Accuracy and reproducibility of duplex ultrasound imaging in a phantom model of femoral artery stenosis

Tuning Viscoelasticity with Minor Changes in Speed of Sound in an Ultrasound Phantom Material

The acoustic properties of ultrasound phantom materials have always been important, but with new applications interrogating tissue mechanical properties, viscoelasticity has also become an interesting feature to consider. Along with Young's modulus, the viscous component of tissue is affected by certain diseases and can therefore be used as a biomarker. Furthermore, viscoelasticity varies between tissue types and individuals, and therefore it would be useful with a phantom material that reflects this physiological range. Here we describe a gel for ultrasound imaging with a range of mechanical properties given by mixing different ratios of two oil-based gels, clear ballistic and styrene-ethylene/butylene-styrene (SEBS). The gels were mixed in five different proportions, ranging from 0-100% of either gel. For each of the gel compositions, we measured time of flight to determine speed of sound, narrowband ultrasound transmission for attenuation, stress-relaxation for viscoelasticity, mass and volume. Analysis of the stress-relaxation data using the generalized Maxwell model suggests that the material can be described by five parameters, E₀, E₁, E₂, η₁ and η₂, and that each of these parameters decreases as more SEBS is incorporated into the mixed material. Instantaneous Young's modulus (the sum of E₀, E₁ and E₂ in our model) ranges between 49 and 117 kPa for the different ratios, similar to values reported for cancerous tissue. Despite the large span of obtainable mechanical properties, speed of sound is relatively constant regardless of composition, with mean value estimates (± 95 % CI) between 1438 ± 9 and 1455 ± 3 m/s for pure and mixed gels. This was attributed to a variation in density and Poisson's ratio, following from the relation linking them to speed of sound and elasticity. Furthermore, both speed of sound and attenuation were within a suitable range for ultrasound phantoms. Combining this ballistic gel with SEBS copolymer in oil allows for control of mechanical properties, both elastic and viscous as evaluated by the material model. Furthermore, it does so without compromising ease of use, longevity and safety of the pre-made gel.

Variation in duplex peak systolic velocity measurement in a multi-site vascular service

Objective:

Duplex US (DUS) is increasingly utilised as a first-line investigation for the assessment of carotid disease. For clinical decision-making, DUS assessment must be accurate and reproducible to ensure reliability. We aimed to investigate the variability in peak systolic velocity (PSV) measurement in a multi-site vascular network.

Methods:

DUS measurements of PSV were taken from continuous and pulsatile flow, generated by a high fidelity phantom, by 12 experienced vascular scientists across four hospitals. Participants were blinded to the actual PSV value (50 cm/s).

Results:

We observed an average error of 13.2% (± 8.3) and 11.6% (± 7.5) in PSV measurements taken from pulsatile and continuous waveforms, respectively. Measurements of PSV using the pulsatile waveform demonstrated statistically significant variation across all hospitals; ((hospital/mean) A 43.9 cm/s, B 61.7 cm/s, C 57.4 cm/s, D 47.7 cm/s, p=0.001). Further analysis demonstrated statistically significant variation in 4 instrumentation-related factors when measuring from a pulsatile waveform (Doppler angle, angle of insonation, velocity range, scale range).

Conclusion:

We observed a significant level of error and variation in PSV measurements across four sites within our vascular network. Variation in instrumentation-related factors may be accountable for this. In light of the centralisation of vascular services, it is increasingly important to unify and implement scanning protocols in order to reduce error and inter-site variability.

