Pulsed Doppler ultrasound system for the measurement of velocity distributions and flow disturbances in arterial prostheses

Optimisation of Pulsed Ultrasonic Velocimetry system and transducer technology for industrial applications

Pulsed Ultrasonic Velocimetry, commonly referred to as Ultrasonic Velocity Profiling (UVP) in research and engineering applications, is both a method and a device to measure an instantaneous one-dimensional velocity profile in opaque fluids along a measurement axis by using Doppler echography. Studies have suggested that the accuracy of the measured velocity gradient close to wall interfaces need to be improved. The reason for this is due to distortion caused by cavities situated in front of ultrasonic transducers, measurement volumes overlapping wall interfaces, refraction of the ultrasonic wave as well as sound velocity variations (Doppler angle changes). In order to increase the accuracy of velocity data close to wall interfaces and solve previous problems a specially designed delay line transducer was acoustically characterised and evaluated. Velocity profiles measured using the delay line transducer, were initially distorted due to the effect of finite sample volume characteristics and propagation through the delay line material boundary layers. These negative effects were overcome by measuring physical properties of the ultrasonic beam and implementing a newly developed deconvolution procedure. Furthermore, custom velocity estimation algorithms were developed, which improved the time resolution and penetration depth of the UVP system. The optimised UVP system was evaluated and compared to standard transducers in three different straight pipes (inner diameters of 16, 22.5 and 52.8mm). Velocity data obtained using the optimised UVP system showed significant improvement close to wall interfaces where the velocity gradients are high. The new transducer technology and signal processing techniques reduced previously mentioned problems and are now more suitable for industrial process monitoring and control.

A new perivascular multi-element pulsed Doppler ultrasound system for in vivo studies of velocity fields and turbulent stresses in large vessels

A pulsed Doppler ultrasound (PDU) multi-element system was developed for perivascular registration of velocity fields and turbulence in large vessels. In vivo evaluation and comparison with hot-film anemometry (HFA) was performed. C-shaped shells were designed with holes to fit five small 10 MHz ultrasonic probes directed at five measuring points along a diameter perpendicular to the vessel axis. By rotating the shell in 45 degrees steps, blood velocities were measured in 17 points covering the entire cross-sectional vessel area. Measurements were performed in the ascending aorta and at three axial locations in the descending thoracic aorta in pigs. Simultaneous PDU and HFA measurements were performed distal to induced vascular stenoses of different degrees. Three-dimensional visualisation of velocity profiles was made, and Reynolds normal stresses (RNS) were calculated for different levels of turbulence intensities based on the simultaneous PDU and HFA measurements. The velocity profiles in the ascending aorta were skewed at top systole with the highest velocities towards the posterior wall. In the descending thoracic aorta at the ligmentum of Botalli, the velocity profiles were skewed throughout the entire systole with the highest velocities at the right anterior vessel wall. Further downstream in the descending aorta the velocity profiles appeared blunter. The frequency response of the modified PDU system was determined by a 'random noise test' revealing an upper -3dB cut-off frequency of approximately 200 Hz. Regression analysis showed a linear relationship between RNS measured with PDU and RNS measured with HFA (r = 0.93). Two vessel diameters distal to a 75% stenosis RNS up to 28 N m-2 were measured.(ABSTRACT TRUNCATED AT 250 WORDS)

Pulsatile velocity distribution and wall shear rate measurement using pulsed Doppler ultrasound

Instantaneous pulsatile velocity profiles were measured in a cylindrical tube using a 20 MHz pulsed Doppler ultrasound system. The spatial resolution of the device was improved with deconvolution using a procedure which employed frequency-domain windowing to suppress high-frequency noise. After deconvolution both the instantaneous velocity profiles and flow waveforms were compared with established theoretical predictions and good agreement was obtained. The average error in a velocity measurement was 7.9 +/- 0.9%, and the average error in the flow rate was 4.7 +/- 1.7%. The wall shear rate was determined from the gradient of the velocity profile both before and after deconvolution. The results were compared to the theoretical value of wall shear rate to quantify the accuracy of the technique under pulsatile flow conditions. Although the velocity profiles agreed well with theory, the wall shear rate was obtained less accurately. The average error in the mean wall shear rate and peak-to-peak wall shear rate was 28 +/- 7% and 11 +/- 3%, respectively. However, some of this discrepancy, was attributed to the method of evaluation. In all cases deconvolution significantly improved the accuracy of the wall shear rate determination.

Quantitative measurement of wall shear rate by pulsed Doppler ultrasound

A 20 MHz pulsed Doppler ultrasound system was used to obtain steady flow velocity profiles in cylindrical tubes. A deconvolution technique was employed to improve the spatial resolution and the corrected results were compared with Poiseuille's theory. Frequency domain windowing was used to suppress noise in the deconvolution procedure, using a method which reduced distortions at the window boundaries. Wall shear rate was calculated from the measured velocity profiles both before and after deconvolution, and compared with theory to quantify the accuracy of the technique. The results illustrate the significant improvement provided by deconvolution, with the average error in measured wall shear rate reducing from 46 to 12% after applying the technique. Some of the discrepancy is in part attributed to the method used to determine wall shear rate.

