Intensity Distortions Research Articles

Characterization of flame-retarded polymer combustion chars by solid-state13C and29Si NMR and EPR

We report here on the results of our continuing effort to study the flame-retardant mechanism of silica gel and potassium carbonate. These additives reduce the flammability of a wide variety of common polymers such as polypropylene, nylon, polymethylmethacrylate, poly(vinyl alcohol), and cellulose. In an effort to determine how these additives reduce polymer flammability, we have used electron paramagnetic resonance spectroscopy and solid-state 13C and 29Si nuclear magnetic resonance spectroscopy to characterize the combustion chars or residues. These data indicate that, in the case of poly(vinyl alcohol), the additives do not change the type of char formed, but they do change the rate of char formation relative to the rate of fuel generation. We also found that, using only CP/MAS 13C NMR, there can be significant intensity distortions which complicate interpretation, if the char is hydrogen depleted and contains paramagnetic centres. © 1998 John Wiley & Sons, Ltd.

Data compression for diffraction patterns.

Efficient coding (lossless) and compression (lossy) of diffraction patterns is important in protein crystallography experiments because of storage and transmission limitations. The goal is to reduce the bit-rate significantly while keeping diffraction peak intensity distortion at an acceptable level. This paper presents an overview of coding and compression techniques more or less adapted to such problems. A large part of this study is dedicated to time-frequency-transform based compression algorithms and some of their extensions. Wavelet based software has been developed and tested. Results are compared with the discrete cosine transform (DCT) and other classical algorithms. These tools seem attractive and very promising for analyzing and compressing signals with singularities and transient phenomena such as diffraction peaks. Tests were performed on a standard protein crystallography data set coming from the CCD detector of D2AM beamline at the European Synchrotron Radiation Facility at Grenoble. These were compressed with DCT and wavelet-based algorithms. It appears that alterations of the result of the processing of restored images remain very weak for compression rates up to 10. These preliminary results indicate that the proposed wavelet method is a good standard technique for efficient compression of diffraction patterns.

Reduction of geometric and intensity distortions in echo-planar imaging using a multireference scan.

Echo-planar imaging (EPI) is very sensitive to patient-induced field inhomogeneity caused by susceptibility changes between different anatomical regions. This results in geometric and intensity distortions in the image, especially near tissue/air and tissue/bone interfaces. A new approach is presented to reduce geometric and intensity distortions in EPI. A phase-encoded multireference scan is used to estimate the amplitude and phase errors in the measured signals due to the field inhomogeneity. The EPI data is corrected using both the amplitude and phase of the measured errors. This technique has been evaluated using EPI pulse sequences implemented with conventional gradients and implemented with imaging systems that have special resonating gradients and fast analog to digital converters. The results in both phantom and human studies show that in the absence of object motion the new correction technique can effectively reduce the geometric and intensity distortions.

Simplified Calibration of Instrument Response Function for Raman Spectrometers Based on Luminescent Intensity Standards

Published Raman spectra are rarely corrected for variations in spectrometer sensitivity across the Raman spectrum, which leads to often severe distortion of relative peak intensities that impede calibration transfer and library searching. A method was developed that uses the known luminescence of standards which fluoresce in response to laser irradiation. Since the standards are observed with the same sampling geometry as the Raman sample of interest, their spectra are subject to the same instrumental response function. After one-time calibration of the standards' fluorescence output against a known tungsten source, the unknown Raman spectrum may be corrected for instrumental response by a simple formula. In practice, the user need only run the standard under the same conditions as the Raman sample, then apply a short GRAMS algorithm. The approach is demonstrated for coumarin 540a and Kopp 2412 glass standards, with 514.5- and 785-nm laser light, respectively. Once the corrected spectrum is in hand, the absolute Raman cross section of a given Raman feature may be determined by comparison to known scatterers such as benzene.

Effect of a cylindrical refracting interface on ultrasound intensity and the CW Doppler spectrum.

