T2-selective magnetization preparation pulses.

The purpose of this work was to present and evaluate a new method for directly designing T2-selective preparation pulses. Using a modified Shinnar-Le-Roux (SLR) transform, the design of T2-selective pulses becomes equivalent to designing a pair of polynomials one of which represents the longitudinal magnetization and the other the transverse magnetization. The polynomials enable one to directly analyze the various tradeoffs involved in the design. To evaluate the new method, a short-T2-selective magnetization preparation pulse was designed. Following the preparation pulse, a 2D Fourier transform (2DFT) multislice gradient echo sequence was used for imaging. For verification Bloch equation simulations were performed along with both in vivo and phantom scans. Phantom scans showed good signal suppression of long-T2 species. This is supported by good long-T2 signal suppression seen on the in vivo images. Simulations indicate that the pulse is robust to +/-150 Hz B0 inhomogeneities and +/-10% B1 inhomogeneities.     

Fast Large-Tip-Angle Multidimensional and Parallel RF Pulse Design in MRI

Large-tip-angle multidimensional radio-frequency (RF) pulse design is a difficult problem, due to the nonlinear response of magnetization to applied RF at large tip-angles. In parallel excitation, multidimensional RF pulse design is further complicated by the possibility for transmit field patterns to change between subjects, requiring pulses to be designed rapidly while a subject lies in the scanner. To accelerate pulse design, we introduce a fast version of the optimal control method for large-tip-angle parallel excitation. The new method is based on a novel approach to analytically linearizing the Bloch equation about a large-tip-angle RF pulse, which results in an approximate linear model for the perturbations created by adding a small-tip-angle pulse to a large-tip-angle pulse. The linear model can be evaluated rapidly using nonuniform fast Fourier transforms, and we apply it iteratively to produce a sequence of pulse updates that improve excitation accuracy. We achieve drastic reductions in design time and memory requirements compared to conventional optimal control, while producing pulses of similar accuracy. The new method can also compensate for nonidealities such as main field inhomogeneties.     

Simultaneous correction of ghost and geometric distortion artifactsin EPI using a multiecho reference scan

A computationally efficient technique is described for the simultaneous removal of ghosting and geometrical distortion artifacts in echo-planar imaging (EPI) utilizing a multiecho, gradient-echo reference scan. Nyquist ghosts occur in EPI reconstructions because odd and even lines of k-space are acquired with opposite polarity, and experimental imperfections such as gradient eddy currents, imperfect pulse sequence timing, B0 field inhomogeneity, susceptibility, and chemical shift result in the even and odd lines of k-space being offset by different amounts relative to the true center of the acquisition window. Geometrical distortion occurs due to the limited bandwidth of the EPI images in the phase-encode direction. This distortion can be problematic when attempting to overlay an activation map from a functional magnetic resonance imaging experiment generated from EPI data on a high-resolution anatomical image. The method described here corrects for geometrical distortion related to B0 inhomogeneity, gradient eddy currents, radio-frequency pulse frequency offset, and chemical shift effect. The algorithm for removing ghost artifacts utilizes phase information in two dimensions and is, thus, more robust than conventional one-dimensional methods. An additional reference scan is required which takes approximately 2 min for a matrix size of 64 X 64 and a repetition time of 2 s. Results from a water phantom and a human brain at 3 T demonstrate the effectiveness of the method for removing ghosts and geometric distortion artifacts.     

NMR velocity-selective excitation composites for flow and motion imaging and suppression of static tissue signal

We propose a technique aimed at increasing the sensitivity of magnetic resonance imaging (MRI) measurements to the signal from moving material. The technique is formulated within the framework of a velocity phase-encoding strategy. The salient new feature of the protocol is the application of an excitation pulse, following the conventional 90 degrees excitation and bipolar phase gradient modulation pulses, which rotates the magnetization through -90 degrees . The whole comprises a velocity-selective excitation composite which leaves only magnetization of moving material in the transverse plane.     

T2 -Selective Magnetization Preparation Pulses

The purpose of this work was to present and evaluate a new method for directly designing T₂-selective preparation pulses. Using a modified Shinnar-Le-Roux (SLR) transform, the design of T₂-selective pulses becomes equivalent to designing a pair of polynomials one of which represents the longitudinal magnetization and the other the transverse magnetization. The polynomials enable one to directly analyze the various tradeoffs involved in the design. To evaluate the new method, a short-T₂-selective magnetization preparation pulse was designed. Following the preparation pulse, a 2D Fourier transform (2DFT) multislice gradient echo sequence was used for imaging. For verification Bloch equation simulations were performed along with both in vivo and phantom scans. Phantom scans showed good signal suppression of long-T₂ species. This is supported by good long-T₂ signal suppression seen on the in vivo images. Simulations indicate that the pulse is robust to plusmn150 Hz B₀ inhomogeneities and plusmn10% B₁ inhomogeneities.     

