Blood pressure estimation in the human fetal descending aorta.

OBJECTIVES: The objectives of this study were to estimate fetal blood pressure non-invasively from two-dimensional color Doppler-derived aortic blood flow and diameter waveforms, and to compare the results with invasively derived human fetal blood pressures available from the literature. METHODS: Aortic pressures were calculated from digitally recorded color Doppler cineloops of the fetal descending aorta by applying the Womersley model in combination with the two-element Windkessel model, assuming constant pulse wave velocity during the second half of pregnancy. The results were compared with invasively derived human fetal blood pressures obtained from the literature. RESULTS: In 21 normal pregnancies the estimated mean aortic pressure regression line increased linearly from 28 mmHg at 20 weeks of gestation to 45 mmHg at 40 weeks of gestation. The pulse pressure based on the regression line increased linearly from 21 mmHg at 20 weeks of gestation to 29 mmHg at 40 weeks of gestation. The aortic compliance exhibited a log linear relationship with the gestational age and a statistically significant eightfold increase was observed between 20 and 40 weeks. The aortic downstream peripheral resistance exhibited an exponentially decaying relationship across the same gestational age range. Non-invasively derived aortic systolic and diastolic aortic pressures were comparable with previously reported invasively derived systolic and diastolic umbilical arterial pressures; however, the mean pressures differed significantly from those reported in the umbilical artery in a separate study. The aortic systolic pressures calculated in this study were significantly higher than invasively derived left ventricular systolic pressures that have been previously reported in the literature. CONCLUSIONS: This study demonstrates the feasibility of estimating arterial blood pressure in the human fetus. The method described is of potential use in assessing fetal blood pressure non-invasively, particularly for studying relative changes with time.

A new method for the noninvasive determination of abdominal muscle feedforward activity based on tissue velocity information from tissue Doppler imaging.

Rapid arm movements elicit anticipatory activation of the deep-lying abdominal muscles; this appears modified in back pain, but the invasive technique used for its assessment [fine-wire electromyography (EMG)] has precluded its widespread investigation. We examined whether tissue-velocity changes recorded with ultrasound (M-mode) tissue Doppler imaging (TDI) provided a viable noninvasive alternative. Fourteen healthy subjects rapidly flexed, extended, and abducted the shoulder; recordings were made of medial deltoid (MD) surface EMG and of fine-wire EMG and TDI tissue-velocity changes of the contralateral transversus abdominis, obliquus internus, and obliquus externus. Muscle onsets were determined by blinded visual analysis of EMG and TDI data. TDI could not distinguish between the relative activation of the three muscles, so in subsequent analyses only the onset of the earliest abdominal muscle activity was used. The latter occurred <50 ms after the onset of medial deltoid EMG (i.e., was feedforward) and correlated with the corresponding EMG onsets (r = 0.47, P < 0.0001). The mean difference between methods was 20 ms and was likely explained by electromechanical delay; limits of agreement were wide (-40 to +80 ms) but no greater than those typical of repeated measurements using either technique. The between-day standard error of measurement of the TDI onsets (examined in 16 further subjects) was 16 ms. TDI yielded reliable and valid measures of the earliest onset of feedforward activity within the anterolateral abdominal muscle group. The method can be used to assess muscle dysfunction in large groups of back-pain patients and may also be suitable for the noninvasive analysis of other deep-lying or small/thin muscles.

Wall shear stress and related hemodynamic parameters in the fetal descending aorta derived from color Doppler velocity profiles.

This paper presents a methodology for estimating the wall shear stress in the fetal descending aorta from color Doppler velocity profiles obtained during the second half of pregnancy. The Womersley model was applied to determine the wall shear stress and related hemodynamic parameters. Our analysis indicates that the aortic diameter can be modeled as a function of the gestational age in weeks as: Diameter (mm) = 0.17.ga + 0.15 (R2 = 0.64, p < 0.001). The aortic volume flow showed a log linear gestational age-related increase that fit the model: F (mL/min) = e(0.08.ga + 3.49) (R2 = 0.61, p < 0.001). The Womersley number increased linearly with gestational age from 3.3 to 6.2 (p < 0.001) and the pressure gradient decreased linearly from 2.68 to 1.16 mPa/mm (p = 0.003) during the second half of pregnancy; the mean wall shear stress for the study group was 2.2 Pa (SD = 0.59) and was independent of gestational age. This study suggests that the size of the fetal aorta adapts to flow demands and maintains constant mean wall shear stress.

