Louis Allard

Limitations of ultrasonic duplex scanning for diagnosing lower limb arterial stenoses in the presence of adjacent segment disease

PURPOSE The purpose of this study was to provide a quantitative evaluation of the effect of adjacent segment lesions on disease classification in lower limb arteries by ultrasonic duplex scanning. METHODS Lower limb arterial duplex scanning from the distal aorta to the popliteal artery was performed in 55 patients. Arterial lesions evaluated by visual interpretation of Doppler spectra were compared blindly with those measured by angiography. RESULTS To recognize severe stenoses (50% to 100% diameter reduction) in any arterial segment, duplex scanning had sensitivity and specificity rates of 74% and 96%, respectively. However, sensitivity and specificity rates increased to 80% and 98%, respectively, when there was no 50% to 100% diameter-reducing lesion in adjacent segments, whereas they decreased to 66% and 94%, respectively, when there was at least one 50% to 100% diameter-reducing lesion in adjacent segments. Moreover, among the 48 duplex misclassifications underestimating or overestimating the degree of arterial stenoses, 30 (62.5%) involved a segment with at least one 50% to 100% lesion in adjacent segments. The segments mostly affected by proximal and distal arterial lesions were the popliteal arteries and the common and deep femoral arteries, where it was found that 86% (24/28) of the misclassifications involved the presence of either proximal or distal severe stenoses. CONCLUSION The results demonstrated that the presence of multiple stenoses was an important limitation of duplex scanning for the detection and quantification of lower limb arterial disease.

Effect of the insonification angle on the Doppler backscattered power under red blood cell aggregation conditions

It has been reported that power color Doppler ultrasound has important advantages over conventional color Doppler flow imaging. Some of these advantages are the aliasing free capability, the increased sensitivity to flow, and the angular independence. This last characteristic of power Doppler ultrasound was evaluated to verify if it was still valid in some well-defined flow conditions where porcine whole blood, calf red cells suspended in saline solution, and carbon fibers suspended in a water-glycerol mixture were used as scattering particles. Experiments were conducted under steady flow conditions (mean shear rates across the tube between 8.5 and 102 s/sup -1/) for insonification angles between 40 and 80/spl deg/. Different hematocrit values (5, 10, 20, and 40%) were specifically tested for porcine whole blood. Results indicated no angular dependence for the saline suspension of calf red cells while a strong anisotropy was observed for the carbon fibers. In this last situation, the Doppler power in decibels increased linearly with the insonification angle. The maximum found at 800 suggests that the fibers were aligned with the direction of the flow. For porcine whole blood, an angular dependence was observed at some specific shear rate conditions. At 40% hematocrit, the anisotropy was about 5 dB for shear rates between 17 and 51 s/sup -1/, while for a lower (8.5 s/sup -1/) or higher shear rate (102 s/sup -1/), the anisotropy was reduced to approximately 2 dB. In all of these situations, the maximum Doppler power was observed for an insonification angle between 45 and 60/spl deg/, For hematocrit values of 5, 10, and 20%, the anisotropy was respectively on the order of 2, 3, and 4 dB or less, depending on the shear rate conditions. Among the possible mechanisms that may explain the anisotropic effect observed in the present study, the structure of the red cell aggregates is believed to be the determinant factor. A hypothesis concerning the structure of the aggregates under flowing conditions in large diameter tubes is proposed.

Changes in ultrasonic doppler backscattered power downstream of concentric and eccentric stenoses under pulsatile flow

The main objective of the present work was to investigate, under pulsatile flow, the patterns of variation of the Doppler power backscattered by blood and Sephadex particles upstream and downstream of concentric and eccentric stenoses ranging from 47% to 91% area reduction. Doppler measurements were performed at 5 diameters upstream and 5, 10, 15 and 20 diameters downstream of the constriction. For the concentric 75% and 85%, and the eccentric 79% and 91% area reduction stenoses, a progressive increase of the power backscattered by red cell suspensions at 40% hematocrit was measured downstream of the narrowing. The maximal power usually occurred around 10 diameters after the stenosis and dropped further downstream. In addition to the increase in the power, a cyclic variation of the backscattered intensity was observed within the flow cycle. For the concentric 52% and eccentric 47% area reduction stenoses, no variation of the Doppler power was measured during flow acceleration and deceleration for all recording sites. A coefficient of correlation of 0.82 was measured between the percentage of area reduction and the ratio of the Doppler mean power at 10 diameters downstream to that at 5 diameters upstream of the stenoses. Using Sephadex particles at low concentration, no increase of the Doppler power was found downstream of the 85% and 91% area reduction stenoses. The possible link between the intensity of turbulence and the power backscattered by blood is discussed along with the influence of the correlation between the scattering particles, under turbulent flow.

