Andrei N. Matlachov

Toward direct neural current imaging by resonant mechanisms at ultra-low field

A variety of techniques have been developed to noninvasively image human brain function that are central to research and clinical applications endeavoring to understand how the brain works and to detect pathology (e.g. epilepsy, schizophrenia, etc.). Current methods can be broadly divided into those that rely on hemodynamic responses as indicators of neural activity (e.g. fMRI, optical, and PET) and methods that measure neural activity directly (e.g. MEG and EEG). The approaches all suffer from poor temporal resolution, poor spatial localization, or indirectly measuring neural activity. It has been suggested that the proton spin population will be altered by neural activity resulting in a measurable effect on the NMR signal that can be imaged by MRI methods. We present here the physical basis and experimental evidence for the resonant interaction between magnetic fields such as those arising from neural activity, with the spin population in ultra-low field (microT) NMR experiments. We demonstrate through the use of current phantoms that, in the case of correlated zero-mean current distributions such as those one might expect to result from neural activity, resonant interactions will produce larger changes in the observed NMR signal than dephasing. The observed resonant interactions reported here might one day form the foundation of a new functional neuroimaging modality ultimately capable of simultaneous direct neural activity and brain anatomy tomography.

Simultaneous magnetoencephalography and SQUID detected nuclear MR in microtesla magnetic fields

A system that simultaneously measures magnetoencephalography (MEG) and nuclear magnetic resonance (NMR) signals from the human brain was designed and fabricated. A superconducting quantum interference device (SQUID) sensor coupled to a gradiometer pickup coil was used to measure the NMR and MEG signals. 1H NMR spectra with typical Larmor frequencies from 100–1000 Hz acquired simultaneously with the evoked MEG response from a stimulus to the median nerve are reported. The single SQUID gradiometer was placed approximately over the somatosensory cortex of a human subject to noninvasively record the signals. These measurements demonstrate, for the first time, the feasibility of simultaneous MRI and MEG. NMR in the microtesla regime provides narrow linewidths and the potential for high spatial resolution imaging, while SQUID sensors enable direct measurement of neuronal activity with high temporal resolution via MEG. Magn Reson Med 52:467–470, 2004.

SQUID-based simultaneous detection of NMR and biomagnetic signals at ultra-low magnetic fields

Nuclear magnetic resonance (NMR) and magnetic resonance imaging (MRI) at ultra-low magnetic fields (ULF, fields of /spl sim//spl mu/T) have several advantages over their counterparts at higher magnetic fields. These include narrow line widths, the possibility of novel imaging schemes such as T/sub 1/ weighted images, and reduced system cost and complexity. In addition, ULF NMR/MRI with superconducting quantum interference devices (SQUIDs) is compatible with simultaneous measurements of biomagnetic signals, a capability conventional systems cannot offer. SQUID-based ULF MRI has already been demonstrated, as have measurements of simultaneous MEG and NMR at ULF. In this paper we will show simultaneous magnetocardiography (MCG) and magnetomyography (MMG) with NMR are also possible. Another compelling application of NMR/MRI at ULF is the possibility of directly measuring magnetic resonance consequences of neuronal signals. In this paper we explore simultaneous MMG/NMR and MCG/NMR for an effect on the NMR signal, in T/sub 2//sup */, that might be associated with the effects of bioelectric currents.

Multi-Channel SQUID System for MEG and Ultra-Low-Field MRI

A seven-channel system capable of performing both magnetoencephalography (MEG) and ultra-low-field magnetic resonance imaging (ULF MRI) is described. The system consists of seven second-order SQUID gradiometers with 37 mm diameter and 60 mm baseline, having magnetic field resolution of 1.2-2.8 fT/radicHz . It also includes four sets of coils for 2D Fourier imaging with pre-polarization. The systems MEG performance was demonstrated by measurements of auditory evoked response. The system was also used to obtain a multi-channel 2D image of a whole human hand at the measurement field of 46 microtesla with 3 by 3 mm resolution.

Magnetic Sensors for Bioassay: HTS SQUIDs or GMRs?

In this paper we compare the detection of magnetic microparticles by HTS SQUIDs and GMR sensors. Our prototype system uses a HTS SQUID array insulated/isolated from a nonmagnetic tube in which the sample flows at room temperature. This necessarily results in a liftoff between sensor and sample in the range of 2 mm. While HTS SQUIDs typically have an intrinsic noise sensitivity that is at least two orders of magnitude better than conventional GMR sensors (~ 1 pTradicHz for washer SQUIDs compared to ~ 100 pTradicHz for the best commercial GMRs), the dipole like response of a magnetic microparticle in flow is such that this difference in noise performance can be compensated for by the reduction in standoff (order of 0.2 mm) when using a magnetoresistive sensor. Here we detail the two different approaches, present comparative results and discuss the relative merits of each setup.