Arterial pulse wave velocity with tissue Doppler imaging.

This paper describes a new noninvasive ultrasonic method for estimating pulse wave velocity (PWV), an important physical parameter for characterizing the elastic properties of the arterial walls. The method utilizes a relatively new color Doppler modality for measuring tissue motion (tissue Doppler imaging or TDI). In contrast to previously proposed methods, the TDI modality offers multiple recording sites along the artery that improve the PWV estimation considerably. The new PWV estimation method was evaluated through an in vitro setup consisting of an elastic vessel supplied with a pulsatile pump. The study concentrated on the effect of different system parameters controlling resolution, sensitivity and the amount of acquired data. It was shown that the system parameters have a significant effect on the PWV variance, whereas the PWV mean remains unchanged. It was also established that high temporal resolution is the most vital parameter for minimizing PWV variance. Finally, the new PWV estimation method was applied to a limited set of human carotid artery data sets, with good results.

Inhibition of red cell aggregation prevents spontaneous echocardiographic contrast formation in human blood.

BACKGROUND: Spontaneous echocardiographic contrast (SEC) is a pattern of blood echogenicity that has been attributed to ultrasonic backscatter from blood cell aggregates that form under low shear conditions. Patients with left atrial SEC have an increased thromboembolic risk. This study examined the role of red cell and platelet aggregates in the pathogenesis of SEC in human blood and the effects on SEC of antithrombotic therapy and red cell disaggregatory agents. METHODS AND RESULTS: Blood echogenicity was examined with the use of quantitative videodensitometry over a controlled range of flow velocities in an in vitro model characterized by nonlaminar flow conditions. One hundred ninety study samples were prepared from single fresh blood donations (40 to 120 mL) from 24 healthy volunteers and 11 patients. Whole blood echogenicity was unaltered by depletion of platelets, stimulation of platelet aggregation with adenosine diphosphate, or inhibition of platelet aggregation with aspirin. Low flow-related echogenicity increased with increasing hematocrit (P<.001) but was abolished when red cells were lysed selectively with saponin (P<.001). In the presence of red cells, low flow-related echogenicity increased with increasing fibrinogen concentration (P<.001) and with plasma paraproteins. Low flow-related echogenicity in whole blood was unaltered by heparin and warfarin but was reduced in a dose-dependent manner by dextran 40 (40 mg/mL, 70% reduction, P<.001) and poloxamer 188 (8 mg/mL, 47% reduction, P<.001), which inhibited red cell aggregation. CONCLUSIONS: These results support protein-mediated red cell aggregation as the mechanism of SEC in human blood. Inhibition of red cell aggregation, indexed by resolution of SEC, may provide an alternative to anticoagulant and antiplatelet therapy to reduce cardiac thromboembolic risk.

Direct effects of DC cardioversion on blood echogenicity: an in vitro study.

Exacerbation of left atrial spontaneous echo contrast (SEC) after cardioversion of atrial fibrillation has been attributed to left atrial mechanical dysfunction induced by the procedure ("atrial stunning"). An in vitro model was devised to determine whether electrically induced changes in blood properties might contribute to SEC formation after cardioversion. Human blood echogenicity was examined quantitatively by videodensitometry before and after shocks of 1, 2, 5, and 20 J. Changes in blood cell numbers, cell morphology, and erythrocyte sedimentation rate were determined by haematological analysis. Immediately following electrical discharges, transient and dose-related, highly echogenic microbubbles were noted, but shocks of increasing intensity did not induce SEC at high blood velocity or alter the severity of SEC at low blood velocity. No quantitative or qualitative changes in haematological parameters were observed. These results suggest that direct effects of electrical shock on blood do not contribute to SEC after cardioversion. Systemic haematological responses to electric shock that might indirectly promote red cell aggregation in vivo cannot be excluded by this in vitro study.

Spectral and color Doppler sonographic applications of a new test object with adjustable moving target spacing.

