboratory of the Helsinki University of Technology since June 1992. The new system allows simultaneous recording of magnetic activity all over the head. The probe employs 122 planar first-order thin-film gradiometers in dual units with two exactly orthogonal channels at 61 measurement sites. The performance of the device is analyzed and compared with more conventional axial gradiometer arrays by considering signal-to-noise ratios, spatial sampling theory, confidence intervals for the estimated equivalent current dipole positions, and information-theoretical channel capacity. The signal-to-noise ratio and the resolution of the planar and axial arrays with the same number of channels are found practically equal. The number of channels and their spacing in our new Neuromag-122 system are found fully adequate for neuromagnetic measurements. An example of whole cortex recordings of auditory evoked brain activity is presented and analyzed.

Activation of the human posterior parietal cortex by median nerve stimulation

We recorded somatosensory evoked magnetic fields from ten healthy, right-handed subjects with a 122-channel whole-scalp SQUID magnetometer. The stimuli, exceeding the motor threshold, were delivered alternately to the left and right median nerves at the wrists, with interstimulus intervals of 1, 3, and 5 s. The first responses, peaking around 20 and 35 ms, were explained by activation of the contralateral primary somatosensory cortex (SI) hand area. All subjects showed additional deflections which peaked after 85 ms; the source locations agreed with the sites of the secondary somatosensory cortices (SII) in both hemispheres. The SII responses were typically stronger in the left than the right hemisphere. All subjects had an additional source, not previously reported in human evoked response data, in the contralateral parietal cortex. This source was posterior and medial to the SI hand area, and evidently in the wall of the postcentral sulcus. It was most active at 70–110 ms.

Visual cortex activation in blind humans during sound discrimination

We used a whole-scalp magnetometer with 122 planar gradiometers to study the activity of the visual cortex of five blind humans deprived of visual input since early infancy. Magnetic responses were recorded to pitch changes in a sound sequence when the subjects were either counting these changes or ignoring the stimuli. In two of the blind subjects, magnetic resonance images were also obtained, showing normal visual cortex macroanatomy. In these subjects, the magnetic responses to counted pitch changes were located at visual and temporal cortices whereas ignored pitch changes activated the temporal cortices almost exclusively. Also in two of the other three blind, the visual-cortex activation was detectable in the auditory counting task. Our results suggest that the visual cortex of blind humans can participate in auditory discrimination.

Auditory evoked magnetic fields to tones and pseudowords in healthy children and adults.

Neuromagnetic responses to tones and pseudowords were measured with a 24-channel magnetometer in nine adults and in 23 children, the latter aged 0.3–15 years. Both stimulus types elicited substantially similar responses in all subjects. At 0.9-s interstimulus interval (ISI), the adult response was a stable P1 m-N1m-P2m-N2m sequence peaking at 50, 100, 200, and 250 ms, respectively. A biphasic Plm-N1,2m response with peaks at 100 and 260 ms occurred in children up to 12 years of age. At longer ISIs (1.2–2.4 s), an adult-type N1m response appeared in most children. N1 m amplitude suppression at short ISIs was stronger in children than in adults and may reflect a longer refractory period of the N1 generator neurons during early childhood than later in life. Peak latencies of PI m, N1 m, and N2m decreased with age, most rapidly <7 years of age. All deflections originated in nearby cortical areas within the posterior sylvian fissure, and may serve as functional landmarks for that anatomic area.

Large-area low-noise seven-channel dc SQUID magnetometer for brain research

The design, construction, and performance of a new high‐sensitivity dc SQUID magnetometer, covering a circular area of 93‐mm diameter, is described. The device, now used routinely in our brain research, comprises seven asymmetric first‐order gradiometers, located on a spherical surface of 125‐mm radius and with the symmetry axis tilted 30° with respect to the vertical. The pickup coil diameter is 20 mm, and the channels are separated by 36.5 mm from each other in a hexagonal array. The overall field sensitivity of the system, measured inside our magnetically shielded room, is 5 fT/(Hz)1/2, mainly limited by the thermal noise in the radiation shields of the Dewar. The optimization of the coil configuration and the measurement system is discussed in detail, and a system to determine automatically the position and orientation of the Dewar with respect to certain fixed points on the subject’s head is described. Finally, some examples of measurements carried out with the new device are given.

