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Published online before print August 2, 2002, 10.1148/radiol.2243011374
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(Radiology 2002;225:295-299.)
© RSNA, 2002

Technical Developments

B-Flow Evaluation of Carotid Arterial Stenosis: Initial Experience1

Robert A. Bucek, MD, Markus Reiter, MD, Irene Koppensteiner, Ramazanali Ahmadi, MD, Erich Minar, MD and Johannes Lammer, MD

1 From the University Clinic for Radiology, Department of Angiography and Interventional Radiology (R.A.B., J.L.), and the University Clinic for Internal Medicine II, Department of Angiology (M.R., I.K., R.A., E.M.), Vienna General Hospital, Währinger Gürtel 18-20, A-1090 Vienna, Austria. Received August 16, 2001; revision requested September 13; final revision received March 20, 2002; accepted March 26. Address correspondence to R.A.B. (e-mail: robert.bucek@akh-wien.ac.at).


   ABSTRACT
TOP
 ABSTRACT
INTRODUCTION
 MATERIALS AND METHODS
RESULTS
 DISCUSSION
 REFERENCES
 
In this prospective pilot trial, B-flow ultrasonographic (US) imaging was compared with color duplex flow US in the evaluation of internal carotid arterial stenosis. Despite almost excellent interobserver variability, none of the investigated B-flow imaging parameters correlated with those of duplex US. In conclusion, the investigated B-flow imaging parameters cannot be used in evaluating internal carotid arterial stenosis.

© RSNA, 2002

Index terms: Arteriosclerosis, 172.7211 • Carotid arteries, angiography, 172.1248, 908.122 • Carotid arteries, stenosis or obstruction, 172.721, 908.721 • Carotid arteries, US, 172.12983, 172.12984, 172.12989 • Ultrasound (US), Doppler studies, 172.12983, 172.12984, 172.12989


   INTRODUCTION
TOP
 ABSTRACT
 INTRODUCTION
MATERIALS AND METHODS
RESULTS
 DISCUSSION
 REFERENCES
 
Cerebral infarction caused by carotid arterial atherosclerosis is related to the degree of stenosis. Toole and Castaldo (1) have reported that a more hemodynamically disturbed flow has a greater propensity for distal cerebrovascular events. Different studies (2,3) have demonstrated better outcomes in patients with symptomatic moderate (50%–69%) and high-grade (70%–99%) internal carotid arterial (ICA) stenoses (ICAS) after carotid endarterectomy, as compared with those treated medically. Even in patients with asymptomatic ICAS of 60%–99%, an absolute risk reduction of 5.9% for stroke has been reported at 5 years (4). These studies gave rise to discussions and kindled the interest in and necessity of accurate measurement of ICAS (5). Selective intraarterial angiography of the carotid artery is still seen as the reference standard but is an invasive method with a morbidity rate of 1%–4% that bears a 1% risk of periinterventional stroke (4). Color duplex flow ultrasonography (US) has become the most widely used noninvasive method of assessing extracranial cerebrovascular occlusive disease (6–8) because it avoids the expense and risk of routine arteriography (9,10). Stenotic lesions are identified and quantified by analyzing Doppler US velocity spectra in combination with real-time B-mode and color-flow images (11).

Despite its broad use, several disadvantages of color duplex flow US have been described. Different studies (6–8,11–17) have shown considerable variation in estimating the degree of stenosis, and even with use of similar equipment, rigid velocity criteria do not have the same validity and predictive values for grading ICAS in different laboratories. It is well recognized that duplex US results are highly dependent on the experience of the operator, which emphasizes the importance of individual evaluation and quality control for each institution (8,12). There are also different technical limitations of US depiction of blood flow. Color duplex flow US is very sensitive to flow signals and can yield quantitative velocity and/or power information, but the price is decreased spatial resolution and frame rate, as well as high angle dependency. Because the color duplex flow US image is presented as an overlay to the B-flow image, any large tissue motion may register as a color flash artifact that can overshadow the true flow data. Conversely, maximizing the color fill-in of vessels will almost always result in some overwriting of the vessel walls on the B-flow image, which can mask any subtle lesion in the vessel under study (18).