A new test object has been developed with the aim of covering a wide range of Doppler quality assurance requirements, with particular emphasis on color Doppler imaging (both velocity and power modes). The test object consists of a rotating string-loop, which provides the single velocity source required for velocity calibration. The loop geometry provides two targets with adjustable spacing, which move in opposite directions within the same scan plane, and the string material offers a number of desirable properties (absence of knot artifacts, small diameter, uniform backscatter as a function of Doppler angle, and low attenuation). Representative results are included to demonstrate applications of the test object in the assessment of velocity accuracy, intrinsic spectral broadening, spatial resolution, and sensitivity, as well as to discuss quality assurance and performance issues in the context of spectral and color Doppler sonography.

Quantification of blood echogenicity: evaluation of a semiquantitative method of grading spontaneous echo contrast.

Spontaneous echo contrast (SEC) is an echogenic, swirling pattern of blood flow which may be observed by transesophageal echocardiography (TEE) in the left atrium in low flow states, such as atrial fibrillation (AF). The presence of SEC has been proposed as a marker of increased thromboembolic risk. Evaluation of the severity of SEC might be useful in stratification of thromboembolic risk. The aim of this study was to validate a semiquantitative method of grading SEC against quantitative videodensitometric analysis. TEE studies were performed in 50 patients with AF. The severity of left atrial SEC was graded by three independent observers and by videodensitometry. There was a strong, positive correlation between the semiquantitative grades of SEC and quantitative videodensitometric scores (r = 0.85, P < 0.0001). Inter- and intraobserver correlations in the grading of SEC were very high (observer 1 vs. 2: r = 0.98, P = 0.0001; observer 1 vs. 3: r = 0.93, P = 0.0001; observer 1 vs. 1: r = 0.97, P = 0.0001). Semiquantitative grading of SEC can be performed rapidly and reliably by experienced observers. These results support the use of semiquantitative grading in studies of the pathogenesis and prognostic implications of SEC.

Computer analysis of the early diastolic notch in Doppler sonograms of the uterine arteries.

This article describes a set of processing and analysis techniques for automated identification and quantification of the early diastolic notch (EDN), a feature of Doppler sonograms from the uterine arteries which has been associated with adverse pregnancy outcomes such as preeclampsia and intrauterine growth retardation. Examples covering different sonogram types are provided to illustrate the effectiveness and reproducibility of the processing/analysis tools. Also, a receiver-operating characteristic-based evaluation of the EDN quantification and pulsatility indexes is presented, which examines the ability to predict hypertension and/or intrauterine growth retardation, using a set of uterine Doppler sonograms from 92 patients acquired at 18 weeks of gestation. In summary, the ROC results confirm the link between the EDN and abnormal pregnancy outcomes, and suggest that EDN quantification has a higher diagnostic accuracy than the pulsatility index, which characterises the flow waveform in a global manner and therefore does not take explicitly into account the localised nature of the EDN. Quantification of the EDN at 18 weeks of gestation appears to best predict the most severely abnormal pregnancy outcomes.

Development of an advanced digital image processor for real-time speckle suppression in routine ultrasonic scanning.

This paper describes a digital image processor for ultrasonic speckle suppression that was explicitly designed to satisfy the requirements of detail preservation, adequate smoothing and real-time operation. The first two of these requirements were addressed by selecting a nonlinear adaptive algorithm, which uses a measure of local homogeneity to adjust the amount of smoothing performed at each point of the scan, and employing a large (9 x 9 pixels) filtering window. Real-time operation was achieved by developing a highly concurrent systolic architecture that allowed the efficient mapping of the algorithm into low-cost high-density hardware. Initial experience, obtained by interfacing the processor to a scanner, suggests that this type of processing is capable of enhancing the visibility of subtle differences in echogenicity while retaining genuine image detail, as judged by the preservation of the small vessels and ducts. In addition to its impact on contrast resolution, this form of real-time speckle suppression should also prove valuable as a preprocessing stage before performing other digital image processing operations that are sensitive to noise, such as segmentation and three-dimensional rendering.

An investigation of simulated umbilical artery Doppler waveforms. I. The effect of three physical parameters on the maximum frequency envelope and on pulsatility index.

