Off-resonance Behavior Research Articles

Improved off-resonance phase behavior using a phase-inverted adiabatic half-passage pulse for 13 C MRS in humans at 7 T.

In vivo 13 C MRS at high field benefits from an improved SNR and spectral resolution especially when using surface coils in combination with adiabatic pulses, such as the adiabatic half-passage (AHP) pulse for 13 C excitation. However, the excitation profile of the AHP pulse is asymmetric relative to the carrier frequency, which could lead to asymmetric excitation of the spectral lines relative to the center of the spectrum. In this study, a pulse-acquire sequence was designed for adiabatic 13 C excitation with a symmetric bandwidth, utilizing a combination of two AHP pulses with inverted phases in alternate scans. Magnetization and phase behavior as a function of frequency offset and RF amplitude of the B1 field, as well as the steady-state transverse magnetization response to off-resonance, were simulated. Excitation properties of the combined pulse sequence were studied by 23 Na imaging and 13 C spectroscopy in vitro on a phantom and in vivo on the human calf at 7T. Simulations demonstrated symmetric transverse magnetization and phase with respect to positive and negative frequency offsets when using two AHP pulses with inverted phases in alternate scans, thereby minimizing baseline distortion and achieving symmetric T1 weighting, as confirmed by in vitro measurements. The intensities of the lipid peaks at 15, 30, 62, 73, and 130 ppm were in agreement with those theoretically predicted using two AHP pulses with inverted phases in alternate scans. We conclude that using two phase-inverted AHP pulses improves the symmetry of the 13 C excitation profile and phase response to off-resonance effects at 7T in comparison with using a single AHP pulse.

Radial fast interrupted steady-state (FISS) magnetic resonance imaging.

To report a highly interrupted radial variant of balanced steady-state free precession (bSSFP) imaging, termed fast interrupted steady-state (FISS), for decreasing flow artifact as well as fat signal conspicuity with respect to bSSFP, and saturation effects vis-à-vis fast low-angle shot (FLASH) imaging. Numerical simulations, phantom studies, and human studies were conducted to examine the imaging contrast, off-resonance behavior, and flow properties of FISS. Human studies applied FISS for cine cardiac imaging and ungated nonenhanced MR angiography (MRA) of the legs, neck, and brain. Comparisons were made with bSSFP and FLASH imaging. Simulations revealed that FISS retains the high signal levels of bSSFP for stationary on-resonant spins, while reducing undesirable signal heterogeneity from flowing spins. Phantom studies agreed with the simulations, and showed that FISS reduces fat signal and flow artifact with respect to bSSFP imaging. FISS imaging in human subjects agreed with the simulations and phantom studies, and showed reduced saturation artifact compared with FLASH imaging. FISS imaging reduces flow artifact and fat signal conspicuity with respect to bSSFP imaging, and ameliorates arterial signal saturation observed with FLASH imaging. Potential clinical applications include fat-suppressed cine imaging and ungated nonenhanced MRA. Magn Reson Med 79:2077-2086, 2018. © 2017 International Society for Magnetic Resonance in Medicine.

Calculated Resonance Vibrational Raman Optical Activity Spectra of Naproxen and Ibuprofen.

Determining the absolute configuration of a molecule is possible by means of various chiroptical spectroscopic methods, one of them being vibrational Raman optical activity (VROA). By adopting a laser excitation wavelength in resonance with an electronic transition, the weak spectral signals can be selectively enhanced. We implement a Kohn-Sham methodology for the calculation of resonance and off-resonance VROA spectra to (S)-naproxen and (S)-ibuprofen and discuss their band patterns. The resonance enhancement of the VROA spectrum of (S)-naproxen at an incident wavelength of 514.5 nm caused by an absorption tail as well as the typical off-resonance behavior of the VROA spectrum of (S)-ibuprofen at the same incident wavelength can be well reproduced. VROA spectra are also predicted under full resonance conditions.