B-flow imaging is a recently introduced flow technology that extends B-mode imaging capabilities to blood flow, including high frame rate and high-spatial-, high-temporal-, and high-contrast-resolution imaging (18,19). It directly depicts blood echoes in a gray-scale presentation, while simultaneously depicting surrounding anatomy, but without the need for overlays. This explains the unobstructed view of the vessel lumen. These attributes of B-flow imaging promise this technique to be an important additional tool in the evaluation of ICAS. Our experience with B-flow imaging has shown that in the poststenotic area, vessel stenoses produce a region of higher gray-scale intensity that we call the jet stream. The rationale for this jet stream seems to be that pixel brightness at B-flow imaging is determined by blood-echo strength and velocity, and both factors are influenced by the grade of vessel stenosis (19). We performed this prospective pilot trial to assess the interobserver variability of different jet stream parameters and their role in the evaluation of ICAS.


   MATERIALS AND METHODS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
RESULTS
 DISCUSSION
 REFERENCES
 
Technical Background of B-flow Imaging
Basics.—B-flow images are generated by using digitally encoded US technology consisting of a transmit encoder and a receive decoder in a digital beam former that provides electronic array focusing (Fig 1). A small number of digitally encoded wideband pulses are transmitted into the body for each scan line. Unlike color imaging techniques, in which the typical packet size is 10–12, B-flow imaging uses a packet size as small as 2–4. Directly after receiving the reversed pulses, the decoder performs pulse-length compression ("coded excitation") on the acoustic data and then performs clutter suppression filtering. The rest of the processing is essentially the same as in conventional B-flow mode (18,19).



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  		Figure 1. Diagram illustrates the basic principle of digitally encoded US technology. EQ = equalization. (Reprinted, with permission, from reference 18.)

 
Coded excitation.—Coded excitation is a technique that increases the transmission energy by as much as one order of magnitude without compromising transverse resolution and is therefore especially suited for high-spatial- and high-temporal-resolution imaging of echo sources that are simply weak (such as red blood cells). Through the digital encoder the scanner transmits not one, but a sequence of N wideband pulses in accordance with a specific pattern referred to as a code; a decoder on the receiving side is used to effectively compress the returning echo back into a single pulse that has nearly the same resolution but N times more energy (Fig 2). If the received coded sequence and the sent code are exactly matched, the response is a pulse of amplitude N times greater than a single uncoded pulse. At in vivo scanning, the returning signal represents a sum of reflections from multiple sources in body tissue, so that the output of the sum should equal the sum of outputs from individual contributing reflectors (18,19).



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  		Figure 2. Diagram illustrates the key principle behind coded excitation. (Reprinted, with permission, from reference 18.)

 
Clutter suppression filtering.—For each of the coded transmissions (eg, two), a stream of acoustic backscatter data from the insonated anatomy is received and coded. These data are stored in a buffer in the equalization filter, which then subtracts a fraction of the second transmission from the first transmission. This process suppresses any large and slow-moving tissue clutter component relative to any moving blood-echo component (18,19).

Study Method
Between March 13 and June 6, 2001, 28 consecutive patients with US-verified ICAS of 30%–99% were included in this prospective pilot study. This trial was approved by the ethics committee of Vienna General Hospital; the additional B-flow evaluation did not require approval, according to our local guidelines. After patient informed consent was obtained, color duplex flow US of the common carotid artery (CCA), ICA, and external carotid arteries (ECA) of both sides was performed by an experienced vascular technician (I.K.) using a model 128 XP scanner with a 5-MHz linear probe (Acuson, Mountain View, Calif). The transducer was placed in the longitudinal plane parallel to the carotid artery, the flow imaging window was electronically angled 20° from the vertical, and the color scale was set at 0.31 m/sec maximal mean velocity. Velocity waveforms were obtained routinely from the CCA in the center stream, approximately 2–3 cm below the bifurcation and the ICA in the area of maximal stenosis in accordance with our local standard protocol, in which an insonation angle of 50°–60° is used. The highest peak systolic velocity (PSV) and the end-diastolic velocity (EDV) of blood flow in the CCA and ICA were recorded in meters per second. On the basis of these values, we evaluated the carotid ratio (PSVICA/PSVCCA) and the ratio of PSVICA/EDVCCA and EDVICA/EDVCCA for each patient. The percentage of ICAS was then calculated on the basis of the cutoff points for highest sensitivity and specificity stated in the publication by Nicolaides et al (15). ICAS greater than 50% were diagnosed in cases of a carotid ratio greater than 2.0, a PSVICA/EDVCCA ratio of 7.0–10.0, or an EDVICA/EDVCCA ratio less than 2.6; corresponding cutoff points for ICAS greater than 70% were greater than 4.0, greater than 15.0, and greater than 2.6, respectively. The quality conditions for US assessment were classified into three stages as follows: (a) good visualization of the CCA and ICA; (b) high bifurcation and/or extensive kinking, and therefore, the ICA visible on only a short track (<5 cm); and (c) extensive calcification.

