Interpulse Delay Time Research Articles

Pulse averaging methods for a laser remote monitoring system using atmospheric backscatter

Three methods for averaging differential absorption lidar (DIAL) measurements are discussed. They are compared using experimental data acquired with a dual-laser direct-detection DIAL system operating in the near UV using atmospheric backscatter. The data set was acquired with the two lasers tuned to the same wavelength to eliminate any spectral variations and fluctuations of differential absorbers from the measurement. The results are compared by evaluating both the standard deviations and the means of the integrated columns. The results suggest that shot noise on the backscattered signals dominates the measurement statistics and considerably reduces the necessity for short interpulse delay times.

A rare example of extremely strong carbon—carbon coupling: Analysis of 13C and 1H NMR spectra of a 13C2‐acetylenic ketone

AbstractNMR spectra are reported for an acetylenic ketone enriched with 13C in the acetylenic carbon atoms that exhibits extremely strong C‐C coupling. Chemical shifts and coupling constants have been extracted from computer simulations of the second‐order 1H and 13C spectra. The two acetylenic carbon atoms exhibit a Δ ν/J ratio of 0.2. Owing to the strong coupling, the double‐quantum coherence 13C spectrum requires an unusually long (58 ms) interpulse delay time in order to obtain a good spectrum. NMR spectra of this compound provide a unqiue example of strong coupling in a CCH (AB or ABX) spin system.

Sampling time effects on signal-to-noise and contrast-to-noise ratios in spin-echo MRI

This work discusses the effect of sampling time on noise, signal-to-noise and contrast-to-noise ratios in magnetic resonance imaging. A simple imaging experiment is performed to demonstrate the effect of sampling time on noise, confirming theoretical expectations that doubling the sampling time while decreasing the read gradient strength by a factor of two reduces statistical noise to 1 2 of its original level. This result suggests that sampling time should be maximized within the constraints of the pulse sequence: namely, that sampling time should be increased and read gradient strength decreased as TE is increased. Revised expressions for signal-to-noise and contrast-to-noise ratios are presented based on the assumption that sampling time increases linearly with echo delay time above a certain minimum TE value. The revised expressions are then used to derive new predictions of the interpulse delay times that maximize signal-to-noise and contrast-to-noise ratios in spin-echo imaging. It is demonstrated that sampling times are critical in determining whether T1-weighted or T2-weighted sequences produce superior tissue contrast in spin-echo imaging.

MR imaging technology: maximizing the signal-to-noise ratio from a single tissue.

The pulse-sequence equations for spin-echo magnetic resonance imaging were used to determine interpulse delay times that give the highest signal-to-noise ratio from a single tissue. This theoretical result was then verified experimentally using 1-, 2-, and 5-mM/l copper sulfate solutions imaged on a 0.15-T resistive system. Theoretical analysis determined the spin-echo interpulse delay times that maximize the signal-to-noise ratio from a single tissue as TEopt = TEmin, the minimum echo delay time permitted by the system, and, to a good approximation, TRopt = 1.27 T1 + 1.90 TEmin, with T1 the longitudinal magnetic relaxation time of the tissue. Phantom measurements of the signal-to-noise ratio in a typical imaging system confirmed the theoretically determined TRopt values to within 7%.

Synthesis of arbitrary frequency domain transmitting pulses applicable to pulsed NMR instruments

The synthesis of specialized excitation waveforms for pulsed NMR spin systems is an integral part of many present-day NMR experiments. Methods in use include various types of constant amplitude rectangular rf pulse trains with controlled interpulse delay times, modulating amplitudes of the envelopes of a chain of rectangular pulses, special long pulse rectangular pulses, and recently, single sideband spectral selections for NMR imaging. This paper describes, in quantitative fashion with experimental results, analysis and implementation of a method to generate pulsed NMR excitation waveforms using AM and FM techniques which yield individual pulses with controllable frequency spectral distributions. Examples given are of simplified cases, but the extension to complex frequency distribution requirements is clear.

Optimizing tissue contrast in magnetic resonance imaging

Magnetic resonance imaging demands that tissue contrast and signal-to-noise advantages be sought in each component of the imaging system. One component of magnetic resonance imaging in which contrast and signal-to-noise ratios are easily manipulated is in the choice of pulse sequences and interpulse delay times. This article provides a general method for determining the best choices of interpulse delay times in pulse sequences and applies that method to saturation recovery, inversion recovery, and spin-echo sequences. Saturation recovery and inversion recovery sequences with rephasing pulses, and tissues with unequal hydrogen densities are considered. Optimization of pulse sequences is carried out for the two distinct cases of (a) a fixed number of sequence repetitions and (b) a fixed total imaging time. Analytic expressions are derived or approximate expressions are provided for the interpulse delay times that optimize contrast-to-noise ratios in each pulse sequence. The acceptable range of interpulse delay times to obtain reasonable contrast using each pulse sequence is discussed.