Slice‐selective adiabatic magnetization T2‐preparation (SAMPA) for efficient T2‐weighted imaging at ultrahigh field strengths

At high field, T2 -weighted (T2 w) imaging is limited by transmit field inhomogeneity and increased radiofrequency power deposition. In this work, we introduce SAMPA (Slice-selective Adiabatic Magnetization T2 PrepAration) and demonstrate its use for efficient brain T2 w imaging at 7 Tesla (T). SAMPA was designed by subsampling an optimized B1 insensitive rotation (BIR4) waveform with small tip angle linear subpulses. To perform T2 w imaging, SAMPA was inserted before a fast gradient echo acquisition. The off-resonance behavior, B1 robustness, and slice selectivity of the novel T2 preparation module were analyzed using Bloch simulations. The performance of SAMPA for T2 w imaging was demonstrated in phantom experiments as well as in the brains of healthy volunteers at 7T. Based on simulations, the proposed design operates at peak B1 of 15 μT and higher, within a 400 Hz bandwidth. T2 values were in strong agreement with spin echo-based T2 mapping in phantom experiments. Whole brain, interleaved multislab three-dimensional imaging could be acquired with 0.8 mm3 isotropic resolution in 5:36 min per T2 weighting. Compared with previous adiabatic T2 preparation techniques, SAMPA allows for slice-selectivity, which can lead to efficient and robust acquisitions for T2 w imaging at high field. Magn Reson Med 76:1741-1749, 2016. © 2015 International Society for Magnetic Resonance in Medicine.

On the generalized Hartman effect for symmetric double-barrier point potentials

We consider the scattering of a non-relativistic particle by a symmetrical arrangement of two identical barriers in one-dimension, with the barriers given by the well-known four-parameter family of point interactions. We calculate the phase time and the stationary Salecker-Wigner-Peres clock time for the particular cases of a double δ and a double δ' barrier and investigate the off-resonance behavior of these time scales in the limit of opaque barriers, addressing the question of emergence of the generalized Hartman effect.

A low power radiofrequency pulse for simultaneous multislice excitation and refocusing.

Simultaneous multislice (SMS) acquisition enables increased temporal efficiency of MRI. Nonetheless, MultiBand (MB) radiofrequency (RF) pulses used for SMS can cause large energy deposition. Power independent of number of slices (PINS) pulses reduce RF power at cost of reduced bandwidth and increased off-resonance dependency. This work improves PINS design to further reduce energy deposition, off-resonance dependency and peak power. Modifying the shape of MB RF-pulses allows for mixing with PINS excitation, creating a new pulse type with reduced energy deposition and SMS excitation characteristics. Bloch Simulations were used to evaluate excitation and off-resonance behavior of this "MultiPINS" pulse. In this work, MultiPINS was used for whole-brain MB = 3 acquisition of high angular and spatial resolution diffusion MRI at 7 Tesla in 3 min. By using MultiPINS, energy transmission and peak power for SMS imaging can be significantly reduced compared with PINS and MB pulses. For MB = 3 acquisition in this work, MultiPINS reduces energy transmission by up to ∼50% compared with PINS pulses. The energy reduction was traded off to shorten the MultiPINS pulse, yielding higher signal at off-resonances for spin-echo acquisitions. MB and PINS pulses can be combined to enable low energy and peak power SMS acquisition.

Coherent monochromatic phonons in highly purified semiconducting single-wall carbon nanotubes

We have used a femtosecond pump-probe impulsive Raman technique to explore the polarization dependence of coherent optical phonons in highly purified and aligned semiconducting single-wall carbon nanotubes (SWCNTs). Coherent phonon spectra for the radial breathing modes (RBMs) exhibit a different monochromatic frequency between the film and solution samples, indicating the presence of differing exciton excitation processes. By varying the polarization of the incident pump beam on the aligned SWCNT film, we found that the anisotropy of the coherent RBM excitation depends on the laser wavelength, which we consider to be associated with the resonant and off-resonant behavior of RBM excitation.

Slice‐selective tunable‐flip adiabatic low peak‐power excitation pulse

Adiabatic pulses are useful in achieving uniform excitation profiles in the presence of B(1)-inhomogeneity. At higher fields, this inhomogeneity becomes more severe, further amplifying the need for B(1)-insensitive excitation. Although gradient modulation techniques for slice-selective adiabatic excitation have been introduced, a pulse that falls within the gradient and RF amplifier limits for most commercial human scanners is currently unavailable. In this work, we present an alternative gradient modulated approach for pulse design that achieves adiabatic slice selection with significantly lower RF peak power requirements. The resulting Slice-selective Tunable-flip AdiaBatic Low peak-power Excitation (STABLE) pulse consists of an oscillating gradient in conjunction with a BIR-4-like RF envelope that is sampled by many short spatial subpulses to achieve spatial selectivity. Simulations show that the expected spatial profile as well as the off-resonance behavior of the pulse remain invariant for a range of B(1) values. Phantom and in vivo results demonstrate the adiabaticity and slice selectivity of the STABLE pulse.