The insonation of axisymmetric flow in a tube by an ultrasound beam which is refracted at the tube surface is considered. A three-dimensional (3-D) model for refraction at a cylindrical interface is developed. The incident beam, which is planar but otherwise arbitrary, is refracted at a single acoustic impedance interface between the medium within the tube and the surrounding medium. A ray approximation is used as the ultrasonic wavelength is small compared to the radius of curvature of the tube. A parametric form for the ray paths is obtained and used to derive an expression for the intensity. The intensity depends on the specific position within the tube, as well as the ratio of sound speeds, N, and the Doppler angle thetaD. A diverging (N = 1.2, thetaD = 45 degrees) and a converging (N = 0.8, thetaD = 45 degrees) case are presented in detail. The converging interface results in more extreme intensity distortion. The regions of maximum and minimum intensity over a tube slice are immediately adjacent in both diverging and converging cases. Once the intensity distribution across a tube slice is known, the previously reported volume integral method for the calculation of continuous wave (CW) Doppler spectral power density (spd) functions may be used. The effect of refraction of the incident beam on the spd function is shown for both an infinite beam and a circular beam with radius equal to the tube radius. A curved acoustic impedance interface, as may be encountered in vitro in flow phantoms or in vivo in vascular disease, leads to nonuniform insonation and spectral distortion which depends on transducer orientation.

Spectral editing in 13C CP/MAS experiments at high magnetic field

We show that the spinning side-bands of protonated and non-protonated carbon atoms can be well separated by means of the standard SCP and LCPD experiments at a relatively slow sample spinning rate or at high magnetic field. These experiments offer a promising way of measuring the principal values of chemical shift anisotropies via spinning side-band analysis in a moderately complex system. General spectral editing in 13C cross-polarization magic-angle spinning (CP/MAS) experiments at high field is achieved by incorporating the total side-band suppression (TOSS) pulse sequence into the standard series of spectral editing pulse sequences. It is confirmed that the relative signal intensity for a certain kind of functional group obtained at different polarization, polarization-in-version and depolarization times is about the same as that obtained at low magnetic field, and that the signal intensity distortion introduced by the TOSS sequence for resonances having different chemical shift anisotropies does not interfere with the spectral editing process. However, quantitative results can only be expected in those cases where full restoration of the intensity of the central band can be achieved by the TOSS sequence. This new strategy at high field is demonstrated by using fumaric acid monoethyl ester as a model compound. A typical application to a Chinese resin is presented, where the relative ratio of each functional group in the aliphatic portion to the total number of aliphatic carbon atoms is determined from only three experimental spectra.

The effect of variations in transducer position and sound speed in intravascular ultrasound: A theoretical study

The intravascular insonation of a blood vessel in the presence of an impedance interface between blood and the inner vessel wall is studied theoretically. The model, which uses a ray approximation, is three dimensional and allows consideration of arbitrary noncircular lumen shapes. Model results are presented for the image geometry, and the insonating intensity over the vessel wall. It is shown that the inner lumen can be imaged accurately with the transducer at any position within the lumen, and at any forward viewing angle, provided the point of origin of the beam is stationary. If it is not stationary but rotating with the same angular velocity as the beam itself, the inner vessel wall is not mapped accurately. A particular geometric distortion which has been observed in practice is predicted if the transducer is near the vessel wall. Acoustic impedance interfaces will be encountered in vascular disease because the speed of sound in fatty plaque is less than in blood, whereas the speed of sound in fibrous and calcified plaque is greater than in blood. A simplified model representation of an atherosclerotic lumen is developed using a cardioid-like curve and a single impedance interface. Model results show that refraction at this interface leads to an intensity distribution which is not uniform around the lumen, and which depends on lumen shape and transducer position. The exception is the special case of a circular lumen with a centrally positioned transducer. Noncircular impedance interfaces encountered in vivo in vascular disease may cause considerable intensity distortion, particularly if the transducer is close to the wall in an irregularly shaped lumen.