Doppler ultrasound compatible plastic material for use in rigid flow models

A technique for the rapid but accurate fabrication of multiple flow phantoms with variations in vascular geometry would be desirable in the investigation of carotid atherosclerosis. This study demonstrates the feasibility and efficacy of implementing numerically controlled direct-machining of vascular geometries into Doppler ultrasound (DUS)-compatible plastic for the easy fabrication of DUS flow phantoms. Candidate plastics were tested for longitudinal speed of sound (SoS) and acoustic attenuation at the diagnostic frequency of 5 MHz. Teflon was found to have the most appropriate SoS (1376 +/- 40 m s(-1) compared with 1540 m s(-1) in soft tissue) and thus was selected to construct a carotid bifurcation flow model with moderate eccentric stenosis. The vessel geometry was machined directly into Teflon using a numerically controlled milling technique. Geometric accuracy of the phantom lumen was verified using nondestructive micro-computed tomography. Although Teflon displayed a higher attenuation coefficient than other tested materials, Doppler data acquired in the Teflon flow model indicated that sufficient signal power was delivered throughout the depth of the vessel and provided comparable velocity profiles to that obtained in the tissue-mimicking phantom. Our results indicate that Teflon provides the best combination of machinability and DUS compatibility, making it an appropriate choice for the fabrication of rigid DUS flow models using a direct-machining method.

Acupuncture Improves Peripheral Perfusion in Patients with Reflex Sympathetic Dystrophy

Patients with stage II reflex sympathetic dystrophy syndrome (algodystrophy) often describe a sensation of beneficial warmth in the affected limb when receiving acupuncture treatment. In a case control study of 10 patients with unilateral algodystrophy stage II, and 10 healthy sex and age-matched controls, we investigated whether acupuncture has an effect in blood volume flow and correlated this with a clinical assessment. Blood volume flow was measured by duplex sonography before, during, and after acupuncture. During acupuncture, blood volume flow increased significantly (+31.1%, p (one-tailed) = 0.024) in the patients' effected limbs (by algodystrophy) compared with the patients' untreated limbs (contralateral). It was also elevated compared with the controls' treated limbs (+23.9%, p = 0.046). The corresponding data after acupuncture were +29.5%, p = 0.057 and +38.3%p = 0.014. All but one patient reported improved symptoms. However, only subjective improvement in function, not pain, was positively correlated to the increase in blood volume flow. In patients with algodystrophy stage II, a significant increase of blood volume flow was attributed to acupuncture and was correlated to functional improvement.

Accuracy of intravascular ultrasound for diameter measurement of phantom arteries

BACKGROUND

Uniplanar quantitative angiography (QA) is the standard method for measuring vessel diameter during surgical and endovascular procedures. Intravascular ultrasound (IVUS), a relatively new technology, is another means of obtaining this measurement. This study was designed to validate the accuracy of these two modalities by comparing each to direct caliper measurement, the gold standard, using phantom femoral artery segments (PAS).

MATERIALS AND METHODS

PAS diameter was measured with a 12.5-MHz mechanically rotating IVUS catheter (Boston Scientific Corp.) and QA (OEC Corp.) was compared to the direct caliper measurement (Mitutoyo Corp.) at 60 different locations within PAS. At each location minimal lumen diameter and perpendicular lumen diameter were measured and their mean was calculated. The intraclass correlation coefficients (ICCC) between direct caliper measurement and IVUS and uniplanar and biplanar angiography were calculated. Fisher's Z transformation was used to compare the correlation coefficients.

RESULTS

The ICCC for IVUS was 0.89. The ICCCs for uniplanar and biplanar angiography were 0.73 and 0.82, respectively. IVUS correlated more closely with direct caliper measurement than uniplanar and biplanar angiography (P = 0.00008, 0.02) Biplanar angiography correlated more closely with direct caliper measurement than uniplanar angiography (P = 0.04).

CONCLUSIONS

IVUS more accurately measures lumen diameter than uniplanar or biplanar angiography. Diameter measurement with biplanar angiography is more accurate than uniplanar angiography.

