Per E. Magnelind

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.

Co-Registration of Interleaved MEG and ULF MRI Using a 7 Channel Low-

In this paper we report the first co-registered, interleaved measurements of ultra-low field (ULF) magnetic resonance imaging (MRI) and magnetoencephalography (MEG). Interleaved measurements are interesting for the ultimate aim of combining MEG and functional MRI at ULF. The measurement system consisted of 7 channels with second-order gradiometers coupled to low transition-temperature superconducting quantum interference devices (SQUIDs). The ULF MRI was acquired at a measurement field of 94 μT after a pre-polarization in a 30 mT field. Our results show that the two modalities can be performed with interleaved measurements. However, due to transients from the walls of the magnetically shielded room a waiting time of more than 3 s had to be introduced between the MRI protocol and the auditory stimulus for the MEG.

T_{\rm c}

A new software package 3D-MLSI was developed for inductance calculation in multilayer superconducting integrated circuits. The key advantages of 3D-MLSI are: a new mathematical model that takes into account the 3D distribution of magnetic field, and a user interface compatible with the Cadence and ACAD design tools. The program is most applicable when both kinetic and magnetic inductances are important. A method of equivalent circuits inductance extraction is suggested.

SQUID System

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.

3D-MLSI: software package for inductance calculation in multilayer superconducting integrated circuits

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.

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

Table of Contents Foreword by John Clarke Preface Acknowledgments Common Acronyms and Abbreviations Common Constants and Conversions 1. Fundamental Principles of NMR and MRI at ULF 1.1. Introduction 1.2. Characteristics of NMR Signal 1.3. Introducing Signal-to-Noise and Contrast-to-Noise 1.4. NMR and MRI at Ultra-Low Fields 1.5. MRI Effects 1.6. New Regimes of Physics Accesible at Ultra-Low Field 1.7. Summary of MRI and ULF 1.8. Summary of HF and ULF MRI Comparisons 2. Nuts and Bolts of ULF MRI 2.1. Introduction 2.2. General Concepts of Pre-Polarization Field Generation 2.3. Generation of Measurement Field, Gradients, and Spin-flip 2.4. Sensing the ULF-MR Signals 2.5. Magnetic Shielding 3. Magnetic Resonance Phenomena at ULF 3.1. Interaction between the Electromagnetic Radiation and the Nuclear Spins at ULF 3.2. Quantum Mechanical Description of Interaction between the Electromagnetic Radiation and the Nuclear Spins 3.3. Theory of Relaxometry (Focused on Magnetic Field Dependence) 4. Imaging Techniques at ULF 4.1. Introduction 4.2. General Imaging Concepts (How to do MRI) 4.3. ULF MRI 4.4. Imaging Techniques: Making the Most of What You Got 4.5. On the Plus Side 4.6. Conclusions 5. Applications in ULF MRI 5.1. Multi-modal imaging of human brain anatomy and function: MEG and ULF MRI 5.2. New approaches to image human brain function uniquely enabled by ULF MRI 5.3. MagViz and ULF NMR relaxometers 5.4. A Final Word Disclosures Index

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

We have developed an HTS SQUID system for both time and frequency domain measurements of magnetic signals from the Brownian relaxation of liquid-suspended magnetic nanoparticles (MNPs) coated with antibodies. A planar YBa2Cu3O7-x dc SQUID gradiometer with a baseline of 3 mm allows stable operation of the measurement system in an unshielded environment. Agglomeration of MNPs induced by prostate specific antigen-antibody binding complexes is characterized with our system and benchmarked with frequency domain measurements performed by Imego AB using an induction coil magnetometer. We estimate an upper bound for our immunoassay analyte sensitivity of 1.8 ng/radicHz.

Ultra-low field nuclear magnetic resonance : a new MRI regime

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.

Fast and Sensitive Measurement of Specific Antigen-Antibody Binding Reactions With Magnetic Nanoparticles and HTS SQUID

We have demonstrated the first ultra-low field (ULF) nuclear magnetic resonance measurements of uranium hexafluoride (UF6), which is used in the uranium enrichment process. A sensitive non-invasive detection system would have an important role in non-proliferation surveillance. A two-frequency technique was employed to remove the transients induced by rapidly switching off the 50 mT pre-polarization field. A mean transverse relaxation time T2 of 24 ms was estimated for the un-enriched UF6 sample measured at a mean temperature of 80degC.

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

A high-Tc dc SQUID (superconducting quantum interference device) gradiometer was developed for magnetic immunoassays where magnetic nanoparticles are used as markers to detect biological reactions. The gradiometer was fabricated on a 5 × 10 mm2 SrTiO3 bicrystal substrate and has a gradiometer resolution of 2.1 pT cm−1 Hz−1/2. A magnetic signal was detected from a sample of 1 μl of Fe3O4 nanoparticles in a 40 mg ml−1 solution kept in a microcavity fabricated on Si wafers with Si3N4 membranes using MEMS (micro-electro-mechanical-systems) technology. It was found that volumes as small as 0.3 nl in principle would be detectable with our present device. This corresponds to a total number of particles of 2.2 × 107. The estimated average dipole moment per particle is 4.8 × 10−22 Am2. We are aiming at reading out immunoassays by detecting the Brownian relaxation of magnetic nanoparticles, and we also intend to integrate MEMS technology into our system.