Simultaneous magnetoencephalography and SQUID detected nuclear MR in microtesla magnetic fields

Abstract

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. 1 H 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. In this article we describe an instrument for and results from simultaneous recording of an evoked magnetoencephalography (MEG) response and nuclear magnetic resonance (NMR) proton signal from the human brain using a superconducting quantum interference device (SQUID) sensor.MEG is a noninvasive technique that measures magnetic fields at the surface of the head that are the direct consequence of neuronal activity in the living brain (1). MEG requires the use of SQUID sensors to measure the extraordinarily low-level magnetic fields, usually in the range from 10 -15 to 10 -12 T, produced by neuronal activity in the brain. While other functional imaging modalities such as functional MRI depend on the relatively slow and indirect hemodynamic response of the brain, MEG (and electroencephalography, or EEG) can provide measurement of the electromagnetic fields arising from the actual neuronal currents with submillisecond temporal resolution.The inverse problem of source localization from MEG (and EEG) is ill posed. These procedures require models of source current distribution and of the effects of geometry and inhomogeneous conductivity within the head volume conductor. Neural electromagnetic data are often combined with anatomical or functional tomographic data obtained with high-field MRI and functional MRI (fMRI).MEG and MRI data are taken with two completely separate systems because the SQUIDs cannot operate in the large magnetic fields (up to 15 orders of magnitude larger than an MEG signal) required by high-field MRI systems. This necessitates colocalization and data fusion techniques to integrate and superpose the two data sets in an accurate and meaningful way.Recording NMR signals in low fields (B Ͻ 10 -5 T) opens up the possibility of acquiring tomographic images simultaneously with high temporal resolution measurements of MEG, as the magnetic fields required for imaging are now compatible with SQUID sensors. An additional motivation for low-field NMR spectroscopy and MRI is the prospect of reduced system cost and size. For a fixed relative inhomogeneity, broadening of the NMR line scales linearly with the strength of the measurement field, providing very narrow NMR lines (limited by the natural linewidth) at ultralow magnetic fields, and raising the possibility of MRI at lower gradients. Susceptibility artifacts caused by spurious field gradients from the sample, which also broaden resonance lines, are si...

Parallel MRI at microtesla fields

“…It has been demonstrated by our group that ULF NMR signals, generated inside a human body, can be measured simultaneously with MEG [21] or MCG [22] signals using the same SQUID sensor.…”

Stimulus-induced Rotary Saturation (SIRS): A potential method for the detection of neuronal currents with MRI

“…An extremely low B 0 measurement field allows resonant interactions between the neuronal currents and spin magnetization if γB 0 is brought into the frequency range of the neuronal currents. Although their first measurements were at significantly higher fields, Kraus and colleagues proposed static measurement fields weaker than 0.025 Gauss using a SQUID magnetometer to detect the MR signals at a Larmor frequency of less than 100 Hz Volegov et al, 2004). In this scenario, the transient neuronal fields can possibly act as resonant excitation pulse providing the initial excitation of the proton magnetization.…”

Correction of concomitant gradient artifacts in experimental microtesla MRI

“…These pulse sequences can supplement or replace the software correction algorithm described above and must be tailored to fit specific applications. For example, consider the proposals [4,26] to combine MSI of the brain with MRI by detecting MR signals with the array of SQUIDs used to measure magnetic fields in MSI equipment. Because neuronal currents in the brain change on a timescale of 1 ms or slower, the bandwidth of existing MSI systems is typically 1-2 kHz, thereby limiting the potential MRI readout frequency.…”