The characterization and calibration of ultrasound imaging systems requires tissue-mimicking phantoms with known acoustic properties, dimensions and internal features. Tissue phantoms are available commercially for a range of medical applications. However, commercial phantoms may not be suitable in ultrasound system design or for evaluation of novel imaging techniques. It is often desirable to have the ability to tailor acoustic properties and phantom configurations for specific applications. A multitude of tissue-mimicking materials and phantoms are described in the literature that have been created using a variety of materials and preparation techniques and that have modeled a range of biological systems. This paper reviews ultrasound tissue-mimicking materials and phantom fabrication techniques that have been developed over the past four decades, and describes the benefits and disadvantages of the processes. Both soft tissue and hard tissue substitutes are explored.

A review of tissue substitutes for ultrasound imaging.

Development of the placental villous tree and its consequences for fetal growth

Time domain formulation of pulse-Doppler ultrasound and blood velocity estimation by cross correlation.

Objective: Accumulating evidence suggests that the balance between vascular endothelial growth factor (VEGF), placental growth factor (PlGF), and their receptors is important for effective vasculogenesis, angiogenesis, and placental development. Recently, the soluble form of VEGFR-1 (sVEGFR-1), an antagonist to VEGF and PlGF, has been implicated in the pathophysiology of pre-eclampsia. Plasma sVEGFR-1 concentration is elevated in pre-eclampsia at the time of clinical diagnosis and correlates with the severity of the disease. The purpose of this study was to determine whether the concentrations of sVEGFR-1 in plasma of pre-eclamptic patients change prior to the clinical manifestations of the disease.Methods: A longitudinal case-control study was conducted in normal pregnant women (n = 44) and patients with pre-eclampsia (n = 44). Blood sampling was performed at six intervals: (1) 7–16 weeks; (2) 16–24 weeks; (3) 24–28 weeks; (4) 28–32 weeks; (5) 32–36 weeks; and (6) more than 37 weeks of gestation. To exam...

Plasma soluble vascular endothelial growth factor receptor-1 concentration is elevated prior to the clinical diagnosis of pre-eclampsia.

A blood-mimicking fluid (BMF) suitable for use in Doppler flow test objects is described and characterised. The BMF consists of 5 μm diameter nylon scattering particles suspended in a fluid base of water, glycerol, dextran and surfactant. The acoustical properties of various BMF preparations were measured under uniform flow to study the effects of particle size, particle concentration, surfactant concentration, flow rate and stability. The physical properties, (density, viscosity and particle size), and acoustical properties (velocity, backscatter and attenuation) of the BMF are within draft International Electrotechnical Commission requirements.

Validation of a new blood-mimicking fluid for use in Doppler flow test objects

A unified approach to modeling the backscattered Doppler ultrasound signal from blood is presented. The approach consists of summing the contributions from elemental acoustic voxels, each containing many red blood cells (RBCs). For an insonified region that is large compared to a wavelength, it is shown that the Doppler signal is a Gaussian random process that arises from fluctuation scattering, which implies that the backscattered power is proportional to the variance of local RBC concentrations. As a result, some common misconceptions about the relationship between the backscattering coefficient and hematocrit can be readily resolved. The unified approach was also used to derive a Doppler signal simulation model which shows that, regardless of flow condition, the power in the Doppler frequency spectrum is governed by the exponential distribution. For finite beamwidth and paraxial flow, it is further shown that the digitized Doppler signal can be modeled by a moving average random process whose order is determined by the signal sampling rate as well as the flow velocity profile. >

A unified approach to modeling the backscattered Doppler ultrasound from blood

A stochastic model of the continuous wave (CW) Doppler ultrasound signal backscattered from blood is developed. The model incorporates the effects due to the flow-dependent packing and aggregation of red blood cells (RBC's) at a given hematocrit. The Doppler signal is shown to be a zero-mean, stationary, Gaussian random process which can be completely specified by its autocorrelation function. From the autocorrelation function, explicit closed-form expressions are derived for the power spectral density and the backscattering coefficient. Both expressions are found to be more general than those derived from previous blood models. Further, the values of the statistical parameters in the model are discussed in relation to recently published data on the backscattering coefficient of bovine RBC's suspended in saline and of bovine whole blood. It is shown that the change in backscatter with flow conditions can be explained by RBC orientation in saline and by RBC aggregation in whole blood.

A Stochastic Model of the Backscattered Doppler Ultrasound from Blood

The performance of four methods for digitally estimating the maximum frequency waveform from the Doppler ultrasound spectrum, are described. The methods investigated are: a percentile method, D'Alessio's threshold crossing method [D'Alessio T. (1985) "Objective" algorithm for maximum frequency estimation in Doppler spectral analysers. Med. Biol. Engng and Comput. 23, 63-68.], a modified threshold crossing method, and a new hybrid algorithm. Evaluations of the variance and bias were performed using stationary simulated continuous wave (CW) Doppler signals of different bandwidths and signal/noise ratios (SNR) of 9 and 17 dB. Furthermore, a simulated nonstationary Doppler signal, similar to that from a normal internal carotid artery, was also used to compare the various methods. Overall, it was found that the modified threshold method and the new hybrid method have the best performance over a wide range of signal and noise hybrid method have the best performance over a wide range of signal and noise conditions; however, D'Alessio's method also performs well for low SNR's.

Comparison of four digital maximum frequency estimators for Doppler ultrasound.

An electrical analogue model of an artery that terminates into a vascular bed is presented. The model consists of an uniform transmission line that represents the artery and a load impedance that represents the vascular bed. The transmission line parameters are based on a well-established first-order approximation of the Navier-Stokes equations for fluid flow in distensible tubes. The model can be used to predict the incident and reflected components of both the arterial pressure and flow waveforms. In addition, it can predict the vessel diameter change and the mean blood velocity waveforms. In this study, the model was applied to the uterine artery so that the characteristics of the utero-placental circulation can be related to Doppler ultrasound recordings. It was found that the presence of the dicrotic notch in the uterine artery time-velocity waveform is the result of wave reflection and that a persistent notch past 20 weeks' gestation may be indicative of an abnormally high placental bed resistance. It is shown that the simulation results are consistent with the known physiological data and the clinically recorded uterine Doppler waveforms.

A transmission line modelling approach to the interpretation of uterine doppler waveforms

A computer model was used to simulate velocity waveforms that can be visualized in the human uterine artery using Doppler ultrasound. It was found that increasing uteroplacental vascular resistance from normal caused an increase in the systolic/diastolic velocity ratio (S/D) and pulsatility index (PI) of the waveform. Increasing uteroplacental resistance also caused the appearance of a dicrotic notch. Reducing the uterine artery radius increased the S/D and PI and this effect was accentuated at high placental resistance. In contrast, increasing mean arterial pressure in the uterine artery had little effect on S/D and PI. Results suggest that waveform shape abnormalities observed in obstetric patients with pregnancy-induced hypertension are primarily caused by high uteroplacental vascular resistance and a reduced uterine arterial diameter.

Larry Y. L. Mo

Papers

A unified approach to modeling the backscattered Doppler ultrasound from blood

A Stochastic Model of the Backscattered Doppler Ultrasound from Blood

Comparison of four digital maximum frequency estimators for Doppler ultrasound.

A transmission line modelling approach to the interpretation of uterine doppler waveforms

Effect of placental resistance, arterial diameter, and blood pressure on the uterine arterial velocity waveform: A computer modeling approach