The effect of three physical parameters on the accuracy of estimation of the maximum frequency envelope and pulsatility index (PI) of simulated umbilical artery Doppler waveforms was investigated. The physical parameters were beam-vessel angle, the offset between the beam axis and vessel axis, and the thickness of overlying attenuating material. Waveforms were acquired using a physiological flow phantom. The maximum frequency envelope was calculated using a threshold maximum frequency follower which was adaptive to the level of background noise. A gold standard maximum frequency envelope was obtained from the ensemble averaged waveform when there was alignment of beam and vessel axis, a 50 degrees beam-vessel angle and 2 cm of attenuating material. Indices of bias, variability and accuracy of estimation of the maximum frequency envelope and PI were calculated by comparing subsequent maximum frequency envelopes with the gold standard maximum frequency envelope. Both the maximum frequency envelope and PI were estimated to a similar degree of accuracy over a wide range of physical conditions. In this study, the error in PI was less than 0.15 for beam-vessel angles less than 80 degrees, for beam-vessel axis offset distances less than 7.5 mm, at a transducer-vessel distance of 5 cm, and for attenuator thicknesses less than 4.5 cm. The percentage root-mean square error for estimation of the maximum frequency envelope was approximately 10% or less for beam-vessel angles less than 75 degrees, for beam-vessel axis offset distances less than 7.5 mm, and for attenuator thicknesses less than 4 cm.

A comparison of three different filters for speckle reduction of Doppler spectra.

This paper is concerned with speckle suppression in Discrete Fourier Transform based Doppler signals by means of digital image processing. The Doppler spectrum is treated as a greyscale image, and three different noise smoothing algorithms are applied to it. These are the Double Window Modified Trimmed Mean filter, which is nonlinear, and Lee's and the directional filter, which are adaptive in the sense that the smoothing performed by them at each point of the image is controlled by a local image measure. In order to evaluate the performance of the filters, they were applied to a variety of regular waveforms obtained from a physiological flow phantom. Ensemble averaging of a large number of unfiltered spectra was used as the "gold standard" in the evaluation, i.e., as the output of an ideal filter which reveals the exact nature of the underlying Doppler spectrum after speckle has been eliminated. Comparison of the "gold standard" with the ensemble averaged filtered data allowed the noise reduction, bias and distortion of the maximum frequency envelope introduced by filtering to be examined. Overall, the best performance was offered by the directional filter whose action was controlled by the combination of the local edge content and the slope of the least-squares-fit line passing through the data points along each particular direction.

Low-order AR models for mean and maximum frequency estimation in the context of Doppler color flow mapping.

Autoregressive (AR) techniques are investigated by developing mean and maximum frequency estimators suitable for use in Doppler color flow mapping systems, where they are most needed. The estimators are based on low-order (for computational efficiency) AR models applied to complex signals whose real and imaginary parts are the in-phase and quadrature components of the analytical Doppler signal, respectively. A large number of simulated data sequences generated by a sinusoidal computer model and having different number of samples, spectral shapes, bandwidths, and signal-to-noise ratios are used to examine the performance (bias and variance) of the estimators in a systematic manner. Comparisons are made with the established autocorrelation technique, whose output is shown to be identical to one of the AR mean frequency estimators described.

A comparison of the Doppler spectra from human blood and artificial blood used in a flow phantom.

A comparison between the Doppler signals from human blood and artificial blood used in a flow phantom is described. The artificial blood used was a suspension of Sephadex particles in a glycerol solution. The Doppler power was measured as a function of Sephadex concentration and found to peak at a concentration of about 40% by volume. The power from blood was less by a factor of 150-250 than the power from Sephadex of a similar concentration. The first and second order statistics of the Doppler spectra from Sephadex were independent of particle concentration, and were very similar to those of spectra from blood.

Deconvolution in medical ultrasonics: practical considerations.

Deconvolution can, in principle, enhance the spatial resolution of specular reflectors in medical ultrasonic imaging but, in practice, the resolution improvement offered is offset by the introduction of undesirable artefacts. In this study, several problems related to deconvolution were identified and practical suggestions for minimising artefacts were made. These include: fitting a three-dimensional surface to experimentally measured beam profiles in order to take into account the depth-dependence of the point spread function (PSF); adaptive detail-preserving noise filtering as a preprocessing tool in order to improve the quality of the data and reduce the speckle enhancement artefact; a histogram modification procedure in order to overcome the problems of ringing, over- and undershooting. Processing of a large number of A-scan data obtained from tissue-mimicking phantoms and the abdomens of normal volunteers demonstrated the efficiency of these techniques in reducing artefacts. The performance of deconvolution in terms of resolution improvement was satisfactory when data from resolution test objects were processed but poor with abdominal scans. This difference in performance raises the question as to how similar the PSF in tissue is to the experimentally measured PSF in water or even a tissue-mimicking material.