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PubMed1996

An in vitro study on the influence of limited frequency resolution on contrast agent-enhanced Doppler signals.

Ultrasound contrast agents for vascular use are gas-filled microspheres and will increase the backscatter from blood and therefore enhance the Doppler signal. Studies have shown that Doppler enhancement may result in an increased maximum velocity when observed as the envelope of the spectral Doppler screen. Change in observed velocity may be due to biological or instrumentational effects. The purpose of this study was to investigate the possible instrumentational effects on the Doppler signal after injection of the contrast agent (Infoson). By performing the measurements on a Doppler phantom, the biological effects was eliminated. Computer simulations of limited frequency resolution were compared to measurements. The conclusion was that the increase in detected maximum velocity after Infoson injection is due to the combined effect of signal enhancement and limited frequency resolution. It is shown that a large change in signal strength can change the estimated maximum frequency from the spectral Doppler whether this is due to a contrast agent or instrument settings. It is therefore important in a clinical situation to have an adequate, but not too strong or too weak signal in order to make the best velocity estimates.

Albumins↗
PubMed1991

Testing of local gamma-ray scatter fractions determined by spectral fitting.

The spectral-fitting method of correction for gamma-ray Compton scattering within objects separates the unscattered and scattered components of locally measured energy spectra. Here, we employ a third-order polynomial for the scattering and an approximately constant fitting window. A scatter fraction, defined as total scattered over total unscattered counts within a 20% window, is calculated for each point in our Anger camera images. These scatter fractions are tested against those from Monte-Carlo simulation for 99mTc and against results from semiconductor detector measurements for 131I. A radioactive sphere at several locations within a non-radioactive cylinder and the inverse are imaged for the testing. For one case, reproducibility of the spectral-fitting scatter fraction as a function of the number of unscattered counts within the 20% acceptance window was also determined. With 99mTc, for all cases, the agreement between spectral fitting and the standard estimation method is within 16%. With 131I, for the 'hot' sphere at two locations, the agreement is within 21%. For the 'hot' sphere at the third location (off the cylinder axis towards the camera), the dependence of scatter fraction on transverse distance is good although the absolute values are too large. Scatter fraction reproducibility is within 10% for 1000 or more counts. Therefore, further testing of spectral fitting and initial application to realistic clinical images seem to be in order.

Gamma Cameras↗
PubMed2005

Double-quantum filtered 1H MAS NMR spectra.

It is shown that straightforward double-quantum filtered (1)H MAS NMR experiments yield spectral lineshapes that permit to estimate the minimum number of (1)H spins in a cluster. The approach may offer an alternative to multiple-quantum experiments for the characterisation of (1)H spin clusters of moderate size. The duration of the double-quantum excitation period has to be chosen suitably, it is necessary to find a practical compromise between optimum double-quantum filtration efficiency and optimum information content of the spectral lineshapes. Some (1)H MAS NMR experiments on partially deuterated maleic acid are reported as well as numerical simulations.

Algorithms↗
PubMed2003

Methods for first-order kernel estimation: simple-cell receptive fields from responses to natural scenes.

Recent studies have recovered receptive-field maps of simple cells in visual cortex from their responses to natural scene stimuli. Natural scenes have many theoretical and practical advantages over traditional, artificial stimuli; however, the receptive-field estimation methods are more complex than for white-noise stimuli. Here, we describe and justify several of these methods-spectral correction of the reverse correlation estimate, direct least-squares solution, iterative least-squares algorithms and regularized least-squares solutions. We investigate the pros and cons of the different methods, and evaluate them in a head-to-head comparison for simulated simple-cell data. This shows that, at least for quasilinear simulated simple cells, a regularized solution ('reginv') is most efficient, requiring fewer stimulus presentations for high-resolution reconstruction of the first-order kernel. We also investigate several practical issues that determine the success of this kind of experiment-the effects of neuronal nonlinearities, response variability and the choice of stimulus regime.

Nature↗
PubMed2004

Illuminant color perception of spectrally filtered spotlights.

