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R S Cobbold

Publications and source records attributed to R S Cobbold.

At least 19 recordsLinked to original sources
PubMed1992

Visualization of complex flow fields, with application to the interpretation of colour flow Doppler images.

The interpretation of colour flow Doppler images in regions of complex flow can be difficult. To aid in such interpretations, we have developed software which accepts as input the results of a fluid flow simulation program and produces simulated colour flow Doppler images on a workstation. This allows direct comparison between well-described flow fields and colour flow Doppler images. Application of this approach is demonstrated for flow in a two-dimensional model of a distal graft-to-vessel anastomosis, and the interpretation of the results for three-dimensional flows is briefly discussed. In addition, a method for presenting two-dimensional (vectorial) velocity data via colour coding is presented.

Algorithms↗
PubMed1992

A unified approach to modeling the backscattered Doppler ultrasound from blood.

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 (RBC's). 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.

Biophysical Phenomena↗
PubMed1991

Experimental study of the effects of pulsed Doppler sample volume size and position on the Doppler spectrum.

With a pulsed Doppler system, the recorded Doppler spectrum is expected to vary depending upon the sample volume size relative to the diameter of the vessel, the position of the sample volume in the vessel and the velocity profile. In the in vitro experiments described in this paper, the velocity profile was kept constant by using steady parabolic flow in a flow model. As the Doppler sample volume size and position were changed, the maximum variations of quantitative measurements from the Doppler spectrum were determined. The maximum, mean and mode frequencies and spectral broadening index (SBI) were affected by the position of the sample volume but to a lesser degree by its length (1.5-5.0 mm) relative to the 9.5 mm beam path length across the tube. When the centre of the Doppler sample volume was moved within the central 25% of the tube, the maximum variations were as follows: maximum frequency 3-5%, mean frequency 8-9%, mode frequency 8-9% and SBI 16-18%, where the range indicates the effect of increasing the sample volume size. Based on these results obtained under steady flow conditions in vitro, it is concluded that quantification of pulsed Doppler spectra may be feasible if the sample volume is positioned within the central 25% of the vessel.

Blood Flow Velocity↗
PubMed1991

Quantitative study of steady flow using color Doppler ultrasound.

The use of color Doppler flow mapping systems for quantitative in vitro studies of flow fields is examined and illustrated. A 5-MHz color Doppler system was used, and the resolution was determined by comparing the results of flow-field measurement for steady parabolic pipe flow with calculated values. The velocity accuracy was about 6% of the velocity corresponding to half the pulse repetition frequency, and the spatial resolution was better than 1 mm. Frame frequency limitations permitted only partial tracking of fast temporal changes in the flow field. However, detection of vortices downstream from a small cylinder placed in the flow tube was significantly enhanced by synchronizing the frame frequency with the vortex shedding frequency and using a velocity-variance mode. Color Doppler aliasing was found to be useful to define streamlines and determine whether the flow was laminar or turbulent. The color Doppler system clearly imaged Poiseuille, transitional and turbulent flow and vortex shedding in vitro. It is concluded that color Doppler ultrasound flow mapping can enable large, complex flow fields to be quantitatively studied in vitro.

Blood Flow Velocity↗
PubMed1991

Computer simulation of blood flow patterns in arteries of various geometries.

The purpose of this study is to illustrate the application of computer simulation to the study of blood flow through arteries and to demonstrate the relationship between geometry of the vessels and local flow patterns. A finite element computer program was developed to simulate steady and pulsatile blood flow by solving the continuity and Navier-Stokes equations. The accuracy of the computational method has been confirmed by comparing the numeric results to analytic solutions and to published experimental data from physical models. The results are presented as plots of the velocity vectors, streamlines, and pressure contours. The computational model has been applied to illustrate flow patterns in the following situations: pulsatile flow in a cylindric artery and an artery with an axisymmetric stenosis, steady flow in cylindric arteries with stenoses of varying severity and with different flow rates, steady flow in an artery containing a fusiform aneurysm, steady flow in a two-dimensional model of a symmetric Y-shaped bifurcation, and steady flow in a two-dimensional model of the carotid bifurcation. Regions that are commonly associated with arterial disease often coincide with zones of reversed or stagnant flow. In conclusion, the versatility and feasibility of computational simulation of blood flow is illustrated by this study. Although this mathematic model is a simplification of the real flow phenomena, it yields results that provide useful insights into the understanding of local blood flow patterns for a variety of complex geometries.