Assessment of collateral perfusion: a pharmacodynamic study with buflomedil hydrochloride

The aim of the study was to assess the influence of Buflomedil hydrochloride on collateral function. Ten patients with isolated superficial femoral occlusions were investigated twice by duplex sonography with measurement sites at the common femoral artery (CF) and the popliteal artery (PA). After the second scan 200 mg of Buflomedil hydrochloride were infused; the infusion was followed by a third duplex examination. Endpoints assessed included the arterial diameter (D(CF), D(PA)), the systolic peak velocity (Vmax), the mean velocity of the maximum envelope (Vmean m.e.), the intensity weighted time average mean velocity (Vmean i.w.), the maximum reverse flow velocity (Vrev), the end-diastolic velocity (Venddiast), the calculated volume flow (Q), the pulsatility and the resistance indices (PI, RI), and PI and RI based segmental damping factors (DF(PI), DF(RI)). For the CF measurement site the infusion of Buflomedil hydrochloride resulted in a significant reduction in Vrev and PI (p<0.05), whereas trends in the opposite direction (increase) were observed for both measures of Vmean and for Q (0.1<p<0.05). No significant changes were reported for the PA measurement site. DF(PI) numerically decreased (0.1<p<0.05). The pattern of changes suggest that Buflomedil hydrochloride induces collateral vasodilation and thus improves collateral function.

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.

Effect of US contrast agents on spectral velocities: in vitro evaluation

PURPOSE

To evaluate the effect of ultrasonographic (US) contrast agents on measurements of peak velocity with spectral Doppler US in stenotic and nonstenotic flow states.

MATERIALS AND METHODS

Nonpulsatile flow was established in a flow phantom with 0%, 50%, 75%, and 90% stenoses. SH U 508A, perflenapent emulsion, and perfluorohexane emulsion were the contrast agents evaluated. Before and after administration of each contrast agent, two peak velocity measurements obtained proximal to, at the site of, and distal to the stenosis in each vessel model were averaged. The percentage difference in peak velocity after contrast agent administration was calculated for each site interrogated. The mean, SD, and coefficient of variation of the percentage difference in peak velocity were calculated.

RESULTS

Percentage differences in peak velocity after contrast agent administration at different sample volume sites were not significantly different irrespective of the degree of stenosis or the contrast agent evaluated.

CONCLUSION

The contrast agents evaluated do not produce a statistically significant increase in peak velocity. If this result is corroborated in clinical practice, contrast agents can be used without reevaluating existing Doppler US thresholds for stenosis.

Can lower limb ultrasonography replace arteriography?

Although duplex ultrasonography offers a number of theoretical and practical advantages over arteriography for the assessment of lower limb arterial disease, it has not yet been widely accepted into clinical practice. This article reviews the current status of lower limb arterial duplex and concludes that arteriography should no longer be regarded as the diagnostic 'gold-standard' and that duplex ultrasonography should now be the first line investigation in patients with lower limb arterial disease.

Interobserver, interequipment and intersubject variability of echo-Doppler examination of the common carotid and vertebral arteries

OBJECTIVE

We evaluated interobserver and interequipment variability in common carotid and vertebral flow measurements, using seven female members of nursing staff as subjects, four different ultrasound machines and three different radiologists as operators.

METHODS

The flow of the common carotid and vertebral arteries in each subject was examined at 0, 10, 20 and 30 min after lying down in a supine position. Peak velocity, end-diastolic velocity, mean velocity, systolic acceleration, cross-sectional area of the vessel, flow volume/min, resistance index and Doppler angle were measured.

RESULTS

A large proportion of the variance of peak velocity, cross-sectional area of the vessel and flow volume was patient-related. Significantly different values of end-diastolic velocity, mean velocity, resistance index, Doppler angle and systolic acceleration were obtained with different equipment. Significantly different values of systolic acceleration, vertebral peak velocity, common carotid artery flow and Doppler angle were obtained by different operators. However, part of the variability may be physiological due to the study protocol.

CONCLUSION

The use of the same operator and equipment is recommended in follow-up examinations.

A comparison between colour duplex ultrasonography and arteriography for imaging infrapopliteal arterial lesions

OBJECTIVE

To investigate the agreement between colour duplex ultrasonography and digital subtraction arteriography of the infrapopliteal arteries.