Characterization of blood flow turbulence with pulsed-wave and power Doppler ultrasound imaging

Blood turbulence downstream of a concentric 86 percent area reduction stenosis was characterized using absolute and relative Doppler spectral broadening measurements, relative Doppler velocity fluctuation, and Doppler backscattered power. Bidimensional mappings of each Doppler index were obtained using a 10 MHz pulsed-wave Doppler system. Calf red cells suspended in a saline solution were used to scatter ultrasound and were circulated in an in vitro steady flow loop model. Results showed that the absolute spectral broadening was not a good index of turbulence because it was strongly affected by the deceleration of the jet and by the shear layer between the jet and the recirculation zones. Relative Doppler spectral broadening (absolute broadening divided by the frequency shift), velocity fluctuation, and Doppler power indices provided consistent mapping of the centerline axial variation of turbulence evaluated by hot-film anemometry. The best agreement between the hot-film and Doppler ultrasound methods was however obtained with the Doppler back-scattered power. The most consistent bidimensional mapping of the flow characteristics downstream of the stenosis was also observed with the Doppler power index. The relative broadening and the velocity fluctuation produced artifacts in the shear layer and in the recirculation zones. Power Doppler imaging is a new emerging technique that may provide reliable in vivo characterization of blood flow turbulence.

DIFFERENCES IN THE ERYTHROCYTE AGGREGATION LEVEL BETWEEN VEINS AND ARTERIES OF NORMOLIPIDEMIC AND HYPERLIPIDEMIC INDIVIDUALS

The objectives of this study were to detect differences in the Doppler power backscattered by blood in vivo, and to identify factors affecting the backscattered power. The main hypothesis was that variations in the erythrocyte aggregation level between veins and arteries of normolipidemic and hyperlipidemic individuals can be detected with power Doppler ultrasound. Doppler measurements were performed at 5 MHz, with an Acuson 128 XP/10 system, over the carotid artery and jugular vein, external iliac artery and vein, common femoral artery and vein and popliteal artery and vein. Doppler signals were recorded at the center of each vessel to optimize the detection of erythrocyte aggregation, and processed off-line to obtain the backscattered power. The power of each recording was compensated for Doppler gain differences, tissue attenuation with depth and transmitted power variations occurring with pulse-repetition interval modifications. Results showed statistically stronger backscattered power in veins compared to arteries for the iliac, femoral and popliteal sites. In comparison with healthy subjects, stronger powers were observed in hyperlipidemic patients for the femoral and popliteal sites. Power differences were also found between peripheral measurements. On the other hand, no difference was observed between the power measured in the carotid artery and jugular vein for both groups of individuals. Multiple linear regression analyses were performed to identify factors affecting the backscattered power. Results showed a correlation (r) of 71.2% between the Doppler power in the femoral vein and the linear combination of two parameters: an erythrocyte aggregation index S10 measured with a laser scattering method, and the diameter of the vessel measured on B-mode images. Statistically significant linear correlation levels were also found between S10 and the Doppler power in various vessels. In conclusion, this study showed that power Doppler differences exist in vivo in large vessels between veins and arteries of normolipidemic and hyperlipidemic individuals. The Doppler power variations were also shown to be related to erythrocyte aggregation.

Effects of a sudden flow reduction on red blood cell rouleau formation and orientation using RF backscattered power

In most studies that were aimed at evaluating the kinetics of red blood cell (RBC) aggregation, human blood was initially circulated at a high shear rate to disrupt the aggregates, and measurements were performed following a complete flow stoppage, during the process of rouleau formation. However, it is known that a very low shear rate can enhance the formation of aggregates, as demonstrated by the modal relationship of the shear-rate dependence of RBC aggregation. The objective of the present study was, thus, to evaluate the influence of sudden flow reductions compared to a complete flow stoppage on the kinetics of rouleau formation, using ultrasound backscattering. Horse blood models, characterized by different aggregation levels, were obtained by diluting the plasma with a saline solution in different proportions. Blood was circulated in a 12.7-mm vertical tube at a flow rate of 1250 mL min-1 (prereduction flow rate) to disrupt the aggregates. The ultrasound radiofrequency (RF) signal was recorded from the center of the tube following different levels of sudden flow reduction or complete stoppage (postreduction flow rate). All measurements were performed over 2 min, using a 10-MHz transducer. No power increase was observed after complete flow stoppage. For postreduction flow rates varying between 20 and 160 mL min-1, the backscattered power increased proportionally with the kinetics of RBC aggregation. The echo buildup was also faster and stronger when the postreduction flow rate was increased. An unexpected pattern of variation of the backscattered power was found for horse RBCs characterized by high kinetics of rouleau formation. The power increased rapidly to a plateau, followed by another rapid increase and another plateau. Rouleau formation, random disorientation and reorientation were postulated to explain the phasic power increases observed.

Review of the assessment of single level and multilevel arterial occlusive disease in lower limbs by duplex ultrasound.