The color perceived to belong to the illumination of objects is often based on cues from the scene within which the objects are perceived, instead of being based on any view of the source itself. We present measurements of illuminant color estimation by human observers for moving, spectrally filtered spotlights. The results show that when only one illuminant is in the field of view, estimates of illuminant color are seriously biased by the chromaticities of the illuminated surfaces. When the surround of the spotlight is illuminated by a dimmer second light, spotlight matching moves toward veridical in most conditions. Simulations show that a gray-world model cannot be rejected as an adequate explanation for illuminant color estimation and provides as good a fit as a model that gives greater weights to the brightest surfaces. When the surrounding illuminant is brighter than the spotlight, the situation is similar to that of a moving filter. Spotlight matches are close to veridical, and the results can be fit by a model based on estimating both illuminants.

Color Perception↗
PubMed2006

Characterization of in vitro healthy and pathological human liver tissue periodicity using backscattered ultrasound signals.

This work studied the periodicity of in vitro healthy and pathologic liver tissue, using backscattered ultrasound (US) signals. It utilized the mean scatterer spacing (MSS) as a parameter of tissue characterization, estimated by three methods: the spectral autocorrelation (SAC), the singular spectrum analysis (SSA) and the quadratic transformation method (SIMON). The liver samples were classified in terms of tissue status using the METAVIR scoring system. Twenty tissue samples were classified in four groups: F0, F1, F3 and F4 (five samples for each). The Kolmogorov-Smirnov test (applied on group pairs) resulted as nonsignificant (p > 0.05) for two pairs only: F1/F3 (for SSA) and F3/F4 (for SAC). A discriminant analysis was applied using as parameters the MSS mean (MSS) and standard deviation (sigmaMSS), the estimates histogram mode (mMSS), and the speed of US (mc(foie)) in the medium, to evaluate the degree of discrimination among healthy and pathologic tissues. The better accuracy (Ac) with SAC (80%) was with parameter group (MSS, sigmaMSS, mc(foie)), achieving a sensitivity (Ss) of 92.3% and a specificity (Sp) of 57.1%. For SSA, the group with all four parameters showed an Ac of 75%, an Ss of 78.6% and an Sp of 66.70%. SIMON obtained the best Ac of all (85%) with group (MSS, mMSS, mc(foie)), an Ss of 100%, but with an Sp of 50%.

Acoustics↗
PubMed1992

[Quantitative topographic characterization of the myoelectric activity distribution of the masseter muscle: mapping of spectral EMG parameters].

A new method for quantitative characterization of myoelectrical masseter activity distribution by mapping of spectral EMG-parameters is described. The surface electromyograms of M. masseter were monopolarly recorded (16 channels). On the basis of registered EMG intervals (512 ms) the spectral EMG power of several frequency bands was calculated (Fast Fourier Transformation). The spectral EMG parameters between the 16 electrode positions were estimated by linear interpolation (4-nearest neighbours algorithm). Afterwards the spectral EMG parameters were fitted in a grey-tone or colour scale with 10 intervals. The so obtained EMG activity maps ("EMG-Maps") permit a quantitative-topographic characterization of myoelectrical masseter activity during different functional load procedures. The frequency range which is to consider in masseter surface-EMG investigations encloses frequencies between 15 and 500 Hz. The topography of EMG activation pattern of M. masseter is only described in a comprehensive manner when the electrode array consists of 16 electrodes and more. During defined motor tasks like clenching with controlled forces the reproducibility of EMG-Maps which respect to the topography of EMG activity pattern is very high. The absolute values of spectral EMG power as well as power changes of selected band ranges during clenching correlate to the extent of chewing forces.

Adult↗
PubMed1989

Nonideal behavior of alkoxyphenoxazone compounds (cytochrome P-450 substrates) in aqueous solution.

