Bulletin of the American Physical Society
2007 APS March Meeting
Volume 52, Number 1
Monday–Friday, March 5–9, 2007; Denver, Colorado
Session V3: Detection & Applications of NMR and MRI at Microtesla Magnetic Fields |
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Sponsoring Units: DCMP Chair: John Clarke, University of California, Berkeley Room: Colorado Convention Center Korbel 2A-3A |
Thursday, March 8, 2007 11:15AM - 11:51AM |
V3.00001: SQUID-detected microtesla MRI Invited Speaker: We have developed a system to detect nuclear magnetic resonance (NMR) and magnetic resonance imaging (MRI) signals in magnetic fields of 1-100 microtesla. At such low fields, the very small nuclear polarization and the frequency dependence of conventional Faraday detection would lead to extremely weak signals. To overcome these problems we use a combination of prepolarization and frequency-independent detection with an untuned superconducting gradiometer coupled to a Superconducting Quantum Interference Device (SQUID). We demonstrate narrow linewidths in NMR spectra of nuclei in liquids and in spectra of J-coupled nuclei in molecules. Our MRI system operates at 132 $\mu $T (proton Larmor frequency 5.6 kHz), uses a prepolarizing field up to 150 mT and has a magnetic field noise below 1 fT/Hz$^{1/2}$. This system demonstrates submillimeter in-plane resolution of phantoms, and can acquire \textit{in vivo} images of the human forearm, wrist and fingers. In high-field MRI, the susceptibility difference between tissue and, for example, a medical implant, can cause severe image distortion. We show that such artifacts are absent at microtesla fields, so that this technique could enable distortion-free MRI of patients with medical implants. Furthermore, microtesla MRI displays a greatly enhanced T$_{1}$-weighted contrast between different concentrations of agarose gel (T$_{1}$ is the longitudinal relaxation time). Preliminary experiments on \textit{ex vivo} prostate specimens containing normal and cancerous tissue demonstrate similarly enhanced contrast, suggesting that this technique could be used to image tumors. [Preview Abstract] |
Thursday, March 8, 2007 11:51AM - 12:27PM |
V3.00002: Low Field Nuclear Magnetic Resonance (NMR) using SQUIDs Invited Speaker: Using a high resolution SQUID system in a magnetically highly shielded room, we measured the precession of 1H nuclei of liquid benzene, distilled water, and chloroform in magnetic fields around a microTesla. We found that the NMR lines of these liquids are in the range of a few hundred milliHertz and increase linearly with the detection field over a Larmor frequency range of two orders of magnitude. The slope is attributed to the inhomogeneity of the detection field and enables the extrapolation of the natural line width to zero magnetic field. For this limit, where any molecular motion is fast with respect to the Larmor frequency, the natural resonance line widths of benzene, chloroform and distilled water were determined to be 120 mHz, 150 mHz, and 170 mHz, respectively. In low magnetic fields, chemical shift and homonuclear coupling become negligible. All that remains as a source of a spectral structure is pure J-coupling between nuclei of different gyromagnetic ratio. We studied pure J-coupling between methylene protons and fluorine nuclei of trifluorethanol and between methyl protons and phosphorus in trimethylphosphate at detection fields from 0.5 microTesla to 4 microTesla. This corresponds to a variation of d=J(H,F)/(f(H)-f(F)) from 8 to 1 and of d=J(H,P)/(f(H)-f(P)) from 0.8 to 0.08, respectively. At very low fields, i.e. at d=8, the spectra of trifluorethanol exhibited only one single resonance line with an irregular structure. With increasing field, more and more individual lines were revealed. For trimethylphosphate, d=0.08 represents the transition to the weak coupling regime. In addition, we employed a 304 SQUID vector magnetometer system for the recording of the magnetic field generated by water protons in two adjacent sample tubes precessing about a magnetic field of a microTesla. From the spatially resolved data, positions and moments of the samples were calculated, yielding a reconstructed moving image of the two precessing magnetic dipoles. [Preview Abstract] |
Thursday, March 8, 2007 12:27PM - 1:03PM |
V3.00003: High resolution NMR spectroscopy in the Earth's magnetic field Invited Speaker: High resolution nuclear magnetic resonance (NMR) spectroscopy at high magnetic fields has developed into a most powerful tool for the determination of molecular structures. The dream is a mobile molecular low field NMR scanner which allows the determination of molecular structures. Until to now at low magnetic fields NMR spectroscopy suffers from the low signal to noise ratio (S/N) and from the lack of access to chemical information in terms of chemical shifts and homo-nuclear $J$-couplings. We demonstrate that chemical analysis of liquids is possible by mobile ultrahigh-resolution $^{1}$H, $^{19}$F and $^{129}$Xe NMR spectroscopy in the Earth's magnetic field. The $^{129}$Xe chemical shift in liquids is determined in the Earth's magnetic field with a precision comparable to that obtained by superconducting magnets. The $^{1}$H and $^{19}$F NMR spectra allow the determination of hetero-nuclear $J$-coupling constants with an accuracy of a few mHz. Very fine details of the molecular structure which are not observable with conventional superconducting magnets can be discriminated. For molecules where a rare spin such as carbon $^{13}$C is present the high-resolution low-field $^{1}$H NMR spectrum indeed reveal all hetero- and homo-nuclear $J$-couplings. All these results open the door for the mobile study of molecular structures as well as for the online monitoring of chemical reactions at ultra-low magnetic fields. [Preview Abstract] |
Thursday, March 8, 2007 1:03PM - 1:39PM |
V3.00004: Simultaneous Measurement of Magnetic Resonance and Neuronal Signals Invited Speaker: Nuclear magnetic resonance (NMR) and magnetic resonance imaging (MRI) at ultra low magnetic fields (ULF, $\sim $ microT) have advantages over their counterparts at higher magnetic fields, despite the reduction in signal strength. Among these advantages are that the instrumentation uses superconducting quantum interference devices (SQUIDs), and is now compatible with simultaneous measurements of biomagnetic signals, such as magnetoencephalography (MEG). This presents a new opportunity for noninvasive simultaneous functional and anatomical brain imaging. We present here the physical basis and experimental evidence for a variety of ULF-MRI techniques being developed at Los Alamos to enable simultaneous anatomical and functional imaging of the human brain. We conclude by presenting a novel technique, based on the resonant interaction between the magnetic fields such as those that arise from neural activity and the spin population in ULF-MRI experiments, that may enable direct tomographic imaging of the consequences of neural activity. [Preview Abstract] |
Thursday, March 8, 2007 1:39PM - 2:15PM |
V3.00005: Optical methods for detection of nuclear magnetic resonance. Invited Speaker: Nuclear magnetic resonance is commonly detected with inductive pick-up coils or, less commonly, with SQUID magnetometers. I will discuss recent work in our group on optical detection of NMR using two separate techniques. In one approach, optically-pumped alkali-metal atoms are used to detect the magnetic fields generated by nuclear magnetic moments. Such atomic magnetometers reach sensitivity similar to low-$T_{c}$ SQUID magnetometers without requiring cryogenic cooling. We recently demonstrated atomic magnetometer detection of NMR and NQR signals at frequencies ranging from 20 Hz to 423 kHz. In the other approach, NMR signals from a transparent substance are obtained by direct optical detection. In this technique the plane of polarization of a linearly polarized light transmitted through the sample is rotated by interaction with nuclear spins. We detected NMR signals from water and liquid $^{129}$Xe using this method. Such nuclear spin optical rotation (NSOR) signals do not rely on measurement of long range dipolar fields and allow new modalities of imaging and spectroscopy. [Preview Abstract] |
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