Algis Urbaitis

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.

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.

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.

Applications of Ultra-Low Field Magnetic Resonance for Imaging and Materials Studies

Recently it has become both possible and practical to perform MR at magnetic fields from muT 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. Here we present the results from several applications of ULF MR which exploit the unique abilities of the method. These include novel ways to image both brain structure and function either by combination of MRI with magnetoencephalography or direct observation of the interaction of neural currents with the spin population, and ULF relaxometry for detection and characterization of materials relevant to numerous non-invasive inspection applications. We briefly describe the motivation, advantages, and recent results of several new applications of the ULF MR method. Specifically, we present recent data measuring the interaction of weak ( ~ 10 muA) currents with a spin-population in a water phantom, as studied by ULF MRI with implications for neural current imaging. We also present data from a ULF MR relaxometer developed inspecting liquids in a check-point for the presence of hazardous material.

Progress on Detection of Liquid Explosives Using Ultra-Low Field MRI

Nuclear magnetic resonance (NMR) spectroscopy and magnetic resonance imaging (MRI) methods are widely used in medicine, chemistry and industry. Over the past several years there has been increasing interest in performing NMR and MRI in the ultra-low field (ULF) regime, with measurement field strengths of 10-100 microTesla and pre-polarization fields of 30-50 mTesla. The real-time signal-to-noise ratio for such measurements is about 100. Our group at LANL has built and demonstrated the performance of SQUID-based ULF NMR/MRI instrumentation for classification of materials and detection of liquid explosives via their relaxation properties measured at ULF, using T1, T2, and T1 frequency dispersion. We are also beginning to investigate the performance of induction coils as sensors. Here we present recent progress on the applications of ULF MR to the detection of liquid explosives, in imaging and relaxometry.

Progress Toward a Deployable SQUID-Based Ultra-Low Field MRI System for Anatomical Imaging

Magnetic resonance imaging (MRI) is the best method for non-invasive imaging of soft tissue anatomy, saving countless lives each year. But conventional MRI relies on very high fixed strength magnetic fields, ≥ 1.5 T, with parts-permillion homogeneity, requiring large and expensive magnets. This is because in conventional Faraday-coil based systems the signal scales approximately with the square of the magnetic field. Recent demonstrations have shown that MRI can be performed at much lower magnetic fields (~100 μT, the ULF regime). Through the use of pulsed prepolarization at magnetic fields from ~10-100 mT and SQUID detection during readout (proton Larmor frequencies on the order of a few kHz), some of the signal loss can be mitigated. Our group and others have shown promising applications of ULF MRI of human anatomy including the brain, enhanced contrast between tissues, and imaging in the presence of (and even through) metal. Although much of the required core technology has been demonstrated, ULF MRI systems still suffer from long imaging times, relatively poor quality images, and remain confined to the R&D laboratory due to the strict requirements for a low noise environment isolated from almost all ambient electromagnetic fields. Our goal in the work presented here is to move ULF MRI from a proof-of-concept in our laboratory to a functional prototype that will exploit the inherent advantages of the approach, and enable increased accessibility. Here we present results from a seven-channel SQUID-based system that achieves pre-polarization field of 100 mT over a 200 cm3 volume, is powered with all magnetic field generation from standard MRI amplifier technology, and uses off the shelf data acquisition. As our ultimate aim is unshielded operation, we also demonstrated a seven-channel system that performs ULF MRI outside of heavy magnetically-shielded enclosure. In this paper we present preliminary images and compare them to a model, and characterize the present and expected performance of this system.

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.

Toward High Resolution Images With SQUID-Based Ultra-Low Field Magnetic Resonance Imaging

Magnetic resonance imaging (MRI) is the state-of-the-art clinical method for imaging soft-tissue anatomy. Because signal scales with the applied magnetic field, the overwhelming trend in MRI has been high magnetic fields, typically 1.5 or 3 T. However, there has been recent interest in ultra-low field (ULF) MRI using 10-100 μT magnetic fields. At ULF there are opportunities for novel imaging applications such as MRI combined with magnetoencephalography in a single device, imaging through or in the presence of metal, and enhanced spin-lattice tissue contrast. Loss in signal is mitigated by sensitive detectors such as superconducting quantum interference devices and sample pre-polarization, typically from 10-100 mT. There have been several proof-of-concept demonstrations based on this approach. However, ULF MRI image quality still suffers from one or more of the following disadvantages compared to high-frequency MRI: lower signal-to-noise ratio, poor spatial resolution, and longer imaging time. Here we present recent progress toward “clinically relevant” ULF MRI parameters: voxel signal-to-noise ratio > 10, voxel size <; 2 × 2 × 4 mm3. Data and simulations from a single channel system are presented and discussed.

Polarization enhancement technique for nuclear quadrupole resonance detection

We demonstrate a dramatic increase in the signal-to-noise ratio (SNR) of a nuclear quadrupole resonance (NQR) signal by using a polarization enhancement technique. By first applying a static magnetic field to pre-polarize one spin subsystem of a material, and then allowing that net polarization to be transferred to the quadrupole subsystem, we increased the SNR of a sample of ammonium nitrate by one-order of magnitude.

Multi-sensor system for simultaneous ultra-low-field MRI and MEG

Magnetoencephalography (MEG) and magnetic resonance imaging at ultra-low fields (ULF MRI) are two methods based on the ability of SQUID (superconducting quantum interference device) sensors to detect femtotesla magnetic fields. Combination of these methods will allow simultaneous functional (MEG) and structural (ULF MRI) imaging of the human brain. In this paper, we report the first implementation of a multi-sensor SQUID system designed for both MEG and ULF MRI. We present a multi-channel image of a human hand obtained at 46 microtesla field, as well as results of auditory MEG measurements with the new system.