DESIGN

Retrospective, blinded study.

SETTING

Vascular laboratory and Radiology Department, University Hospital.

METHODS

The infrapopliteal vasculature was examined in a total of 51 limbs by both colour duplex ultrasound and digital subtraction angiography. By examining all arteries from the distal popliteal to the pedal arteries, a total of 204 individual arterial segments were available for analysis. Each segment was graded as 0-49%, 50-99% diameter reduced or occluded by both modalities. Using ultrasound, classification of stenoses was achieved by observing peak systolic velocity ratios; a doubling of peak systolic velocity indicating a > or = 50% diameter reducing stenosis. Where no Doppler signal could be obtained, the vessel was assumed to be occluded. From angiographic studies, two radiologists separately and blindly assessed the extent of disease for each infrapopliteal artery noting areas of > or = 50% diameter reduction and occlusion. The Kappa statistic was used to examine the level of agreement between angiography and ultrasound as well as between both radiologists.

RESULTS

The Kappa level (95% confidence interval) of agreement between ultrasound and angiographic assessments for distinguishing patent from occluded segments was 0.61 (0.49-0.74) for all segments. The equivalent agreement between radiologists was 0.80 (0.70-0.89). Poorest agreement was observed from ultrasound assessments of the peroneal and tibioperoneal trunk arterial segments.

CONCLUSION

Since agreement between colour duplex scanning and angiography never fell significantly below levels achieved between two radiologists, we conclude that colour duplex ultrasound can be used to assess infrapopliteal artery patency.

Is intermittent claudication improved by percutaneous transluminal angioplasty? A randomized controlled trial

PURPOSE

Percutaneous transluminal angioplasty (PTA) is an increasingly popular invasive treatment for peripheral arterial disease, but there have been very few controlled trials to justify its use. This randomized controlled clinical trial was performed to determine in patients with mild and moderate intermittent claudication differences in outcome between PTA and conventional medical treatment after 2 years.

METHODS

Six hundred patients with claudication were screened at the Peripheral Vascular Clinic, Royal Infirmary of Edinburgh. Sixty-two patients with short femoral artery stenoses or occlusions (47 patients) and iliac stenoses (15 patients) were randomized to either PTA plus medical treatment (PTA group, 30 patients) or to medical treatment alone (control group, 32 patients). Medical treatment consisted of daily low-dose aspirin and advice on smoking and exercise. Outcome measures studied were patient-reported maximum walking distance, exercise treadmill distance until onset of claudication, treadmill maximum walking distance, ankle-brachial pressure index (ABPI), quality of life (Nottingham Health Profile), and duplex ultrasound-measured extent of occlusive disease.

RESULTS

At 2 years of follow-up, the PTA group and control subjects did not differ significantly in patient-reported maximum walking, treadmill onset to claudication, treadmill maximum walking distances, or ABPI (p > 0.05). However, the PTA group had significantly fewer occluded arteries (p = 0.003) and a lesser degree of stenosis (expressed in terms of the velocity ratio; p = 0.004) in patent arteries. Quality of life was not demonstrably different between the two groups (p > 0.05).

CONCLUSIONS

Two years after PTA, patients had less extensive disease than medically treated patients, but this did not translate into a significant advantage in terms of improved walking or quality of life. There are important implications for patient management and future clinical research.

Colour flow duplex imaging of occlusive arterial disease of the lower limb

BACKGROUND

The development of duplex ultrasonography and colour flow imaging has greatly extended the scope of non-invasive assessment of lower limb arterial disease. This review questions whether recent advances might allow colour duplex imaging to displace arteriography as the primary imaging modality for native vessel occlusive disease.

METHODS

A literature review was carried out based predominantly on a Medline database search of English language publications from 1985 to 1996.

RESULTS

Increasing evidence indicates that colour duplex ultrasonography can accurately image the lower limb native arterial tree, and that colour duplex imaging can replace diagnostic arteriography in a large proportion of patients.