Effect of stenosis inlet geometry on shear rates of blood flow in the upstream region

Abnormal shear rates of blood flow have been implicated in the processes of thrombosis, atherosclerosis, and restenosis after angioplasty. However, no study has quantitated the effect of stenosis inlet geometry on the shear rates in the region upstream to the stenosis. To quantitate this effect, we measured the velocity profiles of blood flow at Reynolds numbers 175 and 350 upstream to different axisymmetric model stenoses in an excised canine aorta. Two 40% and two 75% stenoses were tested, one each with a 45-degree inlet angulation and one each with a 90-degree angulation. For the velocity measurements we used a specially developed external single-channel Doppler ultrasound system capable of resolving blood flow velocity at 91 microns radial intervals across the aorta. We found that increasing the severity of stenosis narrowing and angulation resulted in a significant decrease in shear rate at the endothelial surface (40%/45-degree stenosis: 189 +/- 46 sec-1 vs 75%/90-degree stenosis: 49 +/- 12 sec-1 at Reynolds number 350; p < 0.002) and a significant increase in the maximum shear rate within the vessel lumen (189 +/- 46 sec-1 vs 295 +/- 8 sec-1, respectively; p < 0.05) in the region immediately upstream to the stenosis. These effects were less pronounced for Reynolds number 175. We conclude that stenosis inlet geometry has a significant impact on the flow conditions in the region immediately upstream to the stenosis, which is dependent on the Reynolds number. This may be an important determinant of thrombosis and atherogenesis in this particular region.

In situ versus reversed femoropopliteal vein grafts: long-term follow-up of a prospective, randomized trial

In a prospective, randomized trial, 226 patients undergoing femoropopliteal bypass for lower limb ischaemia were allocated to reversed (123 patients) or in situ (103) techniques. The groups were comparable for age, sex, incidence of diabetes, and indications for surgery. Eleven veins were rejected at operation, nine in the reversed group and two in the in situ group, leaving 114 reversed and 101 in situ grafts for study. Cumulative patency rates were not significantly different between reversed and in situ grafts at any time up to 6 years after operation, with primary patency rates at 1, 3 and 5 years of 84.8, 69.5 and 62.4 per cent for reversed grafts and 79.9, 71.2 and 63.5 per cent for in situ grafts. Small vein grafts (< 4 mm in diameter) were associated with patency rates at 1, 3 and 5 years of 63.5, 46.7 and 36.0 per cent compared with 93.9, 82.5 and 75.9 per cent for vein grafts > or = 4 mm in diameter (P < 0.002, log rank test). The patency rates of small veins employed in situ and reversed were similar. The in situ technique confers neither short- nor long-term advantage over reversed vein grafting for femoropopliteal bypass.

The effect of averaging cardiac Doppler spectrograms on the reduction of their amplitude variability

The effect of averaging cardiac Doppler spectrograms on the reduction of their amplitude variability was investigated in 30 patients. Beat-to-beat variations in the amplitude of Doppler spectrograms were also analysed. The quantification of amplitude variability was based on the computation of the area under the absolute value of the derivative function of each spectrum composing mean spectrograms. Fast Fourier transform using a Hanning window was used to compute Doppler spectra. Results obtained over systolic and diastolic periods showed that the reduction of amplitude variability followed an exponentially decreasing curve characterised by the equation f (r) = 100 e-beta(r-1), where r is the number of cardiac cycles, beta the exponentially decreasing rate, and 100 the normalised variability for r = 1. In systole, the decreasing rate beta was 0.165, whereas in diastole it was 0.225. Reductions of the variability in systole for a number of cardiac cycles of 5, 10, 15, and 20 were 48, 77, 90 and 96 per cent, respectively. In diastole, reductions of the variability for the same numbers of cardiac cycles were 59, 87, 96 and 99 per cent, respectively. Based on these results, it can be concluded that no significant improvement in the reduction of amplitude variability may be obtained by averaging more than 20 cardiac cycles.

Effect of compliance mismatch on flow disturbances in a model of an arterial graft replacement

Flow disturbances in a model of an interposition graft in an arterial segment were measured using an ultrasound Doppler velocimeter. The effect of the degree of compliance mismatch between a stiff' graft' and compliant 'arterial' segments was investigated. In steady flow, disturbances were detected when the compliance ratio (stiff to compliant segments) was less than or equal to 0.1 and the Reynolds number greater than or equal to 2200. A recirculation zone just downstream of the distal anastomosis was observed at a Reynolds number greater than or equal to 2400. Disturbances were also measured under pulsatile flow which consisted of a time-varying component superimposed on a steady flow component. The time-varying flow component was either quasiphysiological or sinusoidal in shape. The Reynolds number was 500 but the frequency parameter varied from 4.2 to 8.5. Significant disturbances were observed for conduits with compliance ratio less than or equal to 0.19. The disturbance intensity tended to increase as the compliance ratio decreased and the frequency parameter increased. The magnitude of the disturbance was also greater with the quasiphysiological than the sinusoidal input flow waveform.