Reson Part Research Articles

NMR Concepts

Concepts in Magnetic Resonance Part B: Magnetic Resonance EngineeringVolume 44, Issue 4 p. II-III NMR ConceptsFree to Read NMR Concepts First published: 10 June 2015 https://doi.org/10.1002/cmr.b.21286AboutPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL Share a linkShare onFacebookTwitterLinkedInRedditWechat No abstract is available for this article. Volume44, Issue4October 2014Pages II-III RelatedInformation

Circuit level simulation of MRI receive chain using excitation derived from images

ABSTRACTWe introduce a novel component‐level simulation method with which to characterize the performance of the electronic interfaces and circuitry of a magnetic resonance imaging (MRI) receiver. The Matlab‐based toolbox first reconstructs idealized coil free induction decay signals from real MR images, then uses these signals to drive a transistor‐level and extracted‐layout simulation of the desired receiver architecture. The simulated receiver's outputs are read back into the toolbox, which then reconstructs the MR images. By comparing the reconstructed images with the original ones, important design characteristics, such as the circuit's noise figure, linearity, phase noise, and inter‐channel coupling, can be investigated in terms of image quality. To validate the method, a new MRI receiver architecture is designed and simulated. The architecture is a 16‐channel multiplexer, designed for a commercially available 0.35 μm CMOS technology, and employs both frequency‐division and time‐division multiplexing to form a combined output signal. The SNR degradation and the circuit's linearity determined from the MR images compare well with the SNR and linearity point predictions from conventional circuit simulations. © 2015 Wiley Periodicals, Inc. Concepts Magn Reson Part B (Magn Reson Engineering) 44B: 102–113, 2015

Compressively sampled magnetic resonance image reconstruction using separable surrogate functional method

ABSTRACTAccording to the theory of compressed sensing, magnetic resonance (MR) images can be well reconstructed from randomly sub‐Nyquist sampling of the k‐space data using a nonlinear recovery technique if certain conditions are satisfied. The sparse coefficients of the partial Fourier data can be estimated by minimizing the reconstruction cost function. Separable surrogate functional (SSF) method is one of the effective numerical techniques for minimizing mixed convex optimization problems. This article presents a novel recovery technique for compressively sampled MR images that uses SSF method subject to the data consistency constraints. The experimental results show that the proposed recovery algorithm outperforms the projection onto convex sets and low resolution‐based reconstruction techniques in terms of artifact power, improved signal‐to‐noise ratio, and correlation for the same number of iterations. The results are validated using the phantom and original human head MR images taken from the magnetic resonance imaging scanner at St. Mary's Hospital, London. © 2015 Wiley Periodicals, Inc. Concepts Magn Reson Part A 43A: 157–165, 2015.

AFNMR/NQR signal enhancement via population transfer

Antiferromagnetic nuclear magnetic resonance (AFNMR) and nuclear quadrupole resonance (NQR) are useful techniques to characterize materials in bulk volumes without applying external static magnetic fields. For industrial materials characterization applications, it is crucial to maximize signal‐to‐noise ratio in the available time. We simulate the AFNMR and NQR signal enhancement via population transfer in single crystals and powders and predict the enhancement factors for half‐integer quadrupolar nuclei spin systems. Differences to conventional high‐field NMR population transfer experiments are emphasized. We restrict our discussion to electric field gradients with axial symmetry = 0. Triple resonance tuned crossed coil experiments on CuFeS2 and In2O3 powders support the numerical results. © 2015 Wiley Periodicals, Inc. Concepts Magn Reson Part A 43A: 147–156, 2015.

T1 and T2 estimation in complex domain: First results on clinical data

PurposeThe article provides a practical method for generating T1 and T2 maps from multiple, sequentially acquired Spin Echo images. First results on real data for diagnostic purposes are presented and discussed in order to investigate the possible clinical usages of the proposed technique. The main advantage of the method is its ability in identifying even very small variation in one or both relaxation time maps, allowing a more clear differentiation of tissues across the whole image, instead of highlighting only high vascularized regions, such as in contrast medium based images.Theory and MethodsFor the joint estimation of relaxation time maps, the method exploits the full complex acquired data instead of amplitude acquisitions. This allows having an estimator able to provide unbiased results even in case of tissues characterized by weak Signal to Noise Ratio.ResultsThe effectiveness of the proposed approach is shown on different real case studies: a phantom dataset for quantitative analysis and three human brain datasets for diagnostic purposes.ConclusionThe proposed method shows very interesting and promising results when applied to clinical datasets. The innovative technique is able to evaluate the biological topography and highlight lesions not otherwise easily distinguishable with standard examinations. © 2015 Wiley Periodicals, Inc. Concepts Magn Reson Part A 43A: 166–176, 2015.