CONCLUSION

Arteriography should no longer be considered the gold standard of imaging of peripheral arterial occlusive disease. Future studies should concentrate on the efficacy of colour duplex sonography in guiding clinical decision making.

Randomised controlled trial of percutaneous transluminal angioplasty for intermittent claudication

OBJECTIVES

To determine differences between PTA and conventional medical treatment in treadmill distance until onset of claudication, treadmill maximum walking distance, patient reported maximum walking distance, ankle brachial pressure index (ABPI), quality of life (Nottingham Health Profile, NHP) and Duplex measured extent of occlusive disease.

DESIGN

Randomised controlled clinical trial.

METHODS

Six hundred claudicants were screened. Fifty-one men and 11 women with intermittent claudication due to short femoral stenoses or occlusions (n = 47) and iliac stenoses (n = 15) were randomised to either PTA plus medical treatment (PTA group, n = 30) or to medical treatment alone (control group, n = 32). Medical treatment consisted of daily low dose aspirin and advice on smoking and exercise.

RESULTS

At 6 month follow up: In the PTA group more patients reported no claudication (p < or = 0.05) and were asymptomatic on the treadmill (p < or = 0.01) compared to the control group. The ABPI was significantly higher in the PTA group. More of the PTA group reported lower NHP pain scores (p < or = 0.05). In the control group there were more occluded arteries (p < or = 0.001), and the stenosis velocity ratio of patient arteries was significantly higher (p < or = 0.001).

CONCLUSIONS

Only 10% of claudicants had discrete lesions suitable for PTA. Treatment of these patients with PTA produces a greater short-term improvement in walking and quality of life than medical treatment alone and is associated with less progression of disease.

Reproducibility of Doppler ultrasound measurements

PURPOSES

The reproducibility of Doppler ultrasound measurements was studied in a group of radiologists with different experience of such measurements.

MATERIAL AND METHODS

The 12 participating radiologists measured peak systolic velocity (PSV), resistance index (RI), and pulsatility index (PI) from 3 arteries and PSV alone in one artery of a healthy person under standardized circumstances. Each participant repeated the measurements 10 times.

RESULTS

Significant (p < 0.005) interobserver variation was seen in all of the variables, with an up to 14-fold difference in the PSV values measured and most variation resulting from differences between observers. The reproducibility of RI was better, most variation being within observers. Experience improved the reproducibility of the Doppler US measurements. Significant difference (p < 0.05) in Doppler US measurements was seen between the 3 experience groups. The difference was marked between the highly experienced and the medium experienced group when the variances of the measured values of PSV, RI, and PI from all 4 arteries were combined.

CONCLUSION

The reproducibility of Doppler ultrasound measurements is poor. The value of the method is questionable.

Renal vascular damage in systemic sclerosis patients without clinical evidence of nephropathy

OBJECTIVE

To evaluate the use of color-flow Doppler ultrasonography, a direct, noninvasive technique, for measurement of kidney blood flow in patients with systemic sclerosis (SSc).

METHODS

Twenty-five normal volunteers and 25 SSc patients (median disease duration 8 years, range 2-21 years) were studied. All were free of clinical symptoms of renal damage. The resistance index (RI) was determined on main, interlobar, and cortical vessels.

RESULTS

In SSc patients, the RI was significantly increased at every sampling site examined (P < 0.001). RI values were strongly correlated with disease duration (main artery r = 0.56, P < 0.04; interlobar artery r = 0.63, P < 0.02; cortical artery r = 0.75, P < 0.002). Regression analysis showed no relationship between RI and creatinine clearance values.

CONCLUSION

Color-flow Doppler ultrasonography is a sensitive and noninvasive technique for evaluating vascular damage of the kidney in patients with SSc.

Detection of sub-critical arterial stenoses by hyperaemic Doppler

OBJECTIVES

This study assessed the potential of hyperaemic Doppler to detect sub-critical stenoses using a flowrig model.

