Echo-planar spin-echo and inversion pulses

The echo-planar k-space trajectory can be used as the basis for any two-dimensional selective pulse. The main application is spectral-spatial pulses, which must be based on the echoplanar trajectory. In this paper we show how echo-planar spinecho (EPSE) pulses may be designed.     

Implementation and evaluation of frequency offset corrected inversion (FOCI) pulses on a clinical MR system

FOCI pulses are a variant of hyperbolic secant pulses in which the RF amplitude, RF frequency, and gradient waveform are all modulated by the same function A(t). This increases the usable gradient amplitude without requiring a corresponding increase in RF amplitude. In this paper the implementation and inversion profiles of FOCI pulses on a clinical MR system are described, showing improved slice definition and chemical-shift offset behavior. Their adiabatic behavior is confirmed, and their use with an ISIS sequence for localized MRS is illustrated. Finally the effects of waveform digitization are considered, and the implications for SAR and db/dt are discussed.     

Transmit SENSE.

The idea of using parallel imaging to shorten the acquisition time by the simultaneous use of multiple receive coils can be adapted for the parallel transmission of a spatially-selective multidimensional RF pulse. As in data acquisition, a multidimensional RF pulse follows a certain k-space trajectory. Shortening this trajectory shortens the pulse duration. The use of multiple transmit coils, each with its own time-dependent waveform and spatial sensitivity, can compensate for the missing parts of the excitation k-space. This results in a maintained spatial definition of the pulse profile, while its duration is reduced. This work introduces the concept of parallel transmission with arbitrarily shaped transmit coils (termed "Transmit SENSE"). Results of numerical studies demonstrate the theoretical feasibility of the approach. The experimental proof of principle is provided on a commercial MR scanner. The lack of multiple independent transmit channels was addressed by combining the excitation patterns from two separate subexperiments with different transmit setups. Shortening multidimensional RF pulses could be an interesting means of making 3D RF pulses feasible even for fast T(2)(*) relaxing species or strong main field inhomogeneities. Other applications might benefit from the ability of Transmit SENSE to improve the spatial resolution of the pulse profile while maintaining the transmit duration.     

A new adiabatic inversion pulse

Adiabatic pulses play an important role in magnetization inversion in the presence of RF field inhomogeneity. In this work the authors present an efficient adiabatic inversion pulse that is able to selectively invert magnetization over a large frequency bandwidth in a short time. The pulse is constructed in two steps: (i) the optimal trajectory is determined and (ii) the optimal rate of motion along that trajectory is determined. The resulting pulse enables separately controlling and trading off the pulse duration against the transition width. The superiority of this pulse over the well known sech/tanh adiabatic pulse is demonstrated in a scenario where a large bandwidth should be inverted at a short time using limited B₁ amplitude.     

Chemical Reactions by Pulsed Ultrasound: Memory Effects in the Formation of NO−3 and NO−2 in Aerated Water

Summary

The formation of nitrate and nitrite in the sonolysis of aerated water was studied using pulses of 300 kHz ultrasound. At very low on/off ratios, the yield decreases with decreasing pulse duration. At a pulse length of 3 × 10^?3s, the yield is zero. This time is identified as the ‘activation’ time τ₁ of small gas bubbles formed by cavitation. At larger on/off ratios, a pulse is more effective the shorter the time interval between the pulses. This memory effect is described by a ‘deactivation’ time τ₂ of the system, which amounts to about 6 × 10^?2s. At large on/off ratios (1:3 and 1:1) the yield never becomes zero. It first decreases with decreasing pulse length (increasing modulation frequency) and increases again for very short pulses. The results are also discussed with respect to the use of pulsed ultrasound in diagnostic applications.     

Frequency offset corrected inversion (FOCI) pulses for use in localized spectroscopy

Gradient localized spectroscopy techniques suffer from a well documented spatial localization error caused by the difference in chemical shifts between resonances. This results in the acquisition of spectra from partially overlapping spatial regions of the sample, with each resonance representing a different region. The image-selected in vivo spectroscopy technique uses hyperbolic secant inversion pulses, where the main limitation in reducing this error is in the RF power available for application of the selective RF pulse. This spatial localization error may be dramatically reduced by increasing, and temporally shaping, the gradient pulse during slice-selective spin inversion. The performance of these RF pulses have been experimentally verified.     

Stimulation threshold comparison of time‐varying magnetic pulses with different waveforms

Purpose

To clarify whether sinusoidal pulses possess lower thresholds than rectangular ones at perception threshold, a statement often made that contradicts the theory of stimulation.

Materials and Methods

The results of a nerve stimulation study with 65 volunteers and with trapezoidal and sinusoidal gradient pulses were used to apply the combination of the electric field, induced in the tissue of the human body, with the “Fundamental Law of Electrostimulation.” This law claims that the waveshape of a pulse is not essential as long as the amplitude of the pulse does not decrease below rheobase (rheobase condition).