A comprehensive discussion of HMBC pulse sequences. III. Solving the problem of missing and weakly observed long‐range correlations

ABSTRACTThe intensity of long‐range correlations observed with the classical HMBC pulse sequence using static optimization of the long‐range coupling delay is directly related to the size of the coupling constant and is often set as a compromise. As such, some long‐range correlations might appear with a reduced intensity or might even be completely absent from the spectra. After a short introduction, this third manuscript will give a detailed review of some selected HMBC variants dedicated to improve the detection of long‐range correlations, such as the ACCORD‐HMBC, CIGAR‐HMBC, and Broadband HMBC experiments. Practical details about the accordion optimization, which affords a substantial improvement in both the number and intensity of the long‐range correlations observed, but introduces a modulation in F1, will be discussed. The incorporation of the so‐called constant time variable delay in the CIGAR‐HMBC experiment, which can trigger or even completely suppress 1H–1H coupling modulation inherent to the utilization of the accordion principle, will be also discussed. The broadband HMBC scheme, which consists of recording a series of HMBC spectra with different delays set as a function of the long‐range heteronuclear coupling constant ranges and transverse relaxation times T2, is also examined. © 2015 Wiley Periodicals, Inc. Concepts Magn Reson Part A 43A: 177–206, 2015.

NMR Concepts

Concepts in Magnetic Resonance Part AVolume 43, Issue 5 p. II-III NMR ConceptsFree to Read NMR Concepts First published: 10 June 2015 https://doi.org/10.1002/cmr.a.21331AboutPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL Share a linkShare onFacebookTwitterLinked InRedditWechat No abstract is available for this article. Volume43, Issue5September 2014Pages II-III RelatedInformation

Peripheral nerve stimulation measures in a composite gradient system

ABSTRACTPurpose: This work compared peripheral nerve stimulation (PNS) thresholds for the MRI scanner body gradients (BODY), a head/neck insert gradient (INSERT), and the combination of both gradient sets used simultaneously (COMPOSITE). Methods: For BODY, INSERT and COMPOSITE gradients, PNS thresholds were determined by exposing subject volunteers to stepped increases in gradient strength. For COMPOSITE mode, the INSERT was applied at equal (experiment 1) or double (experiment 2) the BODY gradient strength. Results: The locations and thresholds of peripheral nerve stimulation depended on the gradient system configuration, gradient axis, and gradient strength. Stimulation in the body occurred when using the BODY Y‐gradient axis either singly (110 T/m/s) or in COMPOSITE mode (315 T/m/s experiment 2) and adding the insert gradient had negligible effect on stimulation. Stimulation in the head/sinus area generally occurred when using the INSERT X‐gradient either singly (213 T/m/s) or in COMPOSITE mode (320 T/m/s) and adding the body gradient had negligible effect on stimulation. In the COMPOSITE mode, both the location of stimulation and the limiting gradient strength matched location and strength of the limiting component gradient. Conclusion: Stimulation, to a first‐order approximation, is independent for the two gradient systems. In COMPOSITE mode, PNS can be dominated by either of the individual gradient components, indicating that the contribution of each component can be increased until the threshold limit of each component gradient is reached. COMPOSITE gradients provide increased gradient performance with PNS thresholds higher than either component gradient system operating alone. © 2015 Wiley Periodicals, Inc. Concepts Magn Reson Part B (Magn Reson Engineering) 44B: 66–74, 2015

Estimation of occupational exposure to static magnetic fields due to usual movements in magnetic resonance units

