Robert H. Kraus

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 microT 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.

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.

Parallel MRI at microtesla fields

Parallel imaging techniques have been widely used in high-field magnetic resonance imaging (MRI). Multiple receiver coils have been shown to improve image quality and allow accelerated image acquisition. Magnetic resonance imaging at ultra-low fields (ULF MRI) is a new imaging approach that uses SQUID (superconducting quantum interference device) sensors to measure the spatially encoded precession of pre-polarized nuclear spin populations at microtesla-range measurement fields. In this work, parallel imaging at microtesla fields is systematically studied for the first time. A seven-channel SQUID system, designed for both ULF MRI and magnetoencephalography (MEG), is used to acquire 3D images of a human hand, as well as 2D images of a large water phantom. The imaging is performed at 46 mu T measurement field with pre-polarization at 40 mT. It is shown how the use of seven channels increases imaging field of view and improves signal-to-noise ratio for the hand images. A simple procedure for approximate correction of concomitant gradient artifacts is described. Noise propagation is analyzed experimentally, and the main source of correlated noise is identified. Accelerated imaging based on one-dimensional undersampling and 1D SENSE (sensitivity encoding) image reconstruction is studied in the case of the 2D phantom. Actual threefold imaging acceleration in comparison to single-average fully encoded Fourier imaging is demonstrated. These results show that parallel imaging methods are efficient in ULF MRI, and that imaging performance of SQUID-based instruments improves substantially as the number of channels is increased.

SQUID-based instrumentation for ultralow-field MRI

Magnetic resonance imaging at ultralow fields (ULF MRI) is a promising new imaging method that uses SQUID sensors to measure the spatially encoded precession of pre-polarized nuclear spin populations at a microtesla-range measurement field. In this work, a seven-channel SQUID system designed for simultaneous 3D ULF MRI and magnetoencephalography (MEG) is described. The system includes seven second-order SQUID gradiometers characterized by magnetic field resolutions of 1.2–2.8 fT Hz−1/2. It is also equipped with five sets of coils for 3D Fourier imaging with pre-polarization. Essential technical details of the design are discussed. The systems ULF MRI performance is demonstrated by multi-channel 3D images of a preserved sheep brain acquired at 46 µT measurement field with pre-polarization at 40 mT. The imaging resolution is 2.5 mm × 2.5 mm × 5 mm. The ULF MRI images are compared to images of the same brain acquired using conventional high-field MRI. Different ways to improve imaging SNR are discussed.

MRI with an atomic magnetometer suitable for practical imaging applications

Conventionally implemented MRI is performed in a strong magnetic field, typically >1T. The high fields, however, can lead to many limitations. To overcome these limitations, ultra-low field (ULF) [or microtesla] MRI systems have been proposed and implemented. To-date such systems rely on low-Tc Superconducting Quantum Interference Devices (SQUIDs) leading to the requirement of cryogens. In this letter, we report ULF-MRI obtained with a non-cryogenic atomic magnetometer. This demonstration creates opportunities for developing inexpensive and widely applicable MRI scanners.

Ultra-low-field MRI for the detection of liquid explosives

Recently it has become both possible and practical to perform MR at magnetic fields from µT to mT, the so-called ultra-low field (ULF) regime. SQUID sensor technology allows for ultra-sensitive detection while pulsed pre-polarizing fields greatly enhance signal. The instrumentation allows for unprecedented flexibility in signal acquisition sequences and simplified MRI instrumentation. Here we present the results for a new application of ULF MRI and relaxometry for the detection and characterization of liquids. We briefly describe the motivation and advantages of the ULF MR approach. We then present recent results from a 7- channel ULF MRI/relaxometer system constructed to non-invasively inspect liquids at a security check-point for the presence of hazardous material. The instrument was fielded to the Albuquerque International Airport in December, 2008, and results from that endeavor are also presented.

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.

SQUIDs vs. Induction Coils for Ultra-Low Field Nuclear Magnetic Resonance: Experimental and Simulation Comparison

Nuclear magnetic resonance (NMR) is widely used in medicine, chemistry and industry. One application area is magnetic resonance imaging (MRI). Recently it has become possible to perform NMR and MRI in the ultra-low field (ULF) regime requiring measurement field strengths of the order of only 1 Gauss. This technique exploits the advantages offered by superconducting quantum interference devices or SQUIDs. Our group has built SQUID based MRI systems for brain imaging and for liquid explosives detection at airport security checkpoints. The requirement for liquid helium cooling limits potential applications of ULF MRI for liquid identification and security purposes. Our experimental comparative investigation shows that room temperature inductive magnetometers may provide enough sensitivity in the 3-10 kHz range and can be used for fast liquid explosives detection based on ULF NMR technique. We describe experimental and computer-simulation results comparing multichannel SQUID based and induction coils based instruments that are capable of performing ULF MRI for liquid identification.