Detection of 14N and 35Cl in Cocaine Base and Hydrochloride Using NQR, NMR, and SQUID Techniques

Results from {sup 14}N pure NQR of cocaine in the free base form (cocaine base) yield a nuclear quadrupole coupling constant (NQCC) e{sup 2}Qq/h of 5.0229 ({+-}0.0001) MHz and an asymmetry parameter {eta} of 0.0395 ({+-}0.0001) at 295 K, with corresponding values of 5.0460 ({+-}0.0013) MHz and 0.0353 ({+-}0.0008) at 77 K. Both pure NQR (at 295-77 K) and a superconducting quantum interference device (SQUID) detector (at 4.2 K) were used to measure the very low (<1 MHz) {sup 14}N transition frequencies in cocaine hydrochloride; at 295 K the NQCC is 1.1780 ({+-}0.0014) MHz and the asymmetry parameter is 0.2632 ({+-}0.0034). Stepping the carrier frequency enables one to obtain a powder pattern without the severe intensity distortions that otherwise arise from finite pulse power. A powder pattern simulation using an NQCC value of 5.027 MHz and an asymmetry parameter {eta} of 0.2 agrees reasonably well with the experimental stepped-frequency spectrum. The use of pure NQR for providing nondestructive, quantitative, and highly specific detection of crystalline compounds is discussed, as are experimental strategies. 31 refs., 8 figs., 1 tab.

NMR Spinning Sideband and Spin Echoes. Imaging of Couette Flow

The sidebands generated by a cylindrical sample spinning in a linear gradient under Hahn spin-echo conditions are examined experimentally and theoretically. Explicit expressions are developed for the phase and intensity of the magnetization as a function of time. When the signals are sampled stroboscopically with the echo time matched to multiples of the rotation period, a standard spinning-sideband pattern is observed. Intermediate pulse delays result in a decrease in sideband intensity and nonlinear phase distortions across the pattern. The stroboscopic sampling technique is applied to Couette shear flow to selectively detect slices from the fluid on the basis of angular velocity. The imaged annular slices may then be analyzed spectroscopically.

Overmodulation of intensity modulated signalsdue to stimulated Brillouin scattering

The authors give the first report of stimulated Brillouin scattering (SBS) in optical fibres causing waveform distortion in intensity modulated (IM) optical signals. The effect of SBS on an IM optical signal is experimentally verified. SBS was found to induce overmodulation of the optical signal. >

Anisotropic intrasignal coupling in photorefractive linbo3

AbstractIntrasignaling coupling in a LiNbO3 photorefractive crystal is studied. It is shown that the anisotropy nature of the effective electrooptic coefficient of this crystal results in an orientation‐dependent intrasignal coupling. Analytical solutions for one‐dimensional intrasignal coupling are derived for a‐axis and b‐axis orientations, which enables us to estimate the maximum beam field angle and the maximum crystal thickness for a given intensity distortion due to the coupling. The results are thus illustrative to applications of this crystal in areas such as holographic data storage, interconnection, spatial mode conversion, et cetera. © 1994 John Wiley &amp; Sons, Inc.

Investigation of broad resonances in 31P NMR spectra of the human brain in vivo.

Broad resonances that lie underneath the familiar small molecule profile of in vivo 31P NMR spectra can make accurate spectral integration of these mobile phosphates difficult. The two major broad components are the phosphate contained in the hydroxyapatite in cranial bone and the phosphodiester moiety in partially mobile membrane phospholipids. They can be removed with post-acquisition processing but this results in distortion of lineshapes and intensities and interferes with accurate quantitation. We have employed an off-resonance saturation procedure to eliminate the bone resonance and isolate the signal from the membrane phospholipids by subtraction. Selective saturation of the phospholipid resonance increases the clarity of the sharp peaks downfield from the phosphocreatine peak. Selective saturation 3-D chemical shift imaging techniques were used to create a localized phospholipid profile of the entire brain simultaneously. Monitoring localized phospholipid concentration may be important in studying demyelinating diseases.

Ellipsoidal and hyperboloidal lens aperture functions for computation of fraunhofer diffraction patterns

AbstractThe illumination profile functions of ellipsoidal and hyperboloidal dome refracting lenses are obtained in forms suitable for direct computation of diffraction patterns. The effects of anisotropic radiation from the primary source and intensity distortion on refraction are treated. The resultant Fraunhofer diffraction lobe structures are compared. © 1994 John Wiley &amp; Sons, Inc.

Optoelectronic loop that implements a mean field annealing algorithm for nonlinear noise filtering

We present an optoelectronic feedback loop that uses a mean field annealing algorithm for extracting features from noisy images. The configuration is analyzed and novel methods to reduce optical misalignment problems and intensity distortions are presented. Experimental results are reported and are found to be in good agreement with those obtained in computer simulations.