ABSTRACTThe purpose of this work is to estimate the induced electric field E due to movements of magnetic resonance (MR) workers in static magnetic field. Different operator walking speeds and paths are taken into account, based on real‐world scenarios for a 3T MR scanner. The spectral components of magnetic field gradient are estimated to verify the compliance with the specific International Commission on Non‐Ionizing Radiation Protection safety guidelines for MR workers. The induced electric fields on the chosen paths are estimated using a previously described software tool based on an analytical model. Finally, a real situation is considered in which an operator performs the patient preparation for a brain MR examination. From the spectral components of the motion‐induced magnetic field gradient, it is possible to see that the highest values are concentrated on frequencies below 1 Hz. For the basic paths considered, the worst‐case scenario is a walking perpendicular to the magnet cylinder along the x‐axis. For the example of a real path, the critical zone where the worker exposure is highest is highlighted. The results of this study could be used for training MR workers to follow correct behavior in the scanner room to avoid high‐exposure scenarios. © 2015 Wiley Periodicals, Inc. Concepts Magn Reson Part B (Magn Reson Engineering) 44B: 75–81, 2015

NMR Concepts

Concepts in Magnetic Resonance Part B: Magnetic Resonance EngineeringVolume 44, Issue 3 p. II-III NMR ConceptsFree to Read NMR Concepts First published: 23 April 2015 https://doi.org/10.1002/cmr.b.21274AboutPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL Share a linkShare onFacebookTwitterLinkedInRedditWechat No abstract is available for this article. Volume44, Issue3August 2014Pages II-III RelatedInformation

Zeeman truncation in NMR. I. The role of operator commutation

Hamiltonians are of pivotal importance for describing and analyzing NMR experiments. However, the “exact” spin Hamiltonian operators are in practice not utilized, but merely a simplified form referred to either as the “secular,” “Zeeman‐truncated,” or “high‐field” Hamiltonian. It results after accounting for the dominating role of the Zeeman interaction relative to all other, much smaller NMR interactions, such as chemical shifts, through‐bond, or through‐space spin–spin couplings. In this article and the following one, we introduce the Zeeman truncation process to newcomers to NMR by thoroughly reviewing the options available for reducing the full Hamiltonian of a spin interaction to its Zeeman‐truncated counterpart. The present paper considers time‐independent Hamiltonians, where we discuss the criteria for performing truncation, highlighting the role of operator commutation by a simple formalism that is equivalent to application of lowest‐order static perturbation theory. The validity of the approximations are illustrated by examining the explicit matrix representations of the “exact” and “Zeeman truncated” Hamiltonians, considering the NMR interactions relevant for systems of interacting spin‐1/2 nuclei. © 2015 Wiley Periodicals, Inc. Concepts Magn Reson Part A 43A: 91–108, 2015.

A single phantom to mimic 1H MR spectra of different tissues

ObjectTo design and build a single phantom, encompassing various elementary tubes, that mimics the 1H magnetic resonance (MR) spectra of liver, prostate, muscle and brain.Materials and MethodsHigh‐resolution NMR tubes were filled with water and the metabolite of interest, at a concentration of 50 mM, or with dairy cream or oil. The tubes (5 mm outer diameter) were placed into a plastic support, the ensemble was positioned in the center of a cuboid container filled with water. Simulations of the magnetic field inhomogeneity arising from the susceptibility variations in the current phantom were also performed. MR localized spectroscopy was performed on a 1.5 T clinical scanner.ResultsMR spectra of voxels positioned in four different locations of the phantom closely mimicked the resonance peaks of liver, prostate, muscle and brain in vivo MR spectra. In particular, for the muscle spectrum, the intramyocellular lipid (IMCL) and extramyocellular lipid (EMCL) resonances were obtained using dairy cream and oil, respectively. Simulations of the magnetic field inhomogeneity in the phantom showed that in the case of an NMR tube length five times greater than the thickness of the volume of interest, it is not necessary to fill the container with water. However, to ensure a good reception level and shim settings, the NMR tubes were immersed in water.ConclusionsA single phantom of new design, that is easily built and highly reconfigurable, is here presented, to mimic the in vivo MR spectra of liver, prostate, muscle, brain and other models as needed. © 2015 Wiley Periodicals, Inc. Concepts Magn Reson Part A 43A: 138–145, 2015.