Selection of pulse sequences producing maximum tissue contrast in magnetic resonance imaging

The importance of spin density [ N(H)] and spin-lattice ( T 1) and spin-spin ( T 2) relaxation in the characterization of tissue by nuclear magnetic resonance (NMR) is clearly recognized. This work considers which optimized pulse sequences provide the best tissue discrimination between a given pair of tissues. The effects of tissue spin density and machine-imposed minimum rephasing echo times (TE MIN) for achieving maximum signal tissue contrast are discussed. A long TE MIN sacrifices T 1-dependent contrast in saturation recovery (SR) and inversion recovery (IR) pulse sequences so that spin-echo (SE) becomes the optimum sequence to provide tissue contrast, due to T 2 relaxation. Pulse sequences providing superior performance may be selected based on spin density and T 1 and T 2 ratios for a given pair of tissues. Selection of the preferred pulse sequence and interpulse delay times to produce maximum tissue contrast is strongly dependent on knowledge of tissue spin densities as well as T 1 and T 2 characteristics. As the spin density ratio increases, IR replaces SR as the preferred sequence and SE replaces IR and SR as the pulse sequence providing superior contrast. To select the optimal pulse sequence and interpulse delay times, an accurate knowledge of tissue spin density, T 1 and T 2 must be known for each tissue.

Phase detection and contrast loss in magnetic resonance imaging

Several recent articles have assessed the relative efficiency of nuclear magnetic resonance (NMR) pulse sequences. One consideration that has received little attention is the effect on image contrast of displaying images without information on the sign of the reconstructed signals. The radiofrequency receivers currently used on most NMR imaging systems are quadrature detectors that preserve both the magnitude and sign of the NMR signal. Usually, however, sign or phase information is not used in the final image presentation. We point out that in imaging sequences that may have negative signals, such as inversion recovery, this loss of sign information produces a reduction in contrast between some tissues in an NMR image. We discuss the tissue parameters and interpulse delay times that result in contrast loss in inversion recovery and indicate the extent of contrast loss. We point out that for some tissues with unequal hydrogen spin densities, the region of contrast loss coincides with the region where maximum contrast would occur if sign information were preserved.

Choosing pulse sequences and interpulse delay times for maximum tissue differentiation in hydrogen magnetic resonance imaging

Choosing pulse sequences and interpulse delay times for maximum tissue differentiation in hydrogen magnetic resonance imaging

Pulse compression using angular multiplexing in a high-gain 1.7 kJ amplifier

Pulse compression using angular multiplexing is demonstrated on a large-scale hydrogen-fluoride (HF) laser system. For a train of two 24 ns pulses, the essential elements of this scheme are evaluated as a function of interpulse separation time. Included are energy-extraction efficiency, overall temporal pulse distortion, leading-edge contrast-ratio distortion, and suppression of amplified spontaneous emission (ASE) relative to a single long-duration input pulse. For appropriate interpulse delay time, we show that distortionless amplification is possible with energy-extraction efficiency, as is obtained using a single input beam having a pulsewidth equal to the duration of the amplifier gain.

Driving circuits for copper halide lasers-a parametric study

The influence of circuit inductance and current rise-time on the performance of a longitudinally excited copper halide laser was investigated for (a) double spark gap and (b) thyratron burst-mode driver circuits with low inductance configurations. The inductance of the circuit of the dissociation spark gap was not critical and the output of the laser increased as the inductance of the laser-pulse spark gap circuit was decreased to its minimum value. The effectiveness of the driver circuit was limited principally by the long extinction time of the dissociation spark gap, which restricted the interpulse delay time to values considerably in excess of the intrinsic delay for the laser itself. For the lowest inductance (about 65 nH) of the thyratron burst-mode circuit, the duration of the current pulse is limited by the finite speed of propagation of ionisation waves in the laser discharge tube rather than the inductance and the rate of rise of current in the thyratron significantly exceeds the tube rating. An L-C inverter circuit is described which removes the thyratron from the laser discharge circuit but it is found that the inversion time must be matched to the current growth in the laser tube and this requires the addition of inductance to the thyratron circuit. The most critical factor in determining the pulse energy available from the laser is the restriction on the rate of current rise for presently available thyratrons.

Some characteristics of self-terminating manganese and lead vapor lasers

A study was made of the dependence of the energy output of manganese and lead vapor lasers on the vapor and buffer gas pressures. A difference was discovered between the energetics of the infrared and green transitions in manganese. The method of pulse pairs was used to study the dependence of the maximum pulse repetition frequency on the gas discharge conditions. It was found that, for identical excitation conditions, lasing in the second pulse is achieved for the infrared transitions with considerably shorter interpulse delay times than is the case for the green transitions. The suggestion is put forward that it is mainly the recombination rate of the electrons and not the population of the lower metastable active level which exerts an influence on the maximum pulse repetition frequency.

Single- and double-pulse experiments on the Sr<sup>+</sup>cyclic ion laser

Single- and double-pulse experiments have been conducted on a moderately large bore (19 mm) Sr+ cyclic ion laser oscillating at 1033 and 1092 nm. The dependence of the output energy on the bore temperature, buffer gas (HE) pressure, discharge capacitance, discharge voltage, and timing between successive pulses was determined. Saturation in the output energy as both the capacitance and voltage are increased is observed. An empirical exponential dependence of the energy on the discharge voltage was demonstrated. Results from the double-pulse experiments indicate that the Sr+ laser terminal levels depopulate much faster than do the Sr+ ions in the ground state, so that the efficiency of the laser increases as the interpulse delay time is decreased to 50 μs, and so that this laser system would be capable of more efficient operation at PRF in the region of 20 kHz, even in a large bore tube.