31P transverse relaxation times of ATP in human brain in vivo

31P MRS examinations of the brain of 10 healthy volunteers were performed to determine T2 of the coupled ATP signals by use of the localized 90 degrees-TE/2-2662-TE/2-acq frequency selective spin echo sequence for elimination of phase and intensity distortions. The T2 relaxation times obtained are much longer than usually assumed: gamma-ATP: 89 +/- 9 ms; alpha-ATP: 84 +/- 6 ms; beta-ATP: 62 +/- 3 ms.

Interaction effects on wavelength-multiplexed soliton data packets

Optical soliton interactions cause spectral intensity distortions that may cause performance degradations in wavelength-multiplexed soliton data packet communications systems. We investigate these interactions both numerically and analytically and propose three distinct techniques for determining transmitted data from the spectral intensity functions of received multisoliton ensembles using fast and simple detection components. We also deduce that the spectral effect of collisions between even many solitons can be negligible provided the spacing between their undistorted carrier wavelengths is greater than five times their spectral width.

Cross Polarization with High-Speed Magic-Angle Spinning

Cross-polarization magic-angle-spinning 13C NMR has become an important analytical method for the study of organic solids. At high field strengths, the high magic-angle-sample-spinning (MASS) rates required to suppress spinning sidebands can substantially suppress the dipolar couplings among the abundant protons. This results in dramatic changes in the cross-polarization (CP) dynamics. For samples with both weak homonuclear and heteronuclear dipolar couplings, the matching condition is only 1 ∼ 2 kHz wide at 10 kHz MASS rates. The maximum CP signal for a sample which fills the receiver coil is also rather weak, with an amplitude which is nearly the same as the Bloch decay signal. In addition, intensity distortions are observed in CP spectra due to resonance-offset effects. In this paper it is shown that if the phases of the two RF channels are simultaneously inverted many times during the CP contact, the matching width can be broadened to about 8 kHz, the S magnetization doubled, and intensity distortions significantly reduced. These useful features of rapid simultaneous phase-inversion CP with high-speed MASS are shown by theory to be the result of spin dynamics different than those encountered in static samples where slower phase inversion is employed.

Filament formation in a tapered GaAlAs optical amplifier

Filament formation in a tapered GaAlAs amplifier with an output width of 320 μm is characterized by injecting an input beam with a superimposed sinusoidal intensity modulation (30 μm period, 30% peak-to-peak modulation). Strong seeded filamentation of the output near field, and large far field side lobes are observed for powers above approximately 1 W. Time dependent decay of the main far field lobe, characterized by a time constant of several microseconds, is used to separate the effects of localized carrier density variations from the thermal contribution to intensity and phase distortion of the output beam.

Effect of the wedging error in quantitative analysis of flexible polyurethane foams by FT-IR-microscopy

The absorbance errors in IR-spectra of a polyurethane foam produced by wedging are calculated with the help of a mathematical model resulting from the geometrical form of the struts. It can be shown by experiments with struts and membranes of a flexible polyurethane foam, prepared with wide cell dimensions up to 2.5 mm, that the intensity distortions in the IR-spectra are accounted for by the model. A correction function is obtained for errors in IR-spectra of samples with the special geometry of the struts.

Susceptibility artifacts in 2DFT spin-echo and gradient-echo imaging: The cylinder model revisited

Susceptibility-induced geometry and intensity distortions are a familiar observation in MR imaging. In the past few years several attempts have been made to aid in the understanding of susceptibility artifacts by means of simulation studies. Although these studies, which were mostly carried out with simple test objects, have produced some qualitative insight into χ-artifacts, the results lacked precision in describing finer details. In this paper we show the discrepancy between theory and experiment in previous work to be the result of an inadequate theoretical approach. In most studies so far, ΔB 0 effects are taken into account in the frequency domain, that is, after Fourier transformation of the data. In our view the simulation should follow the actual sequence of events in an imaging experiment and deal with the effect of error fields in the time domain (k-space) already. The correctness of this view is demonstrated here by comparing the results of time and frequency domain simulation against experimental observation for a coaxial cylinder phantom, a widely used model in this type of work. Having established the superiority of the time domain simulation, we demonstrate its use in predicting χ-artifacts under various experimental conditions, for example, in spin-echo and gradient-echo imaging with a reduced number of phaseencoding steps.