Sampling theory for neuromagnetic detector arrays

The sampling theorem for wave-number-limited multivariable functions is applied to the problem of neuromagnetic field mapping. The wave-number spectrum and other relevant properties of these fields are estimated. A theory is derived for reconstructing neuromagnetic fields from measurements using sensor arrays which sample either the field component B/sub z/ perpendicular to the planar grid of measurement points, or the two components partial B/sub z//partial x and partial B/sub z//partial y of its gradient in the xy plane. The maximum sensor spacing consistent with a unique reconstruction is determined for both cases. It is shown that, when two orthogonal components of the gradient are measured at every site of the measurement grid, the density of these sensor-pair units can be reduced, without risk of aliasing, to half of what is necessary for single-channel sensors in an array sampling B/sub z/ alone. Thus the planar and axial gradiometer arrays are equivalent in the sampling sense provided that the number of independent measurements per unit area is equal.<<ETX>>

A 122-channel whole-cortex SQUID system for measuring the brain's magnetic fields

A 122-channel neuromagnetometer with a helmet-shaped detector array covering the entire head allows simultaneous recording of magnetic fields over the whole cortex. The instrument has 122 planar first-order gradiometers in dual units at 61 measurement sites. The SQUIDs are directly coupled to the read-out electronics, with amplifier noise cancellation to eliminate the need for separate preamplifiers inside the magnetically shielded room. The authors analyze the performance of the device and compare it with traditional axial gradiometer arrays by considering signal-to-noise ratios, spatial sampling theory, confidence intervals for equivalent current dipole fits, and information-theoretical channel capacity. The analysis includes the fact that instrument noise is smaller than the background activity of the brain; the signal-to-noise ratio and the resolution of the planar array are in that case equal to or better than that of an axial array. The number of channels and their spacing are very suitable for neuromagnetic measurements. >

NMR experiments on the superfluid phases of3He in restricted geometries

We report transverse cw NMR measurements on the superfluid phases of3He at temperatures between 3 and 0.7 mK. Nuclear demagnetization of copper was used for refrigeration. For thermometry we employed pulsed NMR on platinum powder immersed in the liquid. The measurements on3He were carried out in two NMR coil assemblies in which the liquid was confined between parallel Mylar foils with separations of 0.37 mm and 4 µm. The transition temperatureTcwas measured at pressures between 32 bars and the saturated vapor pressure; a pressure-independent increase of ∼11% was observed inTcwith respect to earlier data obtained with the same apparatus. We found that our temperature scale is not proportional to that used in La Jolla. In the 4-µm stack we observe a reduction in the B → A transition temperature. In our measurements on the orientational anisotropy of the B phase we found qualitative agreement with the theory of Brinkman et al. We also measured the longitudinal resonance frequencies of the A and B phases between 32 bars and the polycritical point. In the 4-µm stack we found a negative NMR shift in the A phase when the field was oriented perpendicular to the Mylar plates, in agreement with the prediction of Takagi. The static susceptibility XB of the B phase was measured as a function of temperature at 18.7 and 29 bars; its low-temperature limiting value was observed to be (0.33±0.02)XN, independent of pressure. We use our data to estimate the strong coupling corrections to the size of the energy gap. The initial slope of the reduced gap in the A phase, ΔA/Tc, was found to increase by ∼25% when the pressure increased from 21.1 bars to the melting curve, whereas in the low-temperature limit ΔB(0)/Tcwas found to be independent of pressure and close to its weak coupling value.

Hybrid ultra-low-field MRI and magnetoencephalography system based on a commercial whole-head neuromagnetometer

Ultra‐low‐field MRI uses microtesla fields for signal encoding and sensitive superconducting quantum interference devices for signal detection. Similarly, modern magnetoencephalography (MEG) systems use arrays comprising hundreds of superconducting quantum interference device channels to measure the magnetic field generated by neuronal activity. In this article, hybrid MEG‐MRI instrumentation based on a commercial whole‐head MEG device is described. The combination of ultra‐low‐field MRI and MEG in a single device is expected to significantly reduce coregistration errors between the two modalities, to simplify MEG analysis, and to improve MEG localization accuracy. The sensor solutions, MRI coils (including a superconducting polarizing coil), an optimized pulse sequence, and a reconstruction method suitable for hybrid MEG‐MRI measurements are described. The performance of the device is demonstrated by presenting ultra‐low‐field‐MR images and MEG recordings that are compared with data obtained with a 3T scanner and a commercial MEG device. Magn Reson Med, 2013.

Effects on DC SQUID characteristics of damping of input coil resonances

The possibility of improving dc SQUID performance by damping the input circuit resonances caused by parasitic capacitances is studied experimentally. A high-quality dc SQUID was coupled to a first-order axial gradiometer built for neuromagnetic research, and a resistor-capacitor shunt was connected in parallel with the input coil of the SQUID. Ten differentRC shunts were studied with the SQUID operating in a flux-locked loop, carefully shielded against external disturbances. It was found that increasing the shunt resistance resulted in smoother flux-voltage characteristics and smaller noise. At best, the minimum obtainable equivalent flux noise level was one-fourth that for the unshunted SQUID. The noise level is a function of the shunt resistanceRsonly, except for shunt capacitance values bringing the low-frequency resonance of the input coil close to the flux modulation frequency. At a constant bias current level, where the amplitude of the flux-voltage characteristics is at maximum, the equivalent flux noise varies asRs/−0.7. The results agree reasonably well with recently published predictions based on numerical simulations where the whole input circuit with parasitic capacitances was taken into account.