RF Intensity Research Articles

Segmentation of 3D radio frequency echocardiography using a spatio-temporal predictor

This paper presents an algorithm for segmenting left ventricular endocardial boundaries from RF ultrasound. Our method incorporates a computationally efficient linear predictor that exploits short-term spatio-temporal coherence in the RF data. Segmentation is achieved jointly using an independent identically distributed (i.i.d.) spatial model for RF intensity and a multiframe conditional model that relates neighboring frames in the image sequence. Segmentation using the RF data overcomes challenges due to image inhomogeneities often amplified in B-mode segmentation and provides geometric constraints for RF phase-based speckle tracking. The incorporation of multiple frames in the conditional model significantly increases the robustness and accuracy of the algorithm. Results are generated using between 2 and 5 frames of RF data for each segmentation and are validated by comparison with manual tracings and automated B-mode boundary detection using standard (Chan and Vese-based) level sets on echocardiographic images from 27 3D sequences acquired from six canine studies.

Experiment on space charge driven nonlinear resonance crossing in an ion synchrotron

Trapping of particles in nonlinear resonances in the presence of space charge and synchrotron motion may be a source of beam halo generation and beam loss in high intensity synchrotrons, in particular for extended storage times at the injection plateau as planned for the SIS100 synchrotron of the FAIR project. Although extensive simulation studies have theoretically demonstrated this mechanism, experimental evidence was so far limited to demonstration experiments at the CERN Proton Synchrotron (PS) in 2002--2003 using an octupolar resonance. Here we describe new experiments at the SIS18 synchrotron at GSI, where the resonance is driven by a sextupolar field error and horizontal static tune scans are taken across the resonance stop band. The new data significantly extend the previous observations by a complete set of measurements comparing beams with and without rf, both at low and high intensity. The correlation between transverse beam loss and simultaneous bunch length shortening provides strong evidence that the measured emittance and the loss in intensity are indeed caused by periodic resonance crossing, leading to the main effect of scattering but also to a lesser extent to the trapping of particles due to the combined effect of the nonlinear resonance and the space charge.

Studies on multiple‐quantum magic‐angle‐spinning NMR of half‐integer quadrupolar nuclei under strong rf pulses with a microcoil

AbstractIn multiple‐quantum magic‐angle‐spinning (MQMAS) NMR experiments, it has been argued that one should use as strong rf irradiation as possible in most situations so as to attain the better sensitivity. In this work, we theoretically derive formulas for the efficiency of MQMAS experiments for spin $ {3 \over 2} $ nuclei under strong rf irradiation, and experimentally examine the validity of the derived formulas. For this purpose, a microcoil was accommodated to magic‐angle‐spinning (MAS) experiments by developing a microcoil‐based, coin‐sized probe, which is attachable to an existing MAS module. The rf intensity of up to ∼1 MHz was realized, and the efficiencies of multiple‐quantum (MQ)‐excitation and conversion were studied for the 23Na (I = 3/2) nucleus in a polycrystalline sample of sodium sulfite, from which the theoretical formulas were confirmed. Moreover, it was found that the effect of rf inhomogeneity is significant for MQ‐excitation when rf irradiation is strong, while it is not appreciable for MQ to single‐quantum conversion. From the insight into the MQ‐excitation and conversion processes, the optimal set of rf intensity and pulse width is proposed for a given quadrupolar coupling constant. © 2008 Wiley Periodicals, Inc. Concepts Magn Reson Part B (Magn Reson Engineering) 33B: 115–123, 2008

Theory of double resonance magnetometers based on atomic alignment

We present a theoretical study of the spectra produced by optical--radio-frequency double resonance devices, in which resonant linearly polarized light is used in the optical pumping and detection processes. We extend previous work by presenting algebraic results which are valid for atomic states with arbitrary angular momenta, arbitrary rf intensities, and arbitrary geometries. The only restriction made is the assumption of low light intensity. The results are discussed in view of their use in optical magnetometers.

On-resonance low B1 pulses for imaging of the effects of PARACEST agents

Application of the exchange-sensitive, low-power RF pulses positioned on the bulk water resonance for imaging of the effects of PARACEST agents is proposed as an alternative to the standard CW off-resonance irradiation. Specifically, we applied a low-power WALTZ-16 RF train, with the 90° pulse unit replaced by a pulse of the fixed length (WALTZ-16*). Using this sequence, the bulk water signal was found to be sensitive to exchange lifetimes with PARACEST complex bound protons, the transverse relaxation time of bulk water, and longitudinal relaxation time of bound protons. In this report, the concept of using WALTZ-16* to “activate” a PARACEST effect is introduced and some of the salient features of this technique with respect to experimental conditions and performance levels are discussed. Computational predictions are verified and explored by comparison with experimental spectroscopic and imaging data. It is shown that WALTZ-16* can be used to detect PARACEST agents with an RF intensity as low as 200 Hz for concentrations as low as a few tens of μM for lanthanide chelates having appropriate water-exchange rates (Tm,Dy).