Zeeman truncation in NMR. II. Time averaging in the rotating frame

The truncation of NMR interactions by the Zeeman Hamiltonian is here examined from a different perspective than that of our previous article [Concepts Magn. Reson., vol. 43A, pages 91–108 (2015)]: a Zeeman‐truncated Hamiltonian is arranged by transforming the initially stationary operator of the NMR interaction into the rotating frame, followed by a time‐averaging procedure over one Larmor period ( ). We review the concept of an interaction frame, with focus on the rotating frame relevant for the Zeeman interaction, as well as the time‐averaging procedure and the criteria for application of the resulting stationary Hamiltonian. A decisive advantage of this approach is its suitability for truncating time‐dependent Hamiltonians, such as that of a radio‐frequency (RF) field. Various options for reducing the time‐dependent RF Hamiltonian into its stationary form are contrasted and thoroughly discussed. The role of Zeeman truncation is illustrated by examining the rotating‐frame RF Hamiltonian associated with both on‐resonance and off‐resonance irradiation, where the time‐independent operator character allows for applying the approximation strategies reviewed in our previous article. © 2015 Wiley Periodicals, Inc. Concepts Magn Reson Part A 43A: 109–126, 2015.

The effective phase of soft RF pulses

ABSTRACTIn this article, the seemingly trivial relation between the spin nutation phase and the phase of radiofrequency (RF) pulses used in NMR is discussed, which in the case of soft (selective) pulses appears more subtle. Unlike hard pulses applied at a common carrier frequency, soft pulses usually have different frequency offsets in the same sequence and their nutation phase cannot be directly linked with the RF phase shift, as the phase shift has no absolute meaning for waves of different frequency. The problem is further complicated in MRI, where RF pulses are accompanied by a time‐dependent slice selection field gradient leading to surprising effects, for example, when two consecutive pulses of identical frequency and phase rotate the spins around different axes depending on slice position. A rule is described that lets the effective nutation phase be derived from the time course the RF phase. Ways to control this time course using various options of digital synthesis are presented. The discussion is illustrated by two popular MRI sequences—fast spin echo (RARE) and balanced steady‐state free precession (true FISP)—whose phase sensitivity easily leads to a pitfall when the frequency synthesis is not programmed with sufficient care. © 2015 Wiley Periodicals, Inc. Concepts Magn Reson Part A 43A: 127–137, 2015.

On the contribution of circumferential resonance modes in acoustic radiation force experienced by cylindrical shells

A body insonified by a constant (time-varying) intensity sound field is known to experience a steady (oscillatory) force that is called the steady-state (dynamic) acoustic radiation force. Using the classical resonance scattering theorem (RST) which suggests the scattered field as a superposition of a resonance field and a background (non-resonance) component, we show that the radiation force acting on a cylindrical shell may be synthesized as a composition of three components: background part, resonance part and their interaction. The background component reveals the pure geometrical reflection effects and illustrates a regular behavior with respect to frequency, while the others demonstrate a singular behavior near the resonance frequencies. The results illustrate that the resonance effects associated to partial waves can be isolated by the subtraction of the background component from the total (steady-state or dynamic) radiation force function (i.e., residue component). In the case of steady-state radiation force, the components are exerted on the body as static forces. For the case of oscillatory amplitude excitation, the components are exerted at the modulation frequency with frequency-dependant phase shifts. The results demonstrate the dominant contribution of the non-resonance component of dynamic radiation force at high frequencies with respect to the residue component, which offers the potential application of ultrasound stimulated vibro-acoustic spectroscopy technique in low frequency resonance spectroscopy purposes. Furthermore, the proposed formulation may be useful essentially due to its intrinsic value in physical acoustics. In addition, it may unveil the contribution of resonance modes in the dynamic radiation force experienced by the cylindrical objects and its underlying physics.

Generalized Beth–Uhlenbeck approach to mesons and diquarks in hot, dense quark matter

An important first step in the program of hadronization of chiral quark models is the bosonization in meson and diquark channels. This procedure is presented at finite temperatures and chemical potentials for the SU(2) flavor case of the NJL model with special emphasis on the mixing between scalar meson and scalar diquark modes which occurs in the 2SC color superconducting phase. The thermodynamic potential is obtained in the Gaussian approximation for the meson and diquark fields and it is given in the Beth–Uhlenbeck form. This allows a detailed discussion of bound state dissociation in hot, dense matter (Mott effect) in terms of the in-medium scattering phase shift of two-particle correlations. It is shown for the case without meson–diquark mixing that the phase shift can be separated into a continuum and a resonance part. In the latter, the Mott transition manifests itself by a change of the phase shift at threshold by π in accordance with Levinson’s theorem, when a bound state transforms to a resonance in the scattering continuum. The consequences for the contribution of pionic correlations to the pressure are discussed by evaluating the Beth–Uhlenbeck equation of state in different approximations. A similar discussion is performed for the scalar diquark channel in the normal phase. Further developments and applications of the developed approach are outlined.

