System for objective assessment of fetal activity.

Fetal activity is an important indicator of fetal well-being. It is proposed to assess this activity using the pulsed wave Doppler method to collect fetal activity data and dedicated software for on-line processing. The system, addressed to 3rd trimester pregnancies, provides information on presence of pseudobreathing, the heart rate trace, the fetal movement trace, the movement velocity spectrogram, histograms of the velocity and acceleration of both the body movements and pseudobreathing, parameters of these histograms (mean values, standard deviations, shape descriptors), and cumulative counts of the velocity histograms. These parameters form the feature vector of the fetal activity. The system was validated by simultaneous echographic and cardiotocographic recordings and during oxytocin challenge tests. Feature vectors obtained from 1h recordings in 61 pregnancies were submitted to multivariate analysis of variance. Activity patterns of physiological cases and "borderline pathologies" were discriminated using reduced feature vectors, containing cumulative counts of velocity histograms.

A dual-chamber, thick-walled cardiac phantom for use in cardiac motion and deformation imaging by ultrasound.

Determination of the mechanical properties of the myocardium is crucial for cardiac diagnosis. Cardiac strain and strain rate imaging may enable such quantification. To further develop these methodologies, an experimental setup allowing the recording of ultrasonic deformation data in a reproducible manner is necessary. Such setup with biventricular polyvinyl alcohol heart phantoms has been built. To test this setup, segmental longitudinal, radial and circumferential displacement, velocity, strain and strain rate in the phantoms were measured using a clinical ultrasound scanner and commercially available deformation imaging algorithms (based on both tissue velocity imaging and speckle tracking). The model deformation was close to that observed in the human left ventricular wall and was highly reproducible (e.g., the average peak longitudinal strain for the mid- and apical phantom segments equals -15.32 +/- 0.53% and -19 +/- 6% for the ventricle wall). The experimental setup is a valuable source of data for the development of algorithms for deformation estimation.

Automatic detection of ultrasonic Doppler signal episodes resulting from fetal breathing movements.

A method for automatic detection of fetal breathing movements has been proposed, based on the time-frequency structure of the corresponding continuous wave ultrasonic Doppler signals. The method uses spectral analysis of the envelope of the directional Doppler signal and cross-correlation analysis of both directional envelopes. Detection rule comprises the following criteria: presence of the peak in the envelope spectrum and of the adequate signal level in the frequency range corresponding to the fetal breathing rhythm, the peak value and the position limits of the peak of the cross-correlation coefficient of the both directional envelopes. The effect of the criteria setting on the rule performance and the tradeoff between the specificity and sensitivity was investigated. The rule is most sensitive to the threshold value of the cross-correlation coefficient of the envelopes. The limits of the position of this peak are crucial for the distinction between the breathing episodes and hiccups. The optimal settings of the criteria, resulting in average sensitivity and specificity exceeding, respectively, 0.70 and 0.80, are proposed.

The discrimination of stenosed carotid bifurcation models with smooth and irregular plaque surface. Part I. Laser and ultrasonic Doppler flow studies.

Irregular carotid lesion surface is considered as a factor increasing the risk of the cerebral embolism. The objective of the study was to investigate the possibility to distinguish models of stenosed carotid bifurcation with lesion irregularity on the basis of the properties of flow velocity distributions. Two groups of elastic replicas of carotid bifurcations with different stenosis degree were investigated. Each group consisted of three models with different severity of plaque surface irregularity and one with smooth wall. Velocity data were collected using a one-component laser Doppler anemometer (LDA) system and a pulsed Doppler flowmeter. The LDA velocity distributions and Doppler spectral broadening index, turbulence intensity index and coefficient of skewness were analysed. The lesion irregularity resulted in change of the size and/or shape of reversed/reduced flow areas and of the position of the jet with respect to those observed in a smooth wall model. The flow features observed in the ultrasonic Doppler spectra were generally coherent with the axial LDA velocity distributions. Doppler spectral parameters demonstrated different sensitivities to the severity of the wall irregularity, however, the complexity of curves of these indices versus time did not allow to draw decisive conclusions and implied use of a more sensitive tool of analysis.

