Shaun Newman

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 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 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.

Toward early cancer detection using superparamagnetic relaxometry in a SQUID-based ULF-MRI system

To detect cancer at a very early state it is essential to detect a very small quantity of cancerous cells. One very sensitive method relies on targeting the cancer cells using antibody labeled single-core magnetic nanoparticles and detecting the relaxation of the magnetization using instruments based on superconducting quantum interference devices (SQUIDs). However, the localization suffers from inverse-problem issues similar to those found in magnetoencephalography. On the other hand, the same magnetic nanoparticles can also work as contrast agents for magnetic resonance imaging. Through the combination of superparamagnetic relaxometry and ultra-low field magnetic resonance imaging (ULF MRI), in one and the same instrument, the accuracy of the magnetic moment localization can be enhanced and anatomical information can also be obtained. Results on superparamagnetic relaxometry and the dipole localization accuracy in our seven-channel low-Tc SQUID-gradiometer array are reported.

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.

Multi-Channel SQUID-Based Ultra-Low Field Magnetic Resonance Imaging in Unshielded Environment

Magnetic Resonance Imaging (MRI) is the best method for non-invasive imaging of soft tissue anatomy. A conventional MRI relies on 1.5-3 T fixed strength magnetic fields, with parts-per-million homogeneity, requiring large and expensive magnets. MRI can be done at ultra-low magnetic fields (ULF) with Larmor frequencies of a few kHz with much more modest magnetic system requirements. However the ULF regime requires a very sensitive detection system. A candidate detection system is based on SQUID gradiometers. A conventional SQUID gradiometer based detection system requires effective shielding from all ambient electromagnetic noise. Large shielded structures, such as magnetically shielded or eddy-current rooms, can be used for proof-of-principles experiments but do not lead to practical deployable instruments. Our goal is to develop a technique in which a SQUID-based detector array could be deployed without the limitation imposed by the requirement for a shielded structure. We have tested a 7-channel ULF MRI system located in unshielded environment inside a modern physics laboratory. It was possible to significantly suppress most of the electromagnetic interference by subtracting the signal from a one-channel reference magnetometer located nearby. We believe that the influence of the pre-polarization coil produces kHz-range frequency noise in gradiometer channels that is very well correlated with the signal from the magnetometer.