Aneurysm↗
PubMed1990

Development of methods to analyse transcranial Doppler ultrasound signals recorded in microgravity.

During space flights, several clinical syndromes may be the result of changes in cerebral circulation. The purpose of the paper is to describe the development and initial evaluation of a system for recording, processing and displaying transcranial Doppler ultrasound (TCD) waveforms from the middle cerebral artery (MCA) in microgravity. Volunteers were repeatedly subjected to 15-20 s intervals of microgravity ('near zero gravity') during flights on the KC-135 military aircraft. Continuous TCD recordings from the MCA were stored on magnetic tape. The paper describes the system that was developed to digitise the Doppler ultrasound data and markers that corresponded to the various levels of microgravity, obtain the maximum and mean Doppler waveforms, identify the waveforms and quantify them. The results demonstrate the feasibility of making TCD recordings in a microgravity environment and illustrate excellent performance of the system and its ease of operation. Quantitative waveform analysis of the recordings from the first subject studied in the supine position showed statistically significant changes in MCA velocity waveforms during microgravity.

Cerebral Arteries↗
PubMed1990

Detailed visualization of pulsatile flow fields produced by modelled arterial stenoses.

A multiple trace photochromic method was used to visualize the pulsatile flow field created by modelled arterial stenoses of 38% and 65% area reductions. Using flow parameters similar to those of a medium sized artery in man, the flow patterns at seven axial locations in relation to the stenosis were simultaneously photographed at various times throughout the flow cycle. With the 65% stenosis, the wall shear stress in the vicinity of the reattachment point was found to fluctuate quite strongly during the turbulent phase of the flow cycle, giving rise to instantaneous shear stresses that were at least eight times larger than those measured upstream. For the 38% stenosis, much smaller shear stresses were observed. These and other results are described in detail.

Arterial Occlusive Diseases↗
PubMed1990

Effects of transducer beam geometry and flow velocity profile on the Doppler power spectrum: a theoretical study.

A theoretical model is used to show how the Doppler spectrum for various axisymmetric velocity profiles is affected by beam misalignment and incomplete insonation. Results are presented for both circular and square beam geometries. Moreover, a closed-form expression is derived for the power spectral density received by an on-axis transducer with a Gaussian beam profile. It is shown that the error incurred in measuring the mean Doppler frequency with such a profile will generally be bounded by the results for the circular and square beam geometries. The effects of an ideal high-pass filter on the mean Doppler frequency and the backscattered Doppler power are examined. It is shown that such a filter can introduce large differences in the measured systolic to diastolic power ratios. Finally, theoretical expressions and results are presented for the spectral broadening index (SBI), normalized spectral variance (NSV), coefficient of kurtosis (CK), the coefficient of skewness (CS) as functions of the axisymmetric velocity profile shape assuming complete uniform insonation.

Blood Flow Velocity↗
PubMed1990

Steady and pulsatile flow fields in an end-to-side arterial anastomosis model.

