In ultrasound elastography, tissue axial strains are calculated from the gradient of the estimated axial displacements. However, the common differentiation operation amplifies the noises in displacement estimation, especially at high frequencies. In this paper, a low-pass digital differentiator (LPDD) is proposed to calculate the axial strain from the estimated tissue displacement. Several LPDDs that have been well developed in the field of digital signal processing are presented. The corresponding performances are compared qualitatively and quantitatively in computer simulations and in preliminary phantom and in vitro experiments. The results are consistent with the theoretical analysis of the LPDDs.

A baroregulation model, based on a previous pulsatile non-linear multielement ardiovascular model, is extended and used to study short-term regulation mechanisms. Using this model, the responses of several cardiovascular variables to ifferent exercise levels are simulated and compared with the experimental data reported in the literature. The impact of physiological or pathological changes on the short-term regulation of arterial pressure under the stimulus of moderate dynamic exercise is then studied. The simulation results indicate that baroreflex feedback plays a critical role in the short-term regulation of arterial pressure. When the baroreflex gain decreases to one-third of the normal value, the response of the mean arterial pressure to moderate dynamic exercise and post-exercise recovery time increases by factors of 1.7 and 2.3, respectively. Clinical data from 36 subjects (two groups: normal and hypertensive) are collected to validate the model. Computer simulations for the hypertensive group show that the elastic modulus of the arterial vessel wall is increased by 1.5 times, and peripheral resistance is increased by 1.3 times the normal value, and the baroreflex gain decreases from 0.55 (for the normal group) to 0.40. The simulation results for normal and hypertensive groups agree well with the clinical data.

Circulation plays an important rule in gas exchange. Therefore, there is an interaction between circulation and gas exchange. To understand the dynamic e ect of these two physiological systems, a computer simulation model of hemodynamics and gas exchange is established in this work. This model includes two physiological systems, namely the respiratory and circulatory systems. It consists of ve parts: the model of gas transport, exchange and storage within the body, the multi-element nonlinear mathematical model of human circulatory system, an alveolar ventilation controller, a cardiac output controller, and a controller of breathing frequency. Model simulations provide results consistent with both dynamic and steady-state responses under hypoxia. Simulation results can re ect the interaction of hemodynamics and gas exchange. Using this model, the changes of pulmonary arterial pressure and right ventricular pressure in high altitude are studied. The optimal mode of breathing extra oxygen using nasal prongs or a facial mask is studied. This model may provide a useful tool to study reaction of hypoxia and the oxygen inhalation mode under hypoxia environments.c 2000 Elsevier Science Ltd. All rights reserved.

A dynamic model was developed for a Hemopump that withdraws blood from the left ventricle and discharges it to the aorta through a miniature axial-flow pump. Incorporation of the Hemopump model in a previously established model for the canine circulatory system enabled the effects of the Hemopump on various haemodynamic variables of the circulatory system to be studied, and the benefit of the Hemopump to the failing heart was investigated. In addition, the influence of the physiological status of the right ventricle on the Hemopump performances was examined, and the synchronous and non-synchronous operations of the Hemopump were compared. Results verified that the Hemopump assists the failing heart by increasing the oxygen supply, while reducing the oxygen consumption of the heart through a reduction in the workload of the left ventricle. These beneficial effects were enhanced when the pump' s rotation speed was increased. When pump speed was increased from 17 000 to 23 000 revolutions min 1, the oxygen supply increased 101%, and the oxygen consumption decreased 60%.However, when the pump rotation speed was too high, the inflow to the pump could be impaired and the pump performance could be negatively affected. Predications from the model were in good agreement with the results previously obtained in animal experiments and in vitro measurements.

The Savitzky-Golay (SG) filters are generally used for smoothing and differentiation in many fields. The properties of the SG smoothing filters have been well studied. However, the properties of the SG differentiation filters or SG digital differentiators (SGDD, for the first order differentiation) are not developed well somehow, although they have been widely used. In the paper, the properties of the SGDD are discussed in detail. The effects of the SGDD on a noise-free single Gaussian line and a noise-free Gaussian doublet are studied via simulation. The results indicate that the contrast and resolution loss of the SGDD depends on the ratio of the width of signal derivative and filter length. The linear SGDD is not preferable for preserving small details of signal derivative from this study. The cubic and quintic SGDD with the choice of appropriate filter length are recommended in order to maintain the resolution of signal derivative.