The discrimination of stenosed carotid bifurcation models with smooth and irregular plaque surface. Part II. The multivariate statistical analysis of ultrasonic Doppler velocity data.

According to the conclusion of the first part of the paper the Doppler spectral indices were analyzed in a detailed manner to provide a means to differentiate bifurcation models with different irregularities. Autoregressive (AR) models using the Akaike FPE criterion were fitted to the curves of the spectral broadening index, turbulence intensity index and coefficient of symmetry. These curves were also submitted to the data reduction significant point extraction algorithm. The multivariate discriminant analysis was performed on parameter vectors containing coefficients of AR models, data reduction rate and data reduction error. The discriminant analysis allowed distinguishing models with the same degree of stenosis and different plaque surface characteristics on the basis of these vectors. The highest discrimination efficiency was observed for parameter vectors obtained from TBI and SKEW curves. The efficiency of discrimination was slightly higher for more severely stenosed models.

Strain rate imaging using two-dimensional speckle tracking.

Strain rate images (SRI) of the beating heart have been proposed to identify non-contracting regions of myocardium. Initial attempts used spatial derivatives of tissue velocity (Doppler) signals. Here, an alternate method is proposed based on two-dimensional phase-sensitive speckle tracking applied to very high frame rate, real-time images. This processing can produce high resolution maps of the time derivative of the strain magnitude (i.e., square root of the strain intensity). Such images complement traditional tissue velocity images (TVI), providing a more complete description of cardiac mechanics. To test the proposed approach, SRI were both simulated and measured on a thick-walled, cylindrical, tissue-equivalent phantom modeling cardiac deformations. Real-time ultrasound images were captured during periodic phantom deformation, where the period was matched to the data capture rate of a commercial scanner mimicking high frame rate imaging of the heart. Simulation results show that SRI with spatial resolution between 1 and 2 mm are possible with an array system operating at 5 MHz. Moreover, these images are virtually free of angle-dependent artifacts present in TVI and simple strain rate maps derived from these images. Measured results clearly show that phantom regions of low deformation, which are difficult to identify on tissue velocity-derived SRI, are readily apparent with SRI generated from two-dimensional phase-sensitive speckle tracking.

The DopFet system: a new ultrasonic Doppler system for monitoring and characterization of fetal movement.

To enable the investigation of fetal movement in a manner similar to fetal heart rate (FHR) monitoring we have developed an apparatus (the DopFet system) that consists of a pair of miniature sensors, a 2-MHz continuous-wave directional Doppler electronic module and a laptop personal computer. One of the sensors is aimed at the fetal limbs and the other at the thorax to detect heart and upper body movements. The signals are analyzed, presented in real-time and postprocessed by software developed by us. The postprocessing software computes a number of parameters (the DopFet parameters) describing fetal movement. These parameters can be divided into two categories: parameters that describe the quantity of fetal movement (i.e., number of movements) and parameters that describe qualitative aspects of fetal movement (i.e., average movement duration). Future studies using the DopFet system will be aimed at discovering which of these parameters or combination of parameters is the best indicator of fetal well-being. We present an example of a 0.5 h recording and the results of testing on 23 volunteer mothers. These results show good sensitivity of the system compared to real-time ultrasound (US). The system detects 96% of rolling movements, 100% of flexion movements and 97% of leg movements.

Analysis of ultrasonic Doppler velocity data obtained in models of stenosed carotid bifurcations with irregular lesion surface.