Spectral properties of the cytochrome P-450 substrates, methoxy-, ethoxy-, pentoxy-, and benzyloxyphenoxazone (MeOPx, EtOPx, PeOPx, and BzOPx, respectively) were investigated from 350 to 600 nm in ethanol and aqueous buffer. In ethanol, each alkoxyphenoxazone displayed a lambda max at 460 nm and a shoulder around 390 nm. Extinction coefficients (EmM) in ethanol were calculated as MeOPx, 20.5; EtOPx, 20.4; PeOPx, 24.7; and BzOPx, 22.4. In aqueous buffer, only MeOPx obeyed the Lambert-Beer law (lambda max = 480 nm, EmM = 22.1). Three substrates, EtOPx, PeOPx, and BzOPx, displayed anomalous behavior in aqueous solution, wherein the lambda max shifted to lower wavelengths (480-430 nm) and EmM (apparent) decreased as the alkoxyphenoxazone concentration increased. This behavior was dependent on the side chain, and the concentrations at which the spectral changes took place were estimated as: BzOPx, 2 microM; PeOPx, 5 microM; EtOPx, 17 microM; and MeOPx, greater than 20 microM. The blue shift and decreased EmM (apparent) observed for PeOPx at high concentration in aqueous buffer was reversed at high temperature. Unlike EtOPx, PeOPx, and BzOPx, and like MeOPx, hydroxyphenoxazone (resorufin) and unsubstituted phenoxazone obeyed the Lambert-Beer law in aqueous buffer and ethanol. The data suggest that the pentoxy and benzyloxy substituents facilitated a self-association process among the phenoxazones in aqueous solution. The data further show that aqueous solutions should be avoided when spectral data are used to determine alkoxyphenoxazone concentrations.

Chemical Phenomena↗
PubMed1998

Spectro-temporal factors in two-dimensional human sound localization.

This paper describes the effect of spectro-temporal factors on human sound localization performance in two dimensions (2D). Subjects responded with saccadic eye movements to acoustic stimuli presented in the frontal hemisphere. Both the horizontal (azimuth) and vertical (elevation) stimulus location were varied randomly. Three types of stimuli were used, having different spectro-temporal patterns, but identically shaped broadband averaged power spectra: noise bursts, frequency-modulated tones, and trains of short noise bursts. In all subjects, the elevation components of the saccadic responses varied systematically with the different temporal parameters, whereas the azimuth response components remained equally accurate for all stimulus conditions. The data show that the auditory system does not calculate a final elevation estimate from a long-term (order 100 ms) integration of sensory input. Instead, the results suggest that the auditory system may apply a "multiple-look" strategy in which the final estimate is calculated from consecutive short-term (order few ms) estimates. These findings are incorporated in a conceptual model that accounts for the data and proposes a scheme for the temporal processing of spectral sensory information into a dynamic estimate of sound elevation.

Adult↗
PubMed1990

Relationship of spectral noise levels to psychophysical scaling of vowel roughness.

Direct magnitude estimation (DME) and equal-appearing interval (EAI) ratings of perceived roughness and spectral noise level (SNL) measurements of isolated sustained vowel samples (/a/ and /i/) were obtained. The linear and curvilinear relationships between the resulting roughness ratings and SNL were then examined. In the present study, the comparison between the two models resulted in conflicting evidence regarding the model that best described the relationship between roughness ratings and SNL measurements. In all comparisons, however, a significant linear component was indicated. Additionally, when a significant curvilinear component was indicated, the difference between the lines and curves that were fit to the data was minimal. The present findings suggest, therefore, that a high degree of linearity exists between both DME and EAI roughness ratings and SNL.

Adult↗
PubMed2006

Spectral response variation of a negative-electron-affinity photocathode in the preparation process.

In order to research the spectral response variation of a negative electron affinity (NEA) photocathode in the preparation process, we have done two experiments on a transmission-type GaAs photocathode. First, an automatic spectral response recording system is described, which is used to take spectral response curves during the activation procedure of the photocathode. By this system, the spectral response curves of a GaAs:Cs-O photocathode measured in situ are presented. Then, after the cathode is sealed with a microchannel plate and a fluorescence screen into the image tube, we measure the spectral response of the tube by another measurement instrument. By way of comparing and analyzing these curves, we can find the typical variation in spectral-responses. The reasons for the variation are discussed. Based on these curves, spectral matching factors of a GaAs cathode for green vegetation and rough concrete are calculated. The visual ranges of night-vision goggles under specific circumstances are estimated. The results show that the spectral response of the NEA photocathode degraded in the sealing process, especially at long wavelengths. The variation has also influenced the whole performance of the intensifier tube.