Feedback control of the nuclear magnetization state: modeling and control design

Magnetic resonance (MR) images are obtained by observing the fluctuations in nuclear magnetization produced by sequences of RF pulses and applied magnetic field gradients. The design of a pulse sequence is based on the expected response of the nuclear magnetization. Currently, all pulse sequence parameters are loaded into the pulse programmer before the start of the sequence and remain unchanged until the completion of the sequence. A fundamentally different approach is considered, whereby the sequence parameters are adjusted between successive RF excitation pulses. In this manner, the nuclear magnetization is regulated to a desired state by using measurements of the magnetization to adjust the amplitude and duration of the RF pulses. Feedback control of the nuclear magnetization state may be advantageous in certain types of MR experiments, for example those requiring a repeated spin-echo or gated MR image acquisition. As a first step in developing this approach a simple scheme for regulating the angle between the bulk magnetization and the axis applied static magnetic field is presented. The behavior of the closed-loop system is explored using computer simulations.     

Passive Nonlinear Pulse Shaping in Normally Dispersive Fiber Systems

We propose a novel approach to characterize the parabolically-shaped pulses that can be generated from more conventional pulses via nonlinear propagation in cascaded sections of commercially available normally dispersive (ND) fibers. The impact of the initial pulse chirp on the passive pulse reshaping is examined. We furthermore demonstrate that the combination of pulse pre-chirping and propagation in a single ND fiber yields a simple, passive method for generating various temporal waveforms of practical interest.      

Radio Frequency Pulsewidth Modulation

The width of a train of square pulses can be varied to produce a modulated carrier at the pulse repetition frequency. When the pulse train is generated by switching (class D) transistors, highefficiency operation is possible. The efficiency of this type of amplifier can be significantly higher than that of conventional pulsewidth modulation amplifiers, since the switching rate is reduced. In addition, the spectrum of a bipolar pulse train so modulated has the highly desirable property of all spurious products being band limited near the odd harmonics of the carrier.     

Optimal control solutions to the magnetic resonance selective excitation problem

Most magnetic resonance imaging sequences employ field gradients and amplitude modulated RF pulses to excite only those spins lying in a specific plane. The fidelity of the resulting magnetization distribution is crucial to overall image resolution. Conventional RF-pulse design techniques rely on the small tip-angle approximation to Bloch's equation, which is inadequate for the design of 90 degrees and 180 degrees pulses. This paper demonstrates the existence of a selective pulse, and provides a sound mathematical and computational basis for pulse design. It is shown that the pulses are optimal in the class of piecewise continuous functions of duration T. An optimal pulse is defined as the pulse on the interval that achieves a magnetization profile "closest" to the desired distribution. Optimal control theory provides the mathematical basis for the new pulse design technique. Computer simulations have verified the efficacy of the 90 degrees and the 180 degrees inversion and "pancake-flip" optimal pulses.     

A new pulse sequence for "fast recovery" fast-scan NMR imaging

This paper describes the "fast recovery" (FR) method for fast NMR imaging. The FR method combines a sequence of four RF pulses-alternating selective 90 degrees nutation pulses and nonselective 180 degrees pulses-with a gradient field pulse sequence which includes "spoiler" pulses to destroy the coherence between successive sequence cycles. We use the 2-D backprojection method of image reconstruction, but other imaging methods could be applied. The paper analyzes the behavior of the macroscopic magnetization-compares the FR method with other methods and proposes "figure of merit" expressions for relative signal-to-noise (S/N) ratios, scan time reduction ratios, and image contrast-and presents experimental results, including backprojection image reconstruction 2-D images and computed T1 and T2 images. For the FR method, in theory and practice, we find that, after each scan sequence cycle, magnetization is restored to equilibrium quickly and exactly; scan time can consequently be less than a tenth that for the saturation recovery method without any penalty in signal-to-noise ratio. Image contrast is even higher than that of the SR method, and compromise "optimum" sequence (interpulse timing) parameters give high image contrast for a wide range of tissue T1 and T2 (spin-lattice and spin-spin relaxation time) values.     

Generation of orthogonal UWB shaping pulses based on compressed chirp signal

This study investigates a novel method to numerically generate orthogonal ultrawide band (UWB) shaping pulses based on compressed chirp signal. First, a pulse template with less than 1 ns duration time, which is used to construct a Hermitian matrix, is produced with a compressed chirp pulse. Sub-nanosecond orthogonal pulses are then generated for UWB by using the Hermitian matrix eigenvectors. The simulation results show that the power spectral density distribution of the UWB shaping pulses met the constraint of Federal communications commissions (FCC) spectral mask. The shaping pulses not only have higher spectrum utilization ratio and very short time duration but also have excellent autocorrelation and cross-correlation properties, which is an advantage to reduce the interference between multiusers. Especially, a method to produce sub-nanosecond orthogonal UWB shaping pulses by using a relatively longer duration chirp signal is presented.     

Simple picosecond pulse generation scheme for injection lasers

A simple scheme is reported for generating picosecond optical pulses from injection lasers based on short electrical pulse excitation. An integrated step recovery diode impulse-train generator (`comb? generator) which gives 50 ps 25 V electrical pulses at 200 to 500 MHz rates is used to drive the injection lasers. Optical pulses as short as 40 ps are generated by the corresponding electrical drive pulses.     