The purpose of this article is to review the performance of duplex ultrasound scanning in assessing lower limb arterial disease with emphasis on patients with multisegmental occlusive lesions. Several studies have reported that duplex scanning can be as accurate as angiography to localize arterial stenoses. In spite of these promising results, there still remain some difficulties and controversies. Among them, it has been reported that multisegmental disease may affect the accuracy of duplex scanning. Indeed, some studies have indicated a lower sensitivity for detecting significant stenoses distal to severe or total occlusions. It also was demonstrated that second-order stenoses were detected with lower sensitivity compared to first-order stenoses. The main reason proposed to explain this lower sensitivity is that the highly reduced flow distal to occluded or highly stenotic segments increases the difficulty of detecting significant Doppler velocity changes in the distal or secondary stenoses. The intrinsic limitations of the peak systolic velocity ratio used as a classification criterion are presented. Finally, new and promising developments in power Doppler imaging and ultrasound contrast agents are discussed, because they may allow expansion of the capabilities of current ultrasound scanning systems and provide more accurate diagnosis of patients with multiple disease.

Computer analysis and pattern recognition of doppler blood flow spectra for disease classification in the lower limb arteries

In the present study, a computer processing method was developed to objectively classify disease in the lower limb arteries evaluated by noninvasive ultrasonic duplex scanning. This method analyzes Doppler blood flow signals, extracts diagnostic features from Doppler spectrograms and classifies the severity of the disease into three categories of diameter reduction (0-19%, 20-49% and 50-99%). The features investigated were based on frequency features obtained at peak systole, spectral broadening indices and normalized amplitudes of the power spectrogram computed in various positive and negative frequency bands. A total of 379 arterial segments studied from the aorta to the popliteal artery were classified using a pattern recognition method based on the Bayes model. Two classification schemes using a two-node decision rule were tested. Both schemes gave similar results, the first one provided an overall accuracy of 83% (Kappa = 0.42) and the second an overall accuracy of 81% (Kappa = 0.35) when compared with conventional biplane contrast arteriography. These performances, especially for the 0 to 19% lesion category, are better than the one obtained by the technologist (accuracy = 76% and Kappa = 0.33), based on visual interpretation of the Doppler spectrograms. To recognize hemodynamically significant stenoses (50-99% lesions), the pattern recognition system has a sensitivity and a specificity of 50% and 99%, respectively, using classification scheme I. With classification scheme II, the sensitivity and the specificity are 50% and 98%, respectively. Visual interpretation of the Doppler spectrograms leads to a sensitivity and a specificity of 50% and 98%, respectively. These results are the first to be obtained by a pattern recognition system in classifying lower limb arterial stenoses.

In vitro evaluation of multiple arterial stenoses using three-dimensional power Doppler angiography.

PURPOSE The study was done to improve quantification of multiple arterial stenoses and to investigate a new imaging technique for lower limb arteries. Three-dimensional power Doppler angiography was used to quantify in vitro arterial stenoses. METHODS We built two types of artery phantoms containing multiple stenoses. One used stenotic porcine arteries, and the other was designed to control the proximal and distal stenoses while we assessed central stenosis of a wall-less agar lumen. Three-dimensional power Doppler angiograms of the flow lumens were generated at different flow rates under steady and pulsatile flow conditions with a PowerPC 8500 computer-based three-dimensional ultrasound imaging system. This experimental system works off-line, performs three-dimensional acquisition, reconstruction, and display of ultrasound images. Images of flow lumens were compared with the measured B-mode images or the true geometry. RESULTS This technique produces good three-dimensional angiographic images of the flow lumen, and multiple stenoses do not affect the diagnosis of arterial stenoses. With this technique, the average errors for estimating 80% and 50% area reduction stenoses were -10% and 4%, respectively. CONCLUSIONS Three-dimensional power Doppler angiography has the potential to quantitatively grade multisegmental stenoses in lower limbs and generate a map for vasculature surgery planning.

DOPPLER VELOCITY RATIO MEASUREMENTS EVALUATED IN A PHANTOM MODEL OF MULTIPLE ARTERIAL DISEASE

The objective of this study was to evaluate in vitro the accuracy of the Doppler velocity ratio (VR) (intrastenotic velocity/prestenotic or poststenotic velocity) under different geometric conditions simulating the presence of multiple stenoses. A steady flow loop model was used to test the influence of the presence of a concentric obstruction of 84% area reduction positioned at a distance of 10, 20 and 30 tube diameters, either proximal or distal to the stenosis under study. The stenosis under evaluation was either concentric or eccentric and had a percentage of area reduction ranging from 20% to 91%. An ultrasound color Doppler system was used to perform both pulsed-wave (PW) Doppler and color-flow velocity measurements. VRs were computed by dividing the maximum velocity of the jet by the velocity at 6 and 10 diameters both proximal and distal to the stenosis under study. A strong correlation was obtained between VR computed using color flow and PW Doppler velocities (r = 0.99). Results indicated that using the prestenotic velocity as a reference velocity generally provided a more sensitive VR index to grade arterial stenosis than using the poststenotic velocity. From a curve fit model, the measured percentages of stenosis were calculated from the VR data and compared to the true percentages. The correlation coefficient, r, was 0.95. When the proximal and distal stenoses were at 10 diameters of the stenosis investigated, r was 0.91, while it increased to 0.98 when the distance was 20 diameters or more. Although VR is theoretically not influenced by hemodynamic factors, we demonstrated that, in practice, the presence of multiple stenoses reduced its sensitivity. Volumetric flow measurements are suggested to obviate this limitation.