Selective 3D ultrashort TE imaging: comparison of “dual-echo” acquisition and magnetization preparation for improving short-T 2 contrast

The objective of this study was to compare two different schemes for long-T (2) component suppression in ultrashort echo-time (UTE) imaging. The aim was to increase conspicuity of short-T (2) components accessible by the UTE technique. A "dual-echo" and a magnetization-preparation approach for long-T (2) and fat suppression were implemented on clinical scanners. Both techniques were compared in 3D UTE exams on healthy volunteers regarding short-T (2) Signal-to-noise ratio (SNR), long-T (2) suppression quality, and scan efficiency. A quantitative SNR evaluation was performed using ankle scans of six volunteers. T (2) suppression profiles were simulated for both approaches to facilitate interpretation of the observations. At 1.5 T, both techniques perform equally well in suppressing long-T (2) components and fat. Magnetization preparation requires more shimming effort due to the use of narrow-band pulses, while the "dual-echo" technique requires a post-processing step to form a subtraction image. For scans with a short repetition time (TR), the "dual-echo" approach is much faster than the magnetization preparation, which depends on slow T (1) recovery between preparation steps. The SNR comparison shows slightly higher short-T (2) SNR for the "dual-echo" approach. At 3.0 T, magnetization preparation becomes more challenging due to stronger off-resonance effects. Both techniques are well suited for long-T (2) suppression and offer comparable short-T (2) SNR. However, the "dual-echo" approach has strong advantages in terms of scan efficiency and off-resonance behavior.

Direct parallel image reconstructions for spiral trajectories using GRAPPA

The use of spiral trajectories is an efficient way to cover a desired k-space partition in magnetic resonance imaging (MRI). Compared to conventional Cartesian k-space sampling, it allows faster acquisitions and results in a slight reduction of the high gradient demand in fast dynamic scans, such as in functional MRI (fMRI). However, spiral images are more susceptible to off-resonance effects that cause blurring artifacts and distortions of the point-spread function (PSF), and thereby degrade the image quality. Since off-resonance effects scale with the readout duration, the respective artifacts can be reduced by shortening the readout trajectory. Multishot experiments represent one approach to reduce these artifacts in spiral imaging, but result in longer scan times and potentially increased flow and motion artifacts. Parallel imaging methods are another promising approach to improve image quality through an increase in the acquisition speed. However, non-Cartesian parallel image reconstructions are known to be computationally time-consuming, which is prohibitive for clinical applications. In this study a new and fast approach for parallel image reconstructions for spiral imaging based on the generalized autocalibrating partially parallel acquisitions (GRAPPA) methodology is presented. With this approach the computational burden is reduced such that it becomes comparable to that needed in accelerated Cartesian procedures. The respective spiral images with two- to eightfold acceleration clearly benefit from the advantages of parallel imaging, such as enabling parallel MRI single-shot spiral imaging with the off-resonance behavior of multishot acquisitions.

On chiral selectivity of enantiomers using a circularly polarized pulsed laser under resonant and off-resonant conditions

The rotating-wave approximation is employed to study the interaction of a pulsed circularly polarized laser with a chiral molecule with a view to controlling state probabilities. For a two-level model system, closed form solutions are available for ground and excited state populations for both resonant and off-resonant one-photon absorption transitions driven by lasers described by a wide class of pulse envelope functions, only one of which, the hyperbolic secant, is symmetric. By appropriate choice of the laser pulse width and envelope function, complete control is possible for molecules with fixed orientation with respect to the laser propagation direction when the optical frequency is equal to the difference in energy levels. In contrast, for off-resonant transitions, full control is lost for asymmetric pulse shapes. When the molecule assumes random orientations, the difference in the excited state relative populations of enantiomers is reduced significantly. Based on the study of off-resonant behavior, a new approach for chiral separation, which can dramatically improve the selectivity, is suggested.