Multi-WDM-channel, Gbit/s pulse generation from a single lasersource utilizing LD-pumped supercontinuum in optical fibers

More than 40 wavelength-division-multiplexed (WDM) channels of 5-10 ps pulse streams are generated over 1530-1570 nm at 6.3 Gbit/s from a single laser source utilizing laser-diode (LD)-pumped supercontinuum in optical fibers for the first time. The time-bandwidth products of the generated pulses are within the range of 0.3-0.6, and it has been verified that the generated WDM picosecond pulses are capable of being interleaved to produce up to 40 WDM channels, each consisting of 50 Gbit/s time-division-multiplexed (TDM) pulse streams, in one optical fiber     

Effect of excitation on pulses and radio interface from a rain drop

Corona-generated pulses and radio interference (RI) from a water drop under five different types of excitation are reported. A basic difference in characteristics of pulse trains generated under normal ac, rectified ac, and dc excitations is presented.     

超短行波放大自发辐射的研究

用泵浦能量具有梯度分布的横向同步泵浦方案,产生超短行波自发辐射光脉冲。以N_2激光为泵浦源,获得约50ps的染料光脉冲输出,以锁模Nd:YAG倍频光作泵浦源,获得8～15ps光脉冲。用条纹照相机和光学多道分析器测量了脉冲波形和光谱。 计算机模拟了行波放大自发辐射的瞬态行为,计算结果与实验结果相符合。     

Optical Pulse Characterization in a Regeneratively Mode-Locked Fiber Ring Laser

We characterize optical pulses generated using a regeneratively mode-locked fiber ring laser (RML-FRL) in terms of pulsewidth and pulse noise. These results are then compared with pulses obtained from a conventional active harmonically mode-locked fiber ring laser (ML-FRL). We establish that under the same operating conditions, optical pulses from the RML-FRL are shorter by more than 8% and exhibit a 15-dB improvement in phase noise compared to those obtained from ML-FRL. In addition, over a frequency range 100 Hz–100 kHz, a reduced amplitude noise of 0.3% and rms timing jitter of 0.26 ps have been estimated for the RML-FRL pulses compared to 0.6% and 0.38 ps, respectively, for the pulses in the ML-FRL. Relaxation oscillations are also completely eliminated in the RML-FRL.     

RE Pulse Shapes for Selective Excitation in NMR Imaging

It is shown that the requirements of an RF pulse needed to produce a slice for use in NMR imaging approximate the requirements of the windows used for calculating the discrete Fourier transform (DFT). This approach gives an interesting insight into the physical background of pulse shape tailoring. One of the more successful DFT windows is the 3 term Blackman-Harris window, which is tailored to produce minimum sidelobes. The results of applying radio frequency (RF) pulses modulated to give this window shape is given. The pulse shape produces a uniform magnetization over a range of frequencies with a quasi-linear phase shift making it suitable for producing a slice for NMR imaging when an echo is produced via an inverse magnetic field gradient.     

Lateral electromagnetic waves and pulses on open microstrip

The propagation of lateral electromagnetic waves and pulses on microstrip is investigated. Interference patterns generated by the superposition of the lateral and direct waves along the air-substrate surface are shown. The field generated by the pulse excitation of a horizontal dipole on the air-substrate boundary is shown to consist of a lateral-wave pulse and a slower direct-wave pulse. Their differences in shape and decay rate are clarified. It is shown that the shape of a Gaussian pulse propagating along an open microstrip transmission line is closely related to the shape of the lateral electric-field pulse generated by a Gaussian current pulse in a dipole on the air-substrate boundary     

Multirate SPAMM tagging

Many magnetic resonance tagging sequences rely on periodicity in order to produce a uniform tagging grid that covers the whole image plane. This, however, is not always desirable, since motion may be restricted to specific parts of the image, and also different motion characteristics may call for different tagging grid densities. In this paper, we present a combination of the spatial modulation of magnetization 1-1 method with selective excitation pulses that can be used in order to restrict the tagging grid only to regions of interest and produce tagging grid of different density in each region. The method is fast and easy to implement even on older or less expensive systems, since it does not require extensive gradient switching.     

Generalization of phase-encoded NMR chemical shift imaging

An echo-time encoding NMR chemical shift imaging proposed by Dixon is generalized to be applicable to multiline chemical shift imaging. The method utilizes the phase differences among nuclei encoded by shifting the position of the rf pulse in the spin echo sequence to resolve chemical shift images. The inevitable phase error caused by the static field inhomogeneity is corrected by using phase images of phantom images measured under the same conditions as the actual measurements. The minimum number of encodings can be reduced to half the number of chemical shifts without sacrificing spatial resolution. The experiments were carried out for 3-line 1H chemical shifts using benzene, water, and acetone at 0.5 T. All the chemical shift images could be clearly resolved with only two scans when the field inhomogeneity was larger than the maximum chemical shift difference.