After color duplex flow US, all patientsunderwent B-flow imaging evaluation of the affected body side by using a scanner with a 5–10-MHz linear probe (Logiq 700; GE Ultrasound Europe, Solingen, Germany) and the following properties: Time-gain compensation was fixed in a medium position for all patients, gain was adapted for optimized image quality (approximately 50%), and the dynamic range was 60 dB, with linear gray-scale calibration. Cine recording of at least one cardiac cycle was performed in each subject. In each patient, one image that showed the stenosis, approximately 1 cm of the prestenotic region, and the complete jet stream at peak systole (when the jet stream was at its maximum) was then stored digitally on the hard drive of the scanner and in tagged image file format on a magneto-optic disk. The jet stream was defined as the poststenotic area of higher gray-scale intensity produced by a vessel stenosis (Figs 3, 4). We analyzed the maximum gray-scale intensity in the prestenotic region, as well as in the jet stream, by using imaging software (Photoshop 6.0; Adobe Systems, San Jose, Calif) and calculated a gray-scale ratio (ICA/CCA). Further image analysis included the length (in centimeters) and area (in square centimeters) of the jet stream (performed directly on the scanner, which uses a calibrated system in the software), which were based on the personal estimate of the observer. All measurements were obtained by two independent readers (R.A.B., M.R.), who were both blinded to the results of color duplex flow US.



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  		Figure 3. B-flow US image (longitudinal plane) obtained in a patient with high-grade stenosis (*) of the left ICA. Note the long poststenotic jet stream with well-defined margins (surrounded by white line) and the difference in gray-scale intensity outside this area.

 


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  		Figure 4. B-flow US image (longitudinal plane) obtained in a patient with moderate stenosis of the right ICA (*). The sharp difference in gray-scale intensity directly after the stenosis is marked with solid white lines, and the ill-defined margins of the poststenotic jet stream are marked with a dotted white line. Also note the short calcified plaque (arrow) with dorsal shadowing on the far margin of the ICA.

 
Additional angiography was performed on the basis of the protocol of an ongoing carotid arterial study, which served as the selection pool for the patients. Eighteen patients (14 of 15 patients with ICAS >70% and four with ICAS of 50%–70%) also underwent intraarterial digital subtraction angiography, which was performed by an experienced angiographer (R.A.). After femoral arterial puncture and selective catheter placement in the carotid arteries, 15 mL of contrast medium (Omnipaque 300; Schering, Vienna, Austria) was infused and views obtained in at least two planes. The diameter of maximum stenosis in the ICA was measured and compared with the diameter of the normal-appearing ICA distally. Graduation of stenosis was based on North American Symptomatic Carotid Endarterectomy Trial criteria (3). The observer (E.M.) was blinded to the results of B-flow and color duplex flow US. In addition, we evaluated the rate of angiography-related neurologic complications and puncture-site complications (bleeding, false aneurysms).

For statistical analysis, we used statistical software (SPSS version 10.0.5; SPSS, Chicago, Ill). All numeric values are expressed as means, with the value range in parentheses. Interobserver variability for the evaluation of gray-scale intensity and the length and area of the jet stream at B-flow imaging was analyzed by using the Pearson correlation coefficient. This coefficient was also used to analyze the correlation of B-flow imaging, color duplex flow US, and angiographic parameters. The possible influence of the quality of assessment conditions on B-flow and color duplex flow US parameters was evaluated by performing the Kruskal-Wallis test. A statistician from the local institute of medical statistics was consulted to ensure use of the appropriate statistical tests.