Journal Article↗
PubMed1987

A new resolution criterion based on spectral signal-to-noise ratios.

A new criterion for the "useful" resolution of electron micrographs of macromolecular particles is introduced. This criterion is based on estimation of the spatial frequency limit beyond which the spectral signal-to-noise ratio (SSNR) falls below an acceptable baseline. Applicable to both periodic and aperiodic specimens, this approach is particularly apposite for sets of correlation-averaged images. It represents a straightforward and intuitively appealing generalization of the traditional method of estimating the resolution of crystalline specimens from the spectral ranges of periodic reflections in their diffraction patterns. This method allows one to assess how closely the resolution of an averaged image based on N individual images approaches the ultimate resolution obtainable from an indefinitely large number of statistically equivalent images. Inter-relationships between the SSNR and two other measures of resolution, the differential phase residual and the Fourier ring correlation coefficient, are discussed, and their properties compared.

Biopolymers↗
PubMed2000

Adaptive noise energy estimation in pathological speech signals.

For pathological voices, spectral noise is closely related to the degree of perceived hoarseness. In this paper, noise variations are tracked during an utterance by means of an adaptive version of the normalized noise energy method [1]. A first step is devoted to pitch estimation, which allows defining the varying optimal time window length for noise retrieval, avoiding empty "dip" regions. The approach is tested on synthetic data and applied to real data coming from cordectomised adult male patients.

Adult↗
PubMed1991

EMG spectral shift- and 31P-NMR-determined intracellular pH in fatigued human biceps brachii muscle.

We have simultaneously recorded human biceps brachii intracellular pH, estimated by 31P-NMR, and EMG spectral shift, during isometric contraction and recovery in six subjects. This method allows us to concurrently study several components of muscle fatigue. The results show a clear dissociation between the recovery of intracellular pH, force-generating capacity, and the shift to low frequency of the EMG power spectrum induced by fatiguing exercise.

Electromyography↗
PubMed2002

Spectral shapes of forward and reverse transfer functions between ear canal and cochlea estimated using DPOAE input/output functions.

It is widely assumed that the distortion characteristics of the cochlea are uniform across its length, or at least across some portion of its length. For each distortion product otoacoustic emission (DPOAE) input/output (I/O) function across frequency, there is a corresponding cochlear I/O function defined over the cochlear source region. An assumption of distortion invariance is adopted such that these cochlear I/O functions are identical across tonotopic place, which is testable in the sense that a single nonlinear function should adequately describe the set of DPOAE I/O functions across frequency. If so, the differences in measured DPOAE I/O functions across frequency are produced by differences in the forward stimulus transmission to the generation site, and reverse DP transmission back to the ear canal. The absolute transfer-function magnitude is not determined by this technique, but the spectral shapes across frequency and between ears are determined. The role of middle-ear functioning is implicit in the I/O functions because of its controlling influence on these transfer functions. Results have been obtained using the average DPOAE I/O functions measured in a population of healthy ears [Gorga et al., J. Acoust. Soc. Am. 107, 2128-2135 (2000)], and support the hypothesis of cochlear-distortion invariance. The measured forward and reverse transfer functions have a general bandpass characteristic, and a more narrow-band structure with similarities to the behavioral threshold curve.

Auditory Perception↗
PubMed2000

Purification and characterization of chorion peroxidase from Aedes aegypti eggs.