Results

If the rheobase condition is applied to sinusoidal waveforms and the pulse duration and amplitude is corrected accordingly, both trapezoidal and sinusoidal gradient pulses have identical threshold amplitudes as a function of pulse duration.

Conclusion

The “Fundamental Law of Electrostimulation,” including the “rheobase condition,” proved to be a good basis for describing magnetic field stimulation (magnetostimulation) and that application of it to magnetostimulation is suitable as the basis for describing magnetic field stimulation with various waveforms. For nonrectangular pulses, pulse durations and pulse amplitudes must be corrected according to the “rheobase condition.” The exponential Blair Equation is less suited to be applied in magnetostimulation. J. Magn. Reson. Imaging 2009;29:229–236. © 2008 Wiley‐Liss, Inc.     

Fat suppressed MRI of articular cartilage with a spatial-spectral excitation pulse

We developed a three-dimensional, gradient-recalled-echo imaging technique that incorporates a short-duration spatial-spectral excitation pulse from the family of binomial pulses. Binomial pulses of different orders were tested on phantoms and on normal volunteers to find the composite pulse that produced in the shortest duration the most reliable fat suppression. Composite pulses employing unipolar slice-selective gradients with explicit rewinder gradients between each radiofrequency (RF) pulse were compared with composite RF pulses employing alternating-polarity, slice-select gradients. The advantage of the sequences using the unipolar gradients is improved fat suppression. Images of the knees of volunteers produced with the composite RF pulse have contrast between fat and articular cartilage equivalent to that on images created by the gradient-recalled-echo imaging technique employing a conventional chemsat pulse. The optimum RF pulse consisted of three amplitude- and phase-modulated pulses combined with unipolar slice-select gradients.     

Understanding pulsed magnetization transfer

Using a two-pool exchange model of magnetization transfer (MT), numeric simulations were developed to predict the time dependence of longitudinal magnetization in both semisolid and liquid pools for arbitrary pulsed radiofrequency (RF) irradiation. Whereas RF excitation of the liquid pool was modeled using the time-dependent Bloch equations, RF saturation of the semi-solid pool was described by a time-dependent rate proportional to both the absorption lineshape of the semisolid pool and the square of the RF pulse amplitude. Simulations show good agreement with experimental results for a 4% agar gel aqueous system in which the two-pool kinetics have been well studied previously. These simulations provide a method for interpreting pulsed MT effects, are easily extended to biologic tissues, and provide a basis for optimizing clinical imaging applications that exploit MT contrast.     

Single-shot,variable flip-angle slice-selective excitation with four gradient-modulated adiabatic half-passage segments

Adiabatic pulses, although useful in generating uniform spin nutation in the presence of inhomogeneous B₁ fields, are limited for NMR imaging applications due to the lack of slice-selective excitation capability. Selective excitation techniques using gradient modulation have been introduced; however, present methods require either a minimum of two excitations or eight adiabatic segments. Here, a scheme is presented that allows single-shot, arbitrary flip-angle, and slice-selective excitation with only four adiabatic half-passage segments. The technique is demonstrated via computer simulation and experimental tests on a phantom. Furthermore, issues associated with the implementation of these gradient-modulated adiabatic pulses are discussed.     

Asymmetric sampling along kslice-select in two-dimensional multislice MRI

Reduction of the slice-select refocusing gradient in two-dimensional multislice imaging results in asymmetry of the k-space representation of collected data along the slice-select direction. Standard methods of partial Fourier reconstruction developed for other methods of asymmetric k-space sampling can be used to reconstruct these data with final through-plane resolution smaller than the collected slice thickness. This method can be used for reducing scan time in the same manner as asymmetric sampling in the phase-encoded direction. In addition, the reduced refocusing gradient reduces minimum TE and motion artifacts in the same manner as for asymmetric sampling in the frequency-encoded direction (fractional echoes). Results using a resolution phantom and a flow phantom illustrate these concepts.     

Efficient design of pulses with trapezoidal magnitude and linear phase response profiles

A variation of the Shinnar-Le Roux (SLR) method of pulse envelope design that allows for control of the phase of the frequency response profile has been developed. The method makes use of the fact that a knowledge of one of the SLR polynomials in combination with a root inversion pattern for the other polynomial is sufficient to fully define the second polynomial. Optimization of the first polynomial, when cast in this form, remains nonlinear. However, it was demonstrated that the relationship between the SLR polynomials and the frequency response profile may be used to generate an initial guess for the SLR polynomials that is sufficiently accurate to allow for the application of linear optimization techniques in most cases. In practice several pulse envelopes having different root inversion patterns are investigated for each target profile. The resulting collection of pulses allows the user to trade off pulse power for profile accuracy. The proposed technique was used to design a large number of amplitude modulated excitation pulses having trapezoidal magnitude and linear phase frequency response profiles. A few examples of the resulting pulses and their response profiles are presented.