Microtesla MRI of the human brain combined with MEG

One of the challenges in functional brain imaging is integration of complementary imaging modalities, such as magnetoencephalography (MEG) and functional magnetic resonance imaging (fMRI). MEG, which uses highly sensitive superconducting quantum interference devices (SQUIDs) to directly measure magnetic fields of neuronal currents, cannot be combined with conventional high-field MRI in a single instrument. Indirect matching of MEG and MRI data leads to significant co-registration errors. A recently proposed imaging method - SQUID-based microtesla MRI - can be naturally combined with MEG in the same system to directly provide structural maps for MEG-localized sources. It enables easy and accurate integration of MEG and MRI/fMRI, because microtesla MR images can be precisely matched to structural images provided by high-field MRI and other techniques. Here we report the first images of the human brain by microtesla MRI, together with auditory MEG (functional) data, recorded using the same seven-channel SQUID system during the same imaging session. The images were acquired at 46 microtesla measurement field with pre-polarization at 30 mT. We also estimated transverse relaxation times for different tissues at microtesla fields. Our results demonstrate feasibility and potential of human brain imaging by microtesla MRI. They also show that two new types of imaging equipment - low-cost systems for anatomical MRI of the human brain at microtesla fields, and more advanced instruments for combined functional (MEG) and structural (microtesla MRI) brain imaging - are practical.Comment: 8 pages, 5 figures - accepted by JM

Neural magnetic field dependent fMRI toward direct functional connectivity measurements: A phantom study

Recently, the main issue in neuroscience has been the imaging of the functional connectivity in the brain. No modality that can measure functional connectivity directly, however, has been developed yet. Here, we show the novel MRI sequence, called the partial spinlock sequence toward direct measurements of functional connectivity. This study investigates a probable measurement of phase differences directly associated with functional connectivity. By employing partial spinlock imaging, the neural magnetic field might influence the magnetic resonance signals. Using simulation and phantom studies to model the neural magnetic fields, we showed that magnetic resonance signals vary depending on the phase of an externally applied oscillating magnetic field with non-right flip angles. These results suggest that the partial spinlock sequence is a promising modality for functional connectivity measurements

Methods for functional brain imaging

Thesis (Ph. D.)--Harvard-MIT Division of Health Sciences and Technology, 2011.Cataloged from PDF version of thesis.Includes bibliographical references.Magnetic resonance imaging (MRI) has demonstrated the potential for non-invasive mapping of structure and function (fMRI) in the human brain. In this thesis, we propose a series of methodological developments towards improved fMRI of auditory processes. First, the inefficiency of standard fMRI that acquires brain volumes one slice at a time is addressed. The proposed single-shot method is capable, for the first time, of imaging the entire brain in a single-acquisition while still maintaining adequate spatial resolution for fMRI. This method dramatically increases the temporal resolution of fMRI (20 fold) and improves sampling efficiency as well as the ability to discriminate against detrimental physiological noise. To accomplish this it exploits highly accelerated parallel imaging techniques and MRI signal detection with a large number of coil elements. We then address a major problem in the application of fMVIRI to auditory studies. In standard fMRI, loud acoustic noise is generated by the rapid switching of the gradient magnetic fields required for image encoding, which interferes with auditory stimuli and enforces inefficient and slow sampling strategies. We demonstrate a fMRI method that uses parallel imaging and redesigned gradient waveforms to both minimize and slow down the gradient switching to substantially reduce acoustic noise while still enabling rapid acquisitions for fMRI. Conventional fMRI is based on a hemodynamic response that is secondary to the underlying neuronal activation. In the final contribution of this thesis, a novel image contrast is introduced that is aimed at the direct observation of neuronal magnetic fields associated with functional activation. Early feasibility studies indicate that the imaging is sensitive to oscillating magnetic fields at amplitudes similar to those observed by magnetoencephalography.by Thomas Witzel.Ph.D

Advanced functional MRI

Abstract Functional Magnetic Resonance Imaging (fMRI) has been widely used to study the responses of the somatosensory cortex, motor cortex and associated neuronal activity in the human cerebral cortex. fMRI is a non-invasive and indirect method for mapping brain activity through measurement of the hemodynamic responses associated with electrical neuronal activity and the neural activity leads directly to changes in blood flow, blood volume and the cerebral metabolic rate of oxygen consumption. Non-invasive neuroimaging technologies such as functional MRI have both advantages, such as good spatial resolution, and disadvantages, such as poor temporal resolution. Some of the disadvantages have been alleviated by incorporating other techniques such as optical spectroscopy or electroencephalography (EEG) which are also non-invasive. All these techniques are sensitive to the vascular response of neuronal activity but in addition we are now investigating the existence of a weak direct electromagnetic effect with advanced fMRI. This neuronal current effect which gives rise to main magnetic field modulation should provide additional information for studying nerve characteristics. In this thesis, methods for fMRI mapping of responses from phantoms, the median nerve, the visual system, the motor sensory cortex and the thalamus are optimised and subsequently quantified. The experimental results strongly support the main hypothesis of the thesis and suggest that the generated magnetic field due to ionic current can be detected by present generation MRI using specific experimental designs and stimulation paradigms. Overall our results show that ionic currents in subjects can generate percentage signal changes in MRI up to 0.1± 0.01% corresponding to mean magnetic axonal fields of 0.7± 0.1nT with a Signal to Noise Ratio (SNR) of 3:1. The responses of the median nerve, motor sensory cortex and thalamus were detected using transcutaneous electrical nerve stimulation (TENS) and the visual cortex using strobe light stimulation in the range of frequencies 2.1 Hz to 4.1 Hz. All these measurements were acquired at 1.5T. Fast fMRI experiments using TENS and finger tapping were also acquired simultaneously. In addition, real and imaginary finger tapping experiments were performed in the motor sensory cortex at 3T. Our results imply that axonal fields that are generated due to action potentials can generate effects on MRI sensitive enough to directly detect neuronal activity using advanced fMRI, although sensitivity is still not fully adequate for clinical use