In this study properties are concerned of ultrasonic Doppler spectra recorded distal to an irregularity localised in the internal carotid artery of a true-to-scale elastic model of a severely stenosed carotid bifurcation. Four models differing in irregularity were obtained by melting graded holes in the original wax cast, about 5-7 mm distal to the bifurcation. Velocity data were collected using an HP Sonos 2500 ultrasonograph working in pulsed Doppler mode. The observed effect of the irregularity with respect to the smooth wall model consisted in: (1) presence of a reduced flow zone 15 mm distal to the severe irregularity situated at the outer wall of the model in the ICA branch; (2) presence of a recirculation zone 15 mm distal to the severe irregularity situated at the inner wall of this model; (3) negative values of the measure of symmetry of the Doppler spectrum and (4) decreased values of relative width of Doppler spectra obtained 25 mm distal to the bifurcation in models with severe irregularity. The alterations of flow distribution and Doppler spectra depend on the geometry of the model under investigation and this study provides an indication as to what phenomena can happen in a model of stenosed carotid bifurcation with irregular lesion surface and how these phenomena may affect the ultrasonic Doppler spectra. Such information could be used in the evaluation of the risk of cerebral embolism.

The effect of wall roughness on velocity distribution in a model of the carotid sinus bifurcation--analysis of laser and ultrasound Doppler velocity data.

The purpose of this work was to find out whether a rough surface in the sinus of a carotid model affects the flow velocity distribution and the properties of the corresponding ultrasonic Doppler spectrum, in view of potential application to the evaluation of risk of cerebral embolism. True-to-scale elastic models of carotid artery were used in the experiments. Experiments were performed in three models with varying severity of roughness and a normal (physiological healthy) model. The different degrees of roughness in the three models were created by melting graded holes in the wax cast used to make the model. Velocity data were collected using 4 Mhz pulsed ultrasonic and single component laser Doppler velocimeters. The roughness resulted in modified velocity distribution, affecting to various extent ultrasonic Doppler spectral parameters, in particular the measure of symmetry of spectral distribution.

Detection of fetal breathing and cardiac movements and rhythms in ultrasonic Doppler signal recorded on the surface of the maternal abdomen.

It is proposed to use information on the direction of reflector movement and extensive filtering in the detection of fetal breathing and cardiac movements in the ultrasonic Doppler signal recorded on the surface of the material abdomen. The method appears fairly insensitive to spurious signals and allows those of interest to be distinguished without any reference technique. A decision rule for breathing and cardiac rhythm detection, incorporating movement direction, amplitude, shape and periodicity criteria, is also proposed.

Effect of method and parameters of spectral analysis on selected indices of simulated Doppler spectra.

The sensitivity of Doppler spectral indices (mean frequency, maximum frequency, spectral broadening index and turbulence intensity) to the conditions of spectral analysis (estimation method, data window, smoothing window or model order) increases with decreasing signal bandwidth and growing index complexity. The bias of spectral estimate has a more important effect on these indices than its variance. A too low order, in the case of autoregressive modeling and minimum variance methods, and excessive smoothing, in the case of the FFT method, result in increased errors of Doppler spectral indices. There is a trade-off between the errors resulting from a short data window and those due to insufficient temporal resolution.

Asymmetry of Doppler spectrum in stenosis differentiation.

The asymmetry of the spectral distribution of ultrasonic Doppler flow velocity signals, assessed using the coefficient of skewness, is discussed as a criterion of stenosis differentiation. Its performance is compared with that of the index of turbulence intensity for both in vitro and in vivo flow Doppler signals, recorded distal to a stenosis. The power spectral distributions are computed using the direct Fourier transform and maximum likelihood method. The asymmetry of spectral distribution has proved to be a more efficient criterion than the turbulence intensity. The maximum likelihood method ensures better stenosis differentiation than the direct FFT method.

Selection of a spectral analysis method for the assessment of velocity distribution based on the spectral distribution of ultrasonic Doppler signals.

The direct Fourier transform method, autoregressive modelling, the maximum likelihood method and the Wigner-Ville distribution were applied to the Doppler signal obtained from a fully insonated laminar model flow. The appreciation of the spectral method was based on the properties of the ratio variance/(fmean)2 (INT) of the spectrum. The basic criterion was the sensitivity of INT to the analysis parameters, especially the data window. The results of spectral analysis, as well as the properties of INT, were strongly affected by the method applied. The maximum likelihood method appeared best suited for the purpose of assessment of velocity distribution and is expected to give the best results in the case of in vivo blood flow. The performances of other discussed methods were inferior, due to their stronger incompatibility with the signal properties.