H 1 to H2 uniform cross-polarization nuclear magnetic resonance using H2 Lee–Goldburg irradiation in static powders

Application of conventional cross polarization (CP) to (2)H results in only a narrowband enhancement of the powder line shape due to the quadrupole interaction. We propose a CP scheme to uniformly enhance (2)H spectra in static powders. In this method, a Hartmann-Hahn matched (2)H rf field is applied on the Lee-Goldburg (LG) condition to remove the zeroth-order quadrupole interaction. In order to achieve a uniformly enhanced (2)H powder line shape with a limited (2)H rf intensity, the (1)H rf amplitude in CP is stepwise altered during the contact time. We develop a spin-thermodynamic theory to describe polarization transfer due to CP with LG irradiation (LG-CP) under the influence of the quadrupole interaction, which can successfully reproduce the LG-CP line shapes observed under various experimental conditions. Experimental and simulated (2)H powder spectra are reported for some compounds.

An optical fibre sensor for dynamic deformation measurements based on the intensity modulation of a low-coherence source

An optical fibre sensing technique for the measurement of dynamic deformations using a Michelson interferometer is reported. The method applied to interferometers with an initial path unbalance of around 1 cm is based on the rf intensity modulation of a low-coherence source. A large measurement range of 1 cm is intrinsic to the method and with an adequate demodulation process sensitivity better than 10 μm is obtained. This novel approach allows for measuring dynamic deformations with a bandwidth up to 100 Hz.

An optical fibre sensor for dynamic deformation measurements based on the intensity modulation of a low-coherence source

An optical fibre sensing technique for the measurement of dynamic deformations using a Michelson interferometer is reported. The method applied to interferometers with an initial path unbalance of around 1 cm is based on the rf intensity modulation of a low-coherence source. A large measurement range of 1 cm is intrinsic to the method and with an adequate demodulation process sensitivity better than 10 μm is obtained. This novel approach allows for measuring dynamic deformations with a bandwidth up to 100 Hz.

Selective Homonuclear Polarization Transfer in the Tilted Rotating Frame under Magic Angle Spinning in Solids

An application of the R2TR method (1995,Chem. Phys. Lett.232,424) to selective homonuclear polarization transfer under magic angle spinning is proposed. It is shown that, for a spinning speed fast enough to remove the maximum homonuclear dipolar coupling constant ωDinvolved, the flip-flop and flop-flop mechanisms are suitable for recoupling the spins with a chemical shift difference larger than ωDand a difference comparable to or smaller than ωD, respectively. It is also shown that, for fast polarization transfer, the off-resonance frequencies should be much higher than the RF intensity in the flip-flop condition, while for the flop-flop condition, the off-resonance frequencies should be much lower than the RF intensity. Some one- and two-dimensional experiments are proposed by utilizing the capability of the R2TR method to abruptly switch on and off the recoupling condition, and are demonstrated for triply13C-enrichedl-alanine. The mixing time required for population transfer was found to be ca. 0.5 ms for the methine and methyl13C spins separated by 1.5 Å and ca. 5 ms for the methyl and the carboxyl carbons separated by 2.5 Å. The experimental results and theoretical simulations show that selective polarization transfer is achieved when the difference in the isotropic chemical shifts between the relevant pair of spins and a neighboring spin is more than 1000 Hz.

Diode-pumped optical parametric oscillators

We report on diode-laser-pumped, continuous-wave (CW) optical parametric oscillators (OPO). The results describe the operation and characterization of doubly and triply resonant CW-OPOs (DRO, TRO) based on the non-linear materials (KTP) and (RTA) and different cavity designs. Stable single-frequency operation with a maximum signal output power of 84 mW was obtained with a pump- and idler-resonant two-mirror RTA-DRO, by electronically locking the OPO's pump cavity to the diode master-oscillator power-amplifier (MOPA) pump source. We present signal wave RF intensity noise spectra of an RTA-TRO, which is pumped with an amplitude-squeezed diode laser. The OPO noise spectra demonstrate, for the first time, the presence of relaxation oscillations in CW-OPOs.