Instrumentation for simultaneous detection of low field NMR and biomagnetic signals

We have built and demonstrated a simple system with open geometry that measures biomagnetic signals such as magnetoencephalogram (MEG), magnetocardiogram (MCG) and magnetomyogram (MMG) simultaneously with low field nuclear magnetic resonance (NMR) free induction decay signals (FID). The system employs LT/sub C/ SQUID gradiometers and can operate with proton Larmor frequency in the 80 Hz-10 kHz range. A pre-polarizing field of up to 60 mT is generated by resistive coils. Two different types of SQUID gradiometers were used: a tangential thin-film planar first-order gradiometer and an axial second-order gradiometer. The gradiometers were placed inside a fiberglass dewar at about 1 cm distance from a subject. All measurements were performed inside a single-layer magnetic shielded room. This system is the prototype for a system that will ultimately be capable of measuring biomagnetic signals together with magnetic resonance images (MRI).

Toward SQUID-Based Direct Measurement of Neural Currents by Nuclear Magnetic Resonance

Modern high field (HF) MRI uses magnetic fields greater than 1.5 T to yield exquisite anatomical features. We have also seen an explosion in functional MRI in the last decade that measures hemodynamic responses that are ultimately sluggish (~one sec) and only indirectly related to electrophysiological processes. Magnetoencephalography (MEG) is a direct measure of the external fields generated by neuronal currents with exquisite temporal information (less than one msec). Spatial localization, however, is inferred from modeling priors, making MEG ldquoimagingrdquo only indirect at best. Ultra low field (ULF) MRI has recently been demonstrated with 2-3 mm resolution using fields in the microtesla regime. While the nuclear magnetic resonance (NMR) signal at ULF is dramatically weaker than at HF, we acquired high signal-to-noise measurements for a variety of samples at ULF using SQUID technology. Several researchers have proposed that electrophysiological activity may interact with the nuclear spins in a volume of interest, causing measurable variations in the NMR signal. We have developed a new approach to directly measuring neuronal activity with SQUID-based ULF-NMR techniques based on the hypothesis that interactions between the spin population and neural activity in cortex can be dominated by resonant mechanisms unique to ULF. We have experimentally demonstrated the feasibility of this approach via ULF-NMR using a single-channel SQUID system.

First-Order Planar Superconducting Quantum Interference Device Gradiometers With Long Baseline

We have developed several types of low-Tc first-order planar gradiometers with long baselines ranging from 3.6 cm to 4 cm. Thin-film planar gradiometers are very attractive for measurements of the off-diagonal components of the magnetic field gradient tensor in noisy environments because of the high intrinsic balance that can be achieved with the precision photolithographic techniques used to fabricate these devices. The gradiometer pickup loops are series configured and connected to the input circuit of a dual-washer gradiometric dc SQUID. Series and parallel SQUID washer configurations have been investigated. For a given chip size, model calculations using a magnetic dipole point source and a distributed current dipole source show that gradiometer performance can be improved by increasing the size of the pickup loops at the expense of a small reduction of the baseline length. Based on these results, an improved planar gradiometer has been developed with 1.175 cm times 1.175 cm square pickup loops integrated with a series-configured dual washer SQUID on a 1.2 cm x 4.8 cm chip. The white flux noise of the improved gradiometer with 3.6 cm baseline is as low as 2.4 muPhi0/Hz1/2 (rms), and the magnetic field sensitivity referred to one pickup loop is 0.63 nT/Phi0. This results in a magnetic field gradient noise of 0.42 fT/cm - Hz1/2. The gradiometers are operable unshielded in typical laboratory environments without losing lock.

Thin-Film Planar Gradiometer with Long Baseline

Gradiometers are attractive for magnetic field measurements in noisy environments. Thin-film planar gradiometers are in particular attractive for measurements of the off-diagonal components of the magnetic field gradient tensor, and they can be fabricated with high intrinsic balance owing to the precision photolithographic techniques used to fabricate these devices. We have developed a low-Tc first-order planar gradiometer with a long baseline of 4 cm. The pickup loops are series configured and connected to the input circuit of a dual washer gradiometric dc SQUID. The white rms flux noise measured in a shielded environment is 1.6 µΦ0/Hz½. The magnetic field sensitivity referred to one pickup loop is 2 nT/Φ0, resulting in an rms magnetic field noise referred to one pickup loop of 3.2 fT/Hz½ and an rms gradient noise of 0.8 fT/cm-Hz½. Based on the lithographic methods used to fabricate the gradiometers, the expected balance level is 1 part in 24, 000 or 0.004%. The gradiometer is operable without shielding in typical laboratory environments without losing lock, with an rms white noise at 10 kHz of 3.5 µΦ0/Hz½.

Weld quality evaluation using a high-temperature SQUID array

This paper presents preliminary data for evaluating weld quality using high temperature SQUIDS. The SQUIDS are integrated into an instrument known as the SQUID Array Microscope, or SAMi. The array consists of ll SQUIDs evenly distributed over an 8.25 mm baseline. Welds are detected using SAMi by using an on board coil to induce eddy currents in a conducting sample and measuring the resulting magnetic fields. The concept is that the induced magnetic fields will differ in parts of varying weld quality. The data presented here was collected from three stainless steel parts using SAMi. Each part was either solid, included a good weld, or included a bad weld. The induced magnetic fields magnitude and phase relative to the induction signal were measured. For each sample considered, both the magnitude and phase data were measurably different than the other two samples. These results indicate that it is possible to use SAMi to evaluate weld quality.