We investigated the flow field within a rigid-walled in vitro model of an end-to-side 45 degree anastomosis in an attempt to identify possible hemodynamic factors that may contribute to the pathogenesis of distal anastomotic intimal hyperplasia. A high-resolution photochromic tracer technique was used to visualize the flow in orthogonal planes and to determine the axial wall shear stress profiles for both steady and pulsatile flows over a range of physiologically relevant conditions. The flow field showed qualitative similarities to those seen in curved vessel: rapidly moving fluid from the graft section affects the bed of the host vessel, that is, the wall opposite the anastomosis, eventually advancing down the host vessel in a spiraling motion. A small mobile separation zone was noted at the toe of the anastomosis. Comparison of wall shear stress profiles with previously reported preferential sites for the development of intimal hyperplasia supported a low wall shear stress and/or flow separation pathogenesis hypothesis. One notable exception was the bed of the host artery that appeared to be subjected to a complex hemodynamic environment.

Arteriovenous Shunt, Surgical↗
PubMed1990

Evidence of a possible link between poststenotic dilation and wall shear stress.

The effects of an axisymmetric 65% area reduction stenosis on a pulsatile flow were investigated by use of an in vitro model that permits simultaneous visualization of the flow velocity profiles at seven sites. By use of seven lenses to focus the ultraviolet light from a nitrogen laser, seven thin blue lines were produced in the photochromic solution flowing through the tube. The displacement profiles of the dye traces were photographed, resulting in the acquisition of the velocity profiles. From these traces, the flow pattern was determined, and the wall shear stresses were measured. Turbulence was generated 3.3 to 6.5 tube diameters downstream from the edge of the stenosis, depending on the time in the pulsatile flow cycle. Maximum wall shear stress fluctuations between positive and negative values appeared to lie within 1.6 to 3.3 tube diameters downstream of the stenosis. In several illustrative clinical cases of thoracic outlet arterial compression, the poststenotic dilation was maximum at 2.0 +/- 0.3 vessel diameters downstream. Based on these observations, it is postulated that wall shear stress fluctuations may be important in the development of poststenotic dilation.

Arteries↗
PubMed1977

An automatic, multi-function pressure cuff control unit.

A fully automatic blood pressure cuff control system is described. The system is designed for use in the assessment of peripheral vascular disease and helps reduce the amount of operator attention required to carry out such measurements properly. It also provides repeatable control of specific functions, thereby eliminating a potential source of variability. The system incorporates a pneumatic control system, an integrated silicon pressure transducer, and a digital readout. The design, modes of operation, safety features, and use of the system are described.

Blood Pressure Determination↗
PubMed1977

Errors and artifacts of Doppler flowmeters and their solution.

Continuous-wave Doppler ultrasound flowmeters are essential instruments for the vascular surgeon. Unfortunately, when used for quantitative purposes, they yield a flow velocity waveform that is substantially in error. Current directional continuous-wave Doppler velocimeters used a O-crossing detector to convert the Doppler signal to a waveform. Unfortunately, there are several inherent problems with this technique, and velocity waveforms are produced that have significant artifacts or errors. In this report, it is shown that a multifilter system is a simple, inexpensive alternative to systems using a O-crossing detector, and furthermore, permits real-time display of the Doppler waveform. The severity of the underlying arterial disease can be quantified by the calculation of pulsatility index. The pulsatility index was calculated from the Doppler waveforms recorded from peripheral arteries in 155 limbs and showed an excellent correlation with the severity of atherosclerosis as assessed by arteriography.

Animals↗
PubMed1977

Systolic pressure slope: system for automated display and calculation.

The rate of rise of arterial blood pressure--the systolic pressure slope--has been measured noninvasively at the ankle and has been shown to be reduced by minor degrees of peripheral arterial stenosis. This report describes an electronic system used to display the systolic part of the arterial pressure wave. The system uses noninvasive data from an ECG, a blood pressure cuff, and a Doppler ultrasound detector. From these data the systolic pressure slope is measured automatically. One hundred and sixty-eight recordings from the limbs of 10 normal subjects were analyzed in detail and confirmed the reproducibility of the system. The points used to calculate the systolic slope fell close to a straight line, with correlation coefficients averaging 0.96. An average of eight repeat measurements were made per limb, and the coefficients of variation averaged 6.5%. Mean values for the normalized systolic pressure slope agreed with previously published data.

Adult↗