63Cu NQR Nutation Spectroscopy and Spin Counting in Copper Oxides

AbstractNutation spectroscopy of 63Cu nuclei at zero static magnetic field is explored as a method for the determination of the quadrupole asymmetry parameters η of Cu sites in copper oxides, with special emphasis on high‐Tc superconductors. Since the nutation spectra reveal the actual radio frequency (rf) intensities experienced by the 63Cu nuclei, the method also provides a verification of a quantitative Cu spin counting procedure based on integration of nuclear quadrupole resonance (NQR) echo‐signal intensities over wide frequency ranges. The paper presents numerical simulations of spectra and signal intensities that were obtained with a density‐matrix formalism whereby the spin dynamics is described as precessions in a 3D operator space. The quantitative effects of the echo refocusing pulse, rf amplitude fluctuations, and variations in η are examined. Examples of 63Cu NQR nutation spectra are shown for randomly oriented powders of Cu2O and YBa2Cu3O7.

Continuous-wave laser without frequency-domain-mode structure: investigation of emission properties and buildup dynamics

The development of a broadband cw laser with no Fabry–Perot-mode structure is detailed. Through insertion of frequency-selective elements a bandwidth of 12 GHz is achieved, resulting in a peak spectral power density of ~50 μW/MHz. With heterodyne techniques the broadband nature of the field is shown to result solely from fluctuations in the phase. A comparison between the output of the broadband laser and that of a conventional multimode laser demonstrates the different nature of the emission. Discrete lines in the rf intensity spectrum are examined with regard to their origin. The temporal evolution of the spectral profile, starting from quantum noise, is investigated. Several features of this system, which incorporates an intracavity acousto-optic modulator, are remarkably different from those of a conventional cw laser.

Mössbauer study of the radio frequency induced effects in amorphous Fe xCo 78−xSi 9B 13 alloys in amorphous Fe xCo 78− xSi 9B 13 alloys

The structural properties and crystallization of amorphous Fe x Co 78− x Si 9B 13 alloys were studied using the Mössbauer technique. The radio frequency (rf) induced collapse of the magnetic hyperfine splitting allowed a direct determination of the quadrupole splitting as a function of composition of ferromagnetic amorphous alloys. It was found that alloys with x < 62 crystallized rapidly in the rf field making the observation of the rf collapse impossible. The rf induced crystallization was studied as a function of time of rf exposure and intensity of the rf field. The dependence of the relative abundance of the crystalline phases α-(FeCo), (FeCo) 2(SiB) and (FeCo) 3(SiB) formed by crystallization is reported. The study of magnetic field distributions and the variation of magnetic field vs. composition for amorphous and crystalline phases permits conclusions about short range order in the amorphous state.

Four-field excitation of multiphoton NMR resonances in spin I=(1/2).

The use of four pulsed rf fields for the excitation of multiphoton resonances in a two-level system is investigated. A general theoretical description of the experiment is provided based on the Floquet formalism and application of basic conservation laws to the overall transition process. The predictions of the theory are compared with computer simulations of the time evolution of the magnetization according to the exact time-dependent Hamiltonian. It is shown that N-photon resonances occur when the fields are applied at frequencies \ifmmode\pm\else\textpm\fi{}k\ensuremath{\omega} and \ifmmode\pm\else\textpm\fi{}l\ensuremath{\omega} from the Larmor frequency, where k and l are positive integers having no common factors and where 0\ensuremath{\le}k&lt;l, k+l=N. For any resonance k photons are taken from the outer field pair and l are taken from the inner field pair. A total of 2[(l+k)!]/(k!l!) mutually interfering transition pathways exist for the overall process and the resultant magnitude of the effective N-photon field is very sensitive to the relative initial phases of the rf fields. The spiral motion observed for multiphoton resonances induced by double-frequency irradiation is absent here when the field intensities are symmetrical. In addition, the fraction of total applied rf intensity available for multiphoton pumping is considerably greater than for two fields. Phase cycling to enhance detection of multiphoton effects and multiphoton spin locking are considered.

Stability of mirrors with inverted pressure profiles

The stability of an axisymmetric tandem mirror with an inverted (or hollow) pressure profile is treated within the framework of scalar-pressure, ideal magnetohydrodynamics. A two-surface sharp-boundary model is formulated to describe the low-m ballooning stability of the annular plasma in the paraxial limit. Included in the analysis are the stabilizing effects of an externally applied force, such as the rf-induced ponderomotive force, and of a perfectly conducting lateral wall. It is shown that there is a stability boundary associated with each of these effects. In critical ranges of the plasma’s annular thickness, the rf intensity and the wall position, the stability regions merge giving complete stability at all values of the plasma pressure.