Previous study has shown that a peroxidase is present in the mature eggs of Aedes aegypti mosquitoes, and the enzyme is involved in the formation of a rigid and insoluble chorion by catalyzing chorion protein crosslinking through dityrosine formation. In this study, chorion peroxidase was solubilized from egg chorion by 1% SDS and 2 M urea and purified by various chromatographic techniques. The enzyme has a relative molecular mass of 63,000 as estimated by SDS-PAGE. Spectral analysis of the enzyme revealed the presence of the Soret band with a lambda(max) at 415 nm, indicating that chorion peroxidase is a hemoprotein. Treatment of the native enzyme with H2O2 in excess in the absence of reducing agents shifted the Soret band from 415 to 422 nm, and reduction of the native enzyme with sodium hydrosulfite under anaerobic conditions changed the Soret band from 415 to 446 nm. These results show that the chorion peroxidase behaves similarly to other peroxidases under oxidative and reductive conditions, respectively. Compared to other peroxidases, the chorion peroxidase, however, is extremely resistant to denaturing agents, such as SDS and organic solvents. For example, chorion peroxidase remained active for several weeks in 1% SDS, while horseradish peroxidase irreversibly lost all its activity in 2 h under the same conditions. Comparative analysis between mosquito chorion peroxidase and horseradish peroxidase showed that the specific activity of chorion peroxidase to tyrosine was at least 100 times greater than that of horseradish peroxidase to tyrosine. Chorion peroxidase is also capable of catalyzing polypeptide and chorion protein crosslinking through dityrosine formation during in vitro assays. Our data suggest that the characteristics of the chorion peroxidase in mosquitoes closely reflect its functions in chorion formation and hardening.

Aedes↗
PubMed1995

Novel three-dimensional 1H-13C-31P triple resonance experiments for sequential backbone correlations in nucleic acids.

Backbone-driven assignment methods that utilize covalent connectivities have greatly facilitated spectral assignments of proteins. In nucleic acids, 1H-13C-31P correlations could play a similar role, and several related experiments (HCP) have recently been presented for backbone-driven sequential assignments in RNA. The three-dimensional extension of 1H-31P Het-Cor (P,H-COSY-H,C-HMQC) and Het-TOCSY (P,H-TOCSY-H,C-HMQC) experiments presented here complements HCP experiments as tools for spectral assignments and extraction of dihedral angle constraints. By relying on 1H-31P rather than 13C-31P couplings to generate cross peaks, the strongest connectivities are observed in different spectral regions, increasing the likelihood of resolving spectral overlap. In addition, semiquantitative estimates of 1H-31P and 13C-31P couplings provide dihedral angle constraints for RNA structure determination.

Base Sequence↗
PubMed1984

Pharmacodynamic modeling of thiopental anesthesia.

We have pharmacodynamically modeled the relationship between the thiopental serum concentration and its effects on the electroencephalogram (EEG). Power spectral analysis was used to calculate the spectral edge, a measure of the underlying EEG frequency that characterizes the progressive slowing of the EEG induced by thiopental. Eight male volunteer subjects had venous thiopental serum concentrations measured, and 10 surgical patients had arterial serum concentrations measured. Thiopental was infused at a rate of 75 to 150 mg/min until a burst suppression EEG pattern was evident. Frequent blood samples were obtained during and after the infusion for measurement of serum thiopental concentrations, and the EEG was recorded for subsequent off-line power spectral analysis to calculate the spectral edge. With venous blood sampling, it was not possible to demonstrate significant hysteresis between the thiopental serum concentration and the spectral edge, allowing thiopental concentrations to be directly related to the spectral edge. With arterial blood sampling, significant hysteresis was present, requiring an effect compartment to relate concentration to effect. The half-time for equilibration (mean +/- SD) between concentration and response for the arterial data was 1.2 +/- 0.30 min. This value for Keo is consistent with known values for cerebral blood flow and thiopental brain: blood partition coefficient. Arterial-venous concentration differences cause the apparent lack of hysteresis with venous blood sampling. An inhibitory sigmoid Emax pharmacodynamic model optimally characterized the relationship between thiopental concentrations and the spectral edge. This model allows estimation of the thiopental serum concentration that causes one-half of the maximal EEG slowing (IC50), which is a measure of an individual's sensitivity to thiopental. Except for the hysteresis, there was no statistical difference in the parameters of the inhibitory sigmoid Emax pharmacodynamic model when venous and arterial blood samplings were compared. Arterial blood sampling offers some distinct advantages when pharmacodynamically modeling continuous, rapidly changing measures of drug effect, such as the EEG.

Adult↗