Resonant transmission through finite-sized carbon nanotubes

We have investigated theoretically the conductance through finite-sized carbon nanotubes coupled to featureless leads in the context of standard tight-binding models. Conduction takes place via resonant tunneling, and the resultant spectrum of peaks may be understood in terms of the band structure of the nanotubes. Specific nanotubes display both on- and off-resonance behavior as a function of nanotube length depending upon whether or not the bands cross at a nonzero k value. We have also investigated the approach to the infinite limit in detail, and, in general, find that the finite-size effects can persist out to hundreds of nanometers. Since the manipulation of nanotubes into devices is likely to induce defects, we have investigated their effects on the conduction. The effects of bending and two broad classes of defects, i.e., defect in the absence and/or presence of addimers, were considered. In general, the presence of defects leads both to a reduction and shifting of the resonant peaks at the Fermi level. However, in the infinite limit, low concentrations of defects have only a relatively modest effect on the transport properties. Finally, we have investigated the effects of an externally imposed magnetic field oriented perpendicular to the nanotube axis. The magnetic field shifts the levels, thereby turning on- and off-resonant devices into each other. All of the effects discussed here are testable experimentally.

On the Conversion of Triple- to Single-Quantum Coherences in MQMAS NMR

A systematic experimental and numerical evaluation of several basic approaches to multiple-quantum magic angle spinning (MQMAS) NMR is presented for spin-32 nuclei. The approaches use identical MQ excitation, via a single RF pulse of high power, and three types of methods for conversion to observable coherence: (a) nutation by strong continuous wave pulse; (b) rotation-induced adiabatic coherence transfer (RIACT), and (c) fast amplitude modulation (FAM-1). The optimization strategies and maximum achievable MQMAS efficiencies of (87)Rb in RbNO(3) and LiRbSO(4) are investigated using several coherence transfer schemes under a wide range of experimental parameters. These parameters include the strength of the RF magnetic field nu(RF), the sample rotation speed nu(R), the length of the conversion period, and the modulation frequency in FAM-1. The data provide new insights into the spin dynamics involved in these techniques and the experimental guidelines for achieving the best sensitivity. The RF requirements for maximum efficiency of conversion depend on the method to be used. In general, FAM-1 performs better than the nutation and RIACT methods in terms of efficiency and off-resonance behavior, especially when nu(RF) is small compared to the quadrupole frequency nu(Q). The experiments performed using nutation, RIACT, and FAM-1 methods yield similar resolution in the isotropic dimension, regardless of nu(RF).

High-resolution bubble sizing through detection of the subharmonic response with a two-frequency excitation technique

Sizing bubbles in fluid using a two-frequency excitation technique is not prone to the same drawbacks of some other sizing methods—it has a global maximum at the bubble resonance frequency and allows good spatial resolution. The bubble is insonated with a high fixed imaging signal and a variable pumping signal tuned to the resonant frequency of the bubble, which are coupled at resonance by the high-amplitude oscillation of the bubble wall, with the formation of sum-and-difference terms. This paper examines both the resonance and off-resonance behavior of such combination frequency signals. A coupling of the subharmonic bubble response with the imaging frequency is shown to be a much more accurate and unambiguous detector of the bubble resonance than couplings involving the fundamental resonance. The characteristics of this subharmonic signal are examined using an automated sizing method, and the dependence of the response on the pumping signal amplitude and the frequency step size between two successive pumping frequencies is examined. The location of a definite subharmonic threshold is reported and quantified both for single bubbles held on a wire and for free rising bubbles moving through the focus of the transducers. This amplitude is found to be orders of magnitude lower than that predicted by traditional volumetric pulsation models, but agrees very closely with the theoretical onset of surface waves.

A homogeneity correction method for magnetic resonance imaging with time-varying gradients

When time-varying gradients are used for imaging, the off-resonance behavior does not just cause geometric distortion as is the case with spin-warp imaging, but changes the shape of the impulse response and causes blurring. This effect is well known for projection reconstruction and spiral k-space scanning sequences. The authors introduce a reconstruction and homogeneity correction method to correct for the zeroth order effects of inhomogeneity using prior knowledge of the inhomogeneity. In this method, the data are segmented according to collection time, reconstructed using some fast, linear algorithm, correlated for inhomogeneity, and then superimposed to yield a homogeneity corrected image. This segmented method is compared to a conjugate phase reconstruction in terms of degree of correction and execution time. The authors apply this method to in vivo images using projection-reconstruction and spiral-scan sequences.