Casimir-Polder force on a two-level atom in a structure containing metamaterials

We study the Casimir-Polder (CP) force on an excited cold two-level atom in structures containing metamaterials. We adopt two kinds of metamaterials: left-handed materials (LHMs) and zero-index materials (ZIMs). The CP force on an excited atom can be divided into two parts: the dispersive force that responds to all frequencies of the electromagnetic mode and the resonant force, which is determined by the frequency at the atomic transition. Left-handed materials and ZIMs can significantly modify the resonant part of the CP force due to their unique character. It is found here that the presence of LHMs can enhance the force on the atom far away from the surface due to its phase compensation, while the presence of ZIMs can lead to a force that is independent of dipole orientation. The Casimir effect within the combination of LHMs and ZIMs leads us to realize a potential well that is insensitive to the orientation of atomic dipole. Due to the spontaneous decay, the resonant part of the CP force disappears eventually; however, the decay at the position with maximum force is inhibited. Therefore, during the time evolution, there are special positions (focuses) at which the force is significant for a longer time. Our results show the trap effect that can work on an atom with arbitrary dipole orientation. This provides a method to either trap or reflect an atom in a position far away from surface.

Relaxation mechanisms and shales

Unconventional petroleum resources present in shales have recently seen increased development due to growing demand for energy worldwide, declining conventional petroleum discoveries, and improved technology for production. Shale reservoirs differ from more conventional reservoirs in both matrix composition and structure, tending to be low in porosity and ultralow in permeability, making traditional laboratory characterization methods difficult to apply. The noninvasiveness of nuclear magnetic resonance (NMR) makes it a favored technique for shale analysis. However, interpretation of NMR relaxometry results for shales is more complex than in traditional reservoirs. Surface relaxivity in conventional reservoirs is predominantly caused by the interaction of fluid molecules with paramagnetic impurities on pore surfaces. However, in shales, small pore sizes coupled with the presence of hydrogen‐rich organic matter can result in more interactions. This article discusses different relaxation mechanisms that may be present in shales and situations where they are relevant. Surface relaxation in organic matter is likely caused by homonuclear dipolar coupling, not paramagnetic impurities, and as such, will have a significant temperature dependence. Due to the small pore sizes in shales, the effect of internal gradients on the signal appears negligible in almost all situations. Analysis of transverse relaxation resulting from organic solids by an inverse Laplace transform may lead to results that are anomalously short and overcall signal intensity. Diffusional coupling between the shale pores confounds interpretation of the NMR response as a pore size distribution. There also appears to be rapid exchange of magnetization between mobile and immobile phases in the samples. © 2014 Wiley Periodicals, Inc. Concepts Magn Reson Part A 43A: 57–78, 2014.

NMR Concepts

Concepts in Magnetic Resonance Part AVolume 43A, Issue 3 p. II-III NMR ConceptsFree to Read NMR Concepts First published: 26 September 2014 https://doi.org/10.1002/cmr.a.21311AboutPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL Share a linkShare onFacebookTwitterLinked InRedditWechat No abstract is available for this article. Volume43A, Issue3May 2014Pages II-III RelatedInformation

Quantization error in magnetic resonance imaging

ABSTRACTDue to the special nature of the acquisition domain, the quantization process of magnetic resonance imaging (MRI) data presents challenges that are not present in other medical imaging techniques. In this article, we demonstrate that the quantization error in MRI cannot be assumed to be a random variable with uniform distribution across the entire acquisition domain. Furthermore, the introduced error is not statistically independent of the input signal. On the contrary, we show that this error is correlated with the object that is being scanned, producing perceptually unpleasant artifacts in the image domain. Although the quantization error is not generally a critical issue in two‐dimensional (2D) MRI acquisition, it could be in the case of 3D acquisitions and in noise estimation measurements. © 2014 Wiley Periodicals, Inc. Concepts Magn Reson Part A 43A: 79–89, 2014.