Travelling-wave nuclear magnetic resonance

Nuclear magnetic resonance (NMR) is one of the most versatile experimental methods in chemistry, physics and biology, providing insight into the structure and dynamics of matter at the molecular scale. Its imaging variant-magnetic resonance imaging (MRI)-is widely used to examine the anatomy, physiology and metabolism of the human body. NMR signal detection is traditionally based on Faraday induction in one or multiple radio-frequency resonators that are brought into close proximity with the sample. Alternative principles involving structured-material flux guides, superconducting quantum interference devices, atomic magnetometers, Hall probes or magnetoresistive elements have been explored. However, a common feature of all NMR implementations until now is that they rely on close coupling between the detector and the object under investigation. Here we show that NMR can also be excited and detected by long-range interaction, relying on travelling radio-frequency waves sent and received by an antenna. One benefit of this approach is more uniform coverage of samples that are larger than the wavelength of the NMR signal-an important current issue in MRI of humans at very high magnetic fields. By allowing a significant distance between the probe and the sample, travelling-wave interaction also introduces new possibilities in the design of NMR experiments and systems

An asymmetrical whole-body birdcage RF coil without RF shield for hyperpolarized 129Xe lung MR imaging at 1.5 T

Purpose This study describes the development and testing of an asymmetrical xenon-129 (129Xe) birdcage radiofrequency (RF) coil for 129Xe lung ventilation imaging at 1.5 Tesla, which allows proton (1H) system body coil transmit–receive functionality. Methods The 129Xe RF coil is a whole-body asymmetrical elliptical birdcage constructed without an outer RF shield to enable 1H imaging. urn:x-wiley:07403194:media:mrm28915:mrm28915-math-0001 field homogeneity and flip angle mapping of the 129Xe birdcage RF coil and 1H system body RF coil with the 129Xe RF coil in situ were evaluated in the MR scanner. The functionality of the 129Xe birdcage RF coil was demonstrated through hyperpolarized 129Xe lung ventilation imaging with the birdcage in both transceiver configuration and transmit-only configuration when combined with an 8-channel 129Xe receive-only RF coil array. The functionality of 1H system body coil with the 129Xe RF coil in situ was demonstrated by acquiring coregistered 1H lung anatomical MR images. Results The asymmetrical birdcage produced a homogeneous urn:x-wiley:07403194:media:mrm28915:mrm28915-math-0002 field (±10%) in agreement with electromagnetic simulations. Simulations indicated an optimal detuning configuration with 4 diodes. The obtained g-factor of 1.4 for acceleration factor of R = 2 indicates optimal array configuration. Coregistered 1H anatomical images from the system body coil along with 129Xe lung images demonstrated concurrent and compatible arrangement of the RF coils. Conclusion A large asymmetrical birdcage for homogenous urn:x-wiley:07403194:media:mrm28915:mrm28915-math-0003 transmission with high sensitivity reception for 129Xe lung MRI at 1.5 Tesla has been demonstrated. The unshielded asymmetrical birdcage design enables 1H structural lung MR imaging in the same exam

Noise figure characterization of preamplifiers at NMR frequencies

A method for characterizing the noise figure of preamplifiers at NMR frequencies is presented. The noise figure of preamplifiers as used for NMR and MRI detection varies with source impedance and with the operating frequency. Therefore, to characterize a preamplifier's noise behavior, it is necessary to perform noise measurements at the targeted frequency while varying the source impedance with high accuracy. At high radiofrequencies, such impedance variation is typically achieved with transmission-line tuners, which however are not available for the relatively low range of typical NMR frequencies. To solve this issue, this work describes an alternative approach that relies on lumped-element circuits for impedance manipulation. It is shown that, using a fixed-impedance noise source and suitable ENR correction, this approach permits noise figure characterization for NMR and MRI purposes. The method is demonstrated for two preamplifiers, a generic BF998 MOSFET module and an MRI-dedicated, integrated preamplifier, which were both studied at 128MHz, i.e., at the Larmor frequency of protons at 3 Tesla. Variations in noise figure of 0.01dB or less over repeated measurements reflect high precision even for small noise figures in the order of 0.4dB. For validation, large sets of measured noise figure values are shown to be consistent with the general noise-parameter model of linear two-ports. Finally, the measured noise characteristics of the superior preamplifier are illustrated by SNR measurements in MRI data

Advances in the ERETIC Method for the Quantification of In-Vivo 1H and 31P Spectra

Absolute quantification is a desirable tool to determine metabolite changes. Calibration with ERETIC (Electric REference To access In vivo Concentrations) has proven to be an accurate method for the assessment of absolute concentration in spectra. In this work, we present a new implementation of ERETIC on a clinical scanner using a low-power transmit channel, thus permitting simultaneous use of proton decoupling and nuclear Overhauser enhancement with the high-power RF channel of the system during the acquisition of 31P or 13C spectra. Stability of the ERETIC signal is demonstrated. Also, ERETIC was for the first time applied to in-vivo 1H measurements