Magnetic domain wall waves, parametrically excited in a stripe domain lattice in a garnet film

Waves of magnetic domain wall motion in an array of straight, aligned stripe domains have been detected by observations of optical diffraction from the array. When the array is excited by a spatially uniform rf drive field of amplitude greater than 0.25 Oe peak, at a frequency near the resonant frequency of the uniform ’’breathing’’ mode of the array, dynamic spots appear in the diffraction pattern. The dynamic spot positions correspond to wave vectors oblique to the stripes, while the rf intensity variation suggests that the waves are parametrically excited at one-half the frequency of the spatially uniform mode.

Double-quantum cross-polarization NMR in solids

Double-quantum NMR is a useful way to obtain spectra of quadrupolar nuclei ($^{2}\mathrm{D}$, $^{14}\mathrm{N}$,...) in solids. This allows measurements of the chemical shifts for these nuclear spins. The theory of Hartmann-Hahn cross polarization between $I=\frac{1}{2}$ and such $S=1$ spins is discussed. Particular attention is drawn to the cross polarization of the double-quantum transition. The thermodynamics and the dynamics of the process are evoked in detail using a fictitious spin-\textonehalf{} formalism. The spin $S=1$ Hamiltonian can always be factored into two commuting parts (independent thermodynamic reservoirs), one of which behaves as a fictitious spin \textonehalf{} which is cross polarized with the $I=\frac{1}{2}$ spins. Modified Hartmann-Hahn conditions emerge from the theory, and the dependence of cross-polarization times ${T}_{\mathrm{IS}}$ on rf intensity and frequency for spin locking and adiabatic demagnetization in the rotating-frame experiments are calculated. Measurements on the $^{1}\mathrm{H}$-$^{2}\mathrm{D}$ double resonance in dilute solid benzene-${d}_{1}$ are reported, verifying the predictions and indicating that cross polarization provides a sensitive means of detecting the $^{2}\mathrm{D}$ double-quantum transition. Values are reported for the thermodynamic parameters and cross-polarization times as a consequence. Three possible versions of double-resonance detection of double-quantum spectra are possible---direct detection of the cross-polarized double-quantum decay, indirect detection of the frequency spectrum following Hartmann and Hahn, and indirect detection of the free-induction decay following Mansfield and Grannell.

Single-frequency cw injection heterolaser tunable by an external dispersive resonator

An investigation was made of the spectral composition of the radiation emitted from a cw stripe AlGaAs–GaAs injection heterolaser operating at room temperature in an external dispersive resonator. Narrow-band (Δλ≤0.15 nm) stimulated emission with an output power up to 20 mW was obtained. The output radiation was tunable in a range of ~10 nm centered on the wavelength of 884 nm and in this case the maximum cw single-frequency power was 4 mW. Excitation of several longitudinal external-resonator modes (within the limits of the narrow emission band) was accompanied by rf intensity fluctuations consisting of mode beats (250–500 MHz) and a noise band at much lower frequencies (30–100 MHz).

Nonlinear interactions of focused resonance cone fields with plasmas

A simple yet novel rf exciter structure has been developed for generating remotely intense rf fields in a magnetoplasma. It is a circular line source of radius R in a plane ⊥B0 driven with an rf signal at ω &amp;lt;ωc≃ωp (where ωc, ωp are the electron cyclotron and electron plasma frequency, respectively) which excites two resonance cones with focal points at z=±R cotΘc (Θc is the cone angle). The rf intensity increases away from the exciter to reach a maximum (approximately 20 dB enhancement) at the remote focal points. This arrangement is ideally suited to study nonlinear interactions free of boundary effects. At large applied rf signals, ε0Erf2/nkTe≳0.2, a strong density depression in the focal region (δn/n≳40%) is observed. The density perturbation modifies the cone angle and field distribution. This nonlinear interaction leads to a rapid growth of ion acoustic wave turbulence and a corresponding random rf field distribution in a broadened focal region. The development of the interaction is mapped in space and time.

Radio-Frequency Resonance Echoes in Magnetic Materials

A resonance-echo effect not involving nuclear spins in magnetically ordered materials has been investigated at radio frequencies. The echo amplitudes and relaxation properties have been studied by varying the temperature, frequency, rf intensity, and external static magnetic field for several magnetic conductors and insulators. In a zero external magnetic field, the echoes occur throughout the available frequency range (5–110 Mc/sec) and relax exponentially with relaxation times roughly inversely proportional to temperature or applied frequency through most of the range of either parameter. With an external field they exhibit peaks in amplitude vs field strength. Physical mechanisms which possibly produce these effects (domain wall motion or intradomain ferromagnetic resonance modes) are discussed.