METHODS

Pulsatile flow of a blood substitute was produced in a compliant circuit. A cadaver carotid artery, constricted by a silk suture produced a variable, focal stenosis. Forty-seven stenoses were created in five arteries. Pressure gradients and Doppler measurements were recorded simultaneously across each stenosis at low (200 ml/min) and high (400 ml/min) flow rates. The change in peak velocities between the arterial segment 2cm proximal to the stenosis (V1), and the stenotic jet (V2) were used to calculate three Doppler indices: (i) V2/V1 ratio, (ii) V2-V1 difference, (iii) a modified 'Bernoulli' value. A high flow pressure gradient of > or = 15% of the resting distal pressure (% delta P), represented a significant stenosis.

RESULTS

There was improved correlation between Doppler indices and % delta P at high flow (r = 0.87 to 0.88) compared to low flow rates (r = 0.81 to 0.84). Optimum V2/V1 cut off values were determined by received operator characteristics (ROC) curve analysis. At low flow five sub-critical stenoses were not detected (sensitivity 82.8%) yet all but one of these lesions were identified at high flow (sensitivity of 96.6%). The V2-V1 and Bernoulli indices did not improve on the discriminant ability of the V2/V1 ratio.

CONCLUSIONS

The V2/V1 ratio is sensitive to haemodynamic changes at enhanced flow rates across ideal arterial stenoses. The potential of hyperaemic Doppler to detect sub-critical lesions and so avoid intraarterial pressure measurements deserves further in vivo study.

Interventionalists' guide to the patient's experience of lower extremity arterial occlusive disease

This article is intended as a guide for all who propose to intervene in patients with symptomatic lower extremity arterial occlusive disease. It reviews the natural history of and therapies for intermittent claudication. The results of a survey of claudicants are summarized; this survey gathered data on claudicants' general health, comorbid conditions, symptoms of claudication, and functional abilities. Respondents predicted a 20% improvement in their ability to perform instrumental activities of daily living (eg, walking several blocks, climbing one flight of stairs, performing household tasks) if lower extremity symptoms were relieved. This level of improvement is consistent with that reported in the literature following revascularization. The conclusion is that patients with claudication have modest goals with respect to improvement following vascular intervention. More data should be collected on the functional outcomes of vascular intervention so that patients can make better informed choices regarding treatment of symptomatic lower extremity arterial occlusive disease.

Hemodynamics of stenotic infrainguinal vein grafts: theoretic considerations

We developed a theoretic model of arterial stenosis to study the relationship between perfusion pressure and regional hemodynamics in stenotic infrainguinal vein grafts in an attempt to identify grafts at high risk for failure. Our model was based on the concept of energy and mass conservation of the flowing blood. We used the modified Bernoulli equation (delta P = 4 delta V2) to calculate the maximum possible intrastenotic peak systolic velocity (PSV) from the systolic blood pressure. PSV was measured by means of duplex ultrasonography in infrainguinal bypasses up to the time of revision (nine grafts) or spontaneous thrombosis (two grafts). We related arm systolic blood pressure, intrastenotic PSV, and prestenotic PSV obtained from duplex examinations conducted prior to graft thrombosis or revision and applied our model to these stenotic vein grafts. Intrastenotic PSV was consistently lower than maximum PSV predicted from the Bernoulli equation. The highest measured intrastenotic PSV of 600 cm/sec would require a minimum perfusion pressure of 144 mm Hg. The lowest measured PSV (20 cm/sec) was considered the minimum "thrombotic threshold velocity." This model predicts that for parabolic profile flow in an 80% diameter-reducing axisymmetric stenosis (96% cross-sectional area reduction), a prestenotic PSV of 20 cm/sec would produce an intrastenotic PSV of 500 cm/sec requiring the equivalent potential energy of 100 mm Hg systolic blood pressure. Our theory implies that in patients with nocturnal hypotension thrombosis of stenotic vein grafts may occur.

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.