John J. Gomez

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.

Tomographic Imaging with Cosmic Ray Muons

Over 120 million vehicles enter the United States each year. Many are capable of transporting hidden nuclear weapons or nuclear material. Currently deployed X-ray radiography systems are limited because they cannot be used on occupied vehicles and the energy and dose are too low to penetrate many cargos. We present a new technique that overcomes these limitations by obtaining tomographic images using the multiple scattering of cosmic radiation as it transits each vehicle. When coupled with passive radiation detection, muon interrogation could contribute to safe and robust border protection against nuclear devices or material in occupied vehicles and containers.

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.

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}

Magnetic resonance imaging at microtesla fields is a promising imaging method that combines the pre-polarization technique and broadband signal reception by superconducting quantum interference device (SQUID) sensors to enable in vivo MRI at microtesla-range magnetic fields similar in strength to the Earth magnetic field. Despite significant advances in recent years, the potential of microtesla MRI for biomedical imaging is limited by its insufficient signal-to-noise ratio due to a relatively low sample polarization. Dynamic nuclear polarization (DNP) is a widely used approach that allows polarization enhancement by 2-4 orders of magnitude without an increase in the polarizing field strength. In this work, the first implementation of microtesla MRI with Overhauser DNP and SQUID signal detection is described. The first measurements of carbon-13 NMR spectra at microtesla fields are also reported. The experiments were performed at the measurement field of 96 μT, corresponding to Larmor frequency of 4 kHz for protons and 1 kHz for carbon-13. The Overhauser DNP was carried out at 3.5-5.7 mT fields using rf irradiation at 120 MHz. Objects for imaging included water phantoms and a cactus plant. Aqueous solutions of metabolically relevant sodium bicarbonate, pyruvate, alanine, and lactate, labeled with carbon-13, were used for NMR studies. All the samples were doped with TEMPO free radicals. The Overhauser DNP enabled nuclear polarization enhancement by factor as large as -95 for protons and as large as -200 for carbon-13, corresponding to thermal polarizations at 0.33 T and 1.1 T fields, respectively. These results demonstrate that SQUID-based microtesla MRI can be naturally combined with Overhauser DNP in one system, and that its signal-to-noise performance is greatly improved in this case. They also suggest that microtesla MRI can become an efficient tool for in vivo imaging of hyperpolarized carbon-13, produced by low-temperature dissolution DNP.

SQUID System

SQUID-based MRI (magnetic resonance imaging) at microtesla fields has developed significantly over the past few years. Here we describe application of this method for magnetic relaxation measurements in the living human brain. We report values of the longitudinal relaxation time T1 for brain tissues, measured in vivo for the first time at microtesla fields. The experiments were performed at 46 muT field using a seven-channel SQUID system designed for microtesla MRI and MEG. Values of T1, measured for different tissues at this field, are found to be close (within 5%) to the corresponding values of the transverse relaxation time T2 at the same field. Implications of this result for imaging contrast in microtesla MRI are discussed.

Microtesla MRI with dynamic nuclear polarization

Detection of high‐Z material hidden inside a large volume of ordinary cargo is an important and timely task given the danger associated with illegal transport of uranium and heavier elements. Existing radiography techniques are inefficient for shielded material, often expensive and involve radiation hazards, real and perceived. We recently demonstrated that radiographs can be formed using cosmic‐ray muons. Here, we show that compact, high‐Z objects can be detected and located in 3 dimensions with muon radiography. The natural flux of cosmic‐ray muons, approximately 10,000 m−2min−1, can generate a reliable detection signal in a fraction of a minute, using large‐area muon detectors as used in particle and nuclear physics.

SQUID-Based Microtesla MRI for In Vivo Relaxometry of the Human Brain

We have built a detector capable of locating high Z objects in the sampling (middle) region of the detector. As atomic number increases, radiation length rapidly decreases, yielding larger variance in scattering angle. Cosmic ray muon tomography works by tracking muons above the sampling region, and tracking them below the region as well. The difference between the two trajectories yield information, via the muon scattering variance, of the materials contained within the sampling region [Borozdin, K, et al., 2003]. One of most important aspects of cosmic ray tomography is minimizing exposure time. The cosmic ray flux is about 1 cm-2 min-1, and the goal is to use them for detecting high-density materials as quickly as possible. This involves using all of the information possible to reconstruct tracks with redundant detectors. Detector scattering residuals yield a low precision measurement of muon energy. Knowing the rough energy of an incoming particle will yield more precisely the expected scattering variance (currently the expectation value of ~3 GeV is used).

Detection of High‐Z Objects using Multiple Scattering of Cosmic Ray Muons

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.