Velocity field studies at surgically imposed arterial stenoses on the abdominal aorta in pigs

Evaluation of distal turbulence intensity for the detection of both plaque ulceration and stenosis grade in the carotid bifurcation using clinical Doppler ultrasound

OBJECTIVES

To determine the interrelationship of stenosis grade and ulceration with distal turbulence intensity (TI) in the carotid bifurcation measured using conventional clinical Doppler ultrasound (DUS) in vitro, in order to establish the feasibility of TI as a diagnostic parameter for plaque ulceration.

METHODS

DUS TI was evaluated in a matched set of ulcerated and smooth-walled carotid bifurcation phantoms with various stenosis severities (30, 50, 60 and 70 %), where the ulcerated models incorporated a type 3 ulceration.

RESULTS

Post-stenotic TI was significantly elevated owing to ulceration in the mild and moderate stenoses (P < 0.001). TI increased with stenosis severity in both the ulcerated and non-ulcerated series, with a statistically significant effect of increasing stenosis severity (P < 0.001). Whereas TI in the mild and non-ulcerated moderate stenoses was less than 20.4 ± 1.3 cm s(-1), TI in the ulcerated moderate and severe models was higher than 25.6 ± 1.3 cm s(-1), indicating a potential diagnostic threshold.

CONCLUSION

We report a two-curve relationship of stenosis grade and ulceration to distal TI measured using clinical DUS in vitro. Clinical DUS measurement of distal TI may be a diagnostic approach to detecting ulceration in the mild and moderately stenosed carotid artery.

KEY POINTS

• Patients with carotid artery plaque ulcerations are at higher risk of stroke. • Clinical Doppler ultrasound is routinely used to detect carotid artery stenosis. • Doppler ultrasound turbulence intensity can detect ulceration in realistic flow models. • Turbulence intensity also increases with stenosis severity independent of ulceration. • Doppler ultrasound should help in assessing both stenosis severity and ulceration.

In vivo Doppler ultrasound quantification of turbulence intensity using a high-pass frequency filter method

The objective of this investigation was to implement a high-pass frequency filter method to analyze Doppler ultrasound velocity waveforms and quantify turbulence intensity (TI) in vivo. Doppler velocity data were analyzed using two techniques, based on either ensemble averaging or high-pass frequency domain filtering of the periodic waveforms. The accuracy and precision of TI measurements were determined with controlled in vitro experiments, using a pulsatile-flow model of a stenosed carotid bifurcation. The high-pass filter technique was also applied in vivo to determine whether this technique could successfully distinguish between pertinent hemodynamic sites within the carotid artery bifurcation. Twenty-five seconds of Doppler audio data were acquired at three sites (common carotid artery [CCA], internal carotid artery [ICA] stenosis and distal ICA) within 10 human carotid arteries, and repeated three times. Doppler velocity data were analyzed using a ninth-order high-pass Butterworth filter with a 12-Hz inflection point. TI measured within the CCA and distal ICA was found to be significantly different (p < 0.0001) for moderate to nearly occluded carotid artery classifications. Also, TI measured within the distal ICA increased with stenosis severity, with the ability to distinguish between each stenosis class (p < 0.05). This investigation demonstrated the ability to precisely quantify TI using a conventional Doppler ultrasound machine in human subjects, without interfering with normal clinical protocols.

Relation of the flow field distal to a moderate stenosis to the Doppler power

An experimental investigation was undertaken to establish how different flow regimes affect the Doppler signal. A rigid tube model consisting of a 70% asymmetric area stenosis was used with steady and pulsatile flow conditions. The characteristics of the flow field at various sites was determined using a photochromic flow visualization method. Continuous-wave Doppler measurements were made using a 41% suspension of human red blood cells (RBCs) in saline as well as a dilute suspension of 4% fixed RBCs. For steady flow, the photochromic results indicated that for Reynolds numbers (Re) of 545 and 1410, turbulence was generated and the length of the turbulent region was found to increase with increasing Re. Under pulsatile flow conditions, turbulence was triggered around peak systole and began to dissipate in late deceleration, and by the end of diastole the flow field almost relaminarized. During the turbulent phase of the flow cycle, the poststenotic flow field was seen to consist of four distinct flow regimes similar to those observed for steady flow. For higher Womersley parameters and Reynolds numbers the turbulent zone was found to be larger and to occupy a greater fraction of the flow cycle. These flow visualization results were compared with the Doppler power measurements made at the same locations and under similar flow conditions. At physiological hematocrits (41%) the onset of turbulence for both steady and pulsatile flow increased the backscattered Doppler power. The location of the peak Doppler power coincided with the region of maximum turbulence observed using the photochromic technique.

Studies by pulsed Doppler ultrasonography of velocity fields downstream of graded stenoses on the abdominal aorta in pigs

PURPOSE

To investigate local hemodynamics downstream of arterial stenoses, a perivascular five-element Doppler ultrasound transducer was used for registration of one-dimensional velocity profiles and estimation of Reynolds (turbulent) normal stresses downstream of smooth, graded stenoses on the abdominal aorta in six 90 kg pigs.

METHODS

Blood velocities were registered by a 10 MHz pulsed Doppler velocimeter that used a modified zero-crossing detector with an upper -3 dB cutoff frequency of 200 Hz. Signal analysis included ensemble averaging, turbulence analysis, and dimensional visualization of velocity profiles.

RESULTS

Velocity profiles downstream of minor (< or = 40%) and moderate (40% to 65%) stenoses were skewed with the highest systolic velocities toward the anterior vessel wall and diastolic flow reversal occasionally present at the posterior vessel wall. Immediately downstream of severe (> or = 65%) stenoses a prominent poststenotic jet and systolic recirculation zones were present. Further downstream, vortices and eddies dominated the flow field. Reynolds normal stresses were highest at locations in the velocity field with high-velocity gradients corresponding to the parajet zone.

CONCLUSIONS

The present study demonstrated that pulsed Doppler ultrasonography can provide detailed and quantitative information of flow phenomena such as jetlike flow, vortices, and recirculation zones in a poststenotic flow field in the abdominal aorta.

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)