Experimental study of one- and two-photon Rabi oscillations.

We report an experimental study of one- and two-photon Rabi oscillations and other coherent effects in microwave transitions between Rydberg states of calcium. Selective field ionization allows us to observe the time evolution of both the initial and final states with high discrimination. The experiments include measurements of the dependence of the one- and two-photon oscillation frequencies on microwave power and detuning from resonance. We have used the off-resonant behavior of the two-photon oscillation frequency to measure the ac Stark shift. In a magnetic field we have also observed the free evolution of the magnetic sublevels and the effect of Zeeman splitting on the one-photon Rabi oscillations. The theory for these experiments is presented; our results are in excellent agreement.

A comprehensive analysis of surface acoustic wave reflections

A thorough study of the perturbative and variational approaches is carried out for the surface acoustic wave reflection problem. We have shown that the perturbation treatment by Datta and Hunsinger and potentially powerful variational formulation by Chen and Haus [IEEE Trans. Sonics Ultrason. SU-32, 395 (1985)] are mutually consistent. In their common region of validity, these two approaches yield nearly identical results for the reflection coefficients and velocity shifts due to metal finger and groove overlays. Term-by-term comparison of the mass- and stress-loading effects, and also the electric shorting effect, is carried out to provide a coherent picture of the reflection phenomena. The on- and off-resonance behavior of the reflection coefficient can be described correctly using either one of these theories, with proper inclusion of the overlay shape dependence. A new term for electric shorting is derived for groove overlays.

The small-signal theory of the cyclotron maser and other gyrotron-type devices

The simplified cyclotron maser model presented by Lau (ibid., vol.ED-29, p.320-35, 1982) is analyzed under small-signal conditions with an exact analytical solution to the proper boundary condition taken across a thin, hollow rotating beam. Not only is the transverse electric field discontinuous but so are the axial and radial magnetic field components. The simultaneous presence of TE and TM modes are considered, leading to modifications in the dispersion relationships. The simultaneous inclusion of both modes and space charge leads to the main contribution to the theory: TE-TM coupling, off-resonance behavior, and subcutoff operation in the drift spaces of klystron-type devices. Also, since radial displacement of the beam is taken into account, the theory predicts a peniotron interaction in the cyclotron maser. Examples are presented of gyrotron or peniotron interactions in either the TE and TM modes as well as off-resonance and subcutoff. >

Receiving and Scattering Characteristics of Circular Patch Antenna Array

This paper examines the receiving characteristics of an infinite array of circular patch antennas. The patches are located on a layer of dielectric, which is situated on a ground plane. Each patch is connected to a load impedance at the ground plane through a feed pin. The entire structure is illuminated by an incident plane wave. The receiving characteristics are obtained over a frequency range of 3 to 1 or more, so that off-resonance behavior and the effect of higher order resonances on the antenna characteristics are important, and need to be accurately determined. The receiving array is decomposed into scattering and transmitting cases which are solved separately, and reconstituted to give the receiving case. Accurate solutions of these cases over a wide frequency bandwidth are pursued, including full effects of the feed pin. The solution is approached through a Green's dyadic formulation for sources and induced fields both in the dielectric region and on the dielectric-cover interface. Galerkin's met...

Theory of a generalized Helmholtz resonator

Based on the jet-flow model which is manifested in the nonlinearity of the orifice flow upon the passage of a high intensity wave, the theory of a generalized Helmholtz resonator has been developed. The results for some special devices, such as the conventional Helmholtz resonator and the quarter-wave tube, can be obtained through this general approach. The performance of the acoustic damper is characterized by a quantity known as the real part of the admittance coefficient. It is known that the conventional Helmholtz resonator has the lowest peak performance but the response is quite flat over a certain frequency range. On the other hand, a quarter-wave tube offers the largest peak in energy absorption with rather poor off-resonance behavior. Once the orifice length goes beyond the quarter-wave tube configuration, the performance of the so-called long damping device is even less attractive away from resonance.