Correlation of Upper Triassic sections throughout the Lagonegro Basin

The present paper is a preliminary contribution to the stratigraphy and conodont biostratigraphy of the Upper Triassic (Norian/Rhaetian) of the Lagonegro Basin (Southern Apennines, Italy).Four stratigraphic sections (Sasso di Castalda, Madonna del Sirino, Lagonegro, and Pignola-Abriola) were measured in basinal successions belonging to the Calcari con Selce (>) and Scisti Silicei (>) Fms. The former stratigraphic unit yielded a rich Alaunian (Middle Norian) to the Rhaetian conodont fauna.A red-clay horizon, widely used in literature as a physical correlation tool throughout the Lagonegro Basin, is here identified and dated to the Sevatian (upper Norian).The present paper is a preliminary contribution to the stratigraphy and conodont biostratigraphy of the Upper Triassic (Norian/Rhaetian) of the Lagonegro Basin (Southern Apennines, Italy). Four stratigraphic sections (Sasso di Castalda, Madonna del Sirino, Lagonegro, and Pignola-Abriola) were measured in basinal successions belonging to the Calcari con Selce («Cherty Limestones») and Scisti Silicei («Radiolarites») Fms. The former stratigraphic unit yielded a rich Alaunian (Middle Norian) to the Rhaetian conodont fauna. A red-clay horizon, widely used in literature as a physical correlation tool throughout the Lagonegro Basin, is here identified and dated to the Sevatian (upper Norian)

An RF-Over-Fiber System for Reliable Signal Injection in ERETIC Spectroscopy

We present an optical link used to transmit a reference signal in spectroscopy (ERETIC method). The optical link has the peculiarity to be safety and inexpensive since it is made of only a low noise preamplifier, a high speed LED, polymer optical fiber and a photodiode. The reference signal is generated outside the MR room, it travels along the optical fiber, enters the scanner room’s Faraday shield through a waveguide and is injected into a birdcage coil by inductive coupling. The link is shown to be reliable and stable over time consenting so, good fidelity quantitative measurements of metabolite concentrations

Heart beats brain: the problem of detecting alpha waves by neuronal current imaging in joint EEG-MRI experiments

It has been suggested recently that the influence of the neuro-magnetic field should make electrical brain activity directly detectable by MRI. To test this hypothesis, we performed combined EEG-MRI experiments which aim to localize the neuronal current sources of alpha waves (8-12 Hz), one of the most prominent EEG phenomena in humans. A detailed analysis of cross-spectral coherence between simultaneously recorded EEG and MRI time series revealed no sign of alpha waves. Instead the EEG-MRI approach was found to be hampered by artefacts due to cardiac pulsation, which extend into the frequency band of alpha waves. Separate brain displacement mapping experiments confirmed that not only the EEG but also the MRI signal is confounded by harmonics of the cardiac frequency even at 10 Hz and beyond. This well-known ballistocardiogram artefact cannot be avoided or eliminated entirely by available signal processing techniques. Therefore we must conclude that current EEG-MRI methodology based on correlation analysis lacks not only the sensitivity but also the specificity required for the reliable detection of alpha waves

Preliminary experience with visualization of intracortical fibers by focused high-resolution diffusion tensor imaging

BACKGROUND AND PURPOSE: The inherent low anisotropy of gray matter and the lack of adequate imaging sensitivity and resolution has, so far, impeded depiction of axonal fibers to their intracortical origin or termination. We tested the hypothesis that an experimental approach with high-resolution diffusion tensor imaging (DTI) provides anisotropic data for fiber tractography with sufficient sensitivity to visualize in vivo the fine distribution of white matter bundles at the intracortical level. MATERIALS AND METHODS: We conducted phantom measurements of signal-to-noise ratio (SNR) and obtained diffusion tensor maps of the occipital lobe in 6 healthy volunteers using a dedicated miniature phased array detector at 3T. We reconstructed virtual fibers using a standard tracking algorithm. RESULTS: The coil array provided a SNR of 8.0 times higher at the head surface compared with a standard quadrature whole head coil. Diffusion tensor maps could be obtained with an in-plane resolution of 0.58 x 0.58 mm(2). The axonal trajectories reconstructed from the diffusion data penetrate into the cortical ribbon perpendicular to the pial surface. This is the expected pattern for the terminations of thalamocortical afferent fibers to the middle layers of the occipital cortex and is consistent with the known microstructural organization of the mammalian cerebral cortex. CONCLUSION: High-resolution DTI reveals intracortical anisotropy with a distinct parallel geometrical order, perpendicular to the pial surface, consistent with structures that may be identified as the terminal afferents in cortical gray matter