   RESULTS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
RESULTS
DISCUSSION
 REFERENCES
 
Of 28 consecutive patients, 17 (61%) were men and 11 (39%) were women; all underwent color duplex flow and B-flow assessment. The mean patient age was 70.7 years (range, 45–85 years). Fifteen (54%) left and thirteen (46%) right ICAs were evaluated. The quality of the conditions for US assessment was staged at color duplex flow US as follows: good visualization of the CCA and ICA in 17 (61%) patients, high bifurcation and/or extensive kinking in five (18%) patients, and extensive calcification in six (21%) patients. The quality of assessment conditions was not significantly inferior (P = .56) in patients with a higher percentage of ICAS. Color duplex flow US revealed that five (18%) patients had ICAS of less than 50%, eight (29%) had ICAS of 50%–70%, and the remaining 15 (54%) had ICAS of more than 70%. The mean PSV in patients with ICAS was 2.3 m/sec (range, 0.4–4.8 m/sec), with a mean carotid ratio of 4.0 (range, 1.1–10.0).

Results of angiography (n = 18) correlated well with those of color duplex flow US (r = 0.85, P < .001). There were no periinterventional mortalities or angiography-related neurologic complications, and there was one false aneurysm at the puncture site, which was treated with US-guided compression therapy.

B-flow images obtained in one patient with high-grade ICAS and in one patient with moderate ICAS are shown in Figures 3 and 4, respectively. B-flow imaging results concerning the length and area of the jet stream, as well as the gray-scale intensity of the ICA and gray-scale ratio for both observers, including interobserver variability, are shown in Table 1. There was no systematic measurement error between observers (for all parameters, P >.05). The Pearson correlation coefficients of B-flow imaging versus color duplex flow US and angiography are shown in Table 2. Scatterplots depicting the length (Fig 5c) and area (Fig 5d) of the jet stream (each vs ICAS percentages, which were evaluated with color duplex flow US), depicting the gray-scale intensity of ICA versus the PSV of the ICA, and depicting gray-scale ratio versus carotid ratio for both observers are shown in Figure 5. The length (P = .46) and area (P = .69) of the jet stream, the maximum gray-scale intensity in the ICA (P = .20), and the gray-scale ratio (P = .34) were not significantly influenced by the quality of the assessment conditions.


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  		TABLE 1. B-flow Imaging Parameters of Both Observers, Expressed as Means

 

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  		TABLE 2. Pearson Correlation Coefficients of B-Flow versus Color Duplex Flow US Parameters for All 28 Patients and Angiography for 18 Patients

 


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  		Figure 5a. Scatterplots indicate that there is no significant correlation of (a) maximum gray-scale intensity versus PSV in the ICA, (b) gray-scale versus the carotid ratio, (c) the length of the jet stream versus ICAS, and (d) the area of the jet stream versus ICAS. * = observer 2, {square} = observer 1.

 


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  		Figure 5b. Scatterplots indicate that there is no significant correlation of (a) maximum gray-scale intensity versus PSV in the ICA, (b) gray-scale versus the carotid ratio, (c) the length of the jet stream versus ICAS, and (d) the area of the jet stream versus ICAS. * = observer 2, {square} = observer 1.

 


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  		Figure 5c. Scatterplots indicate that there is no significant correlation of (a) maximum gray-scale intensity versus PSV in the ICA, (b) gray-scale versus the carotid ratio, (c) the length of the jet stream versus ICAS, and (d) the area of the jet stream versus ICAS. * = observer 2, {square} = observer 1.

 


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  		Figure 5d. Scatterplots indicate that there is no significant correlation of (a) maximum gray-scale intensity versus PSV in the ICA, (b) gray-scale versus the carotid ratio, (c) the length of the jet stream versus ICAS, and (d) the area of the jet stream versus ICAS. * = observer 2, {square} = observer 1.

 

   DISCUSSION
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
RESULTS
 DISCUSSION
REFERENCES
 
The practice of performing carotid endarterectomy without arteriography is a growing trend in vascular surgery and has been shown to be safe and to depend heavily on reliable US results (13). Doppler US velocity spectra in combination with real-time B-mode and color duplex flow images of the ICA are used to quantify stenotic lesions (11). Several teams have evaluated the correlation of different velocity parameters of color duplex flow US with those of angiography, proving sensitivity rates of 85%–87% and specificity rates of 89%–97%, with high interobserver correlation (15). Corresponding results, with excellent correlation between the results of angiography and color duplex flow US, were observed in the current study. On the other hand, clinicians caring for patients with carotid arterial disease should be aware that the duplex US criteria used for noninvasive estimation of the extent of ICAS may vary considerably (14).

Despite the broad use of color duplex flow US, there are a number of inherent limitations for visualization of blood flow. Velocity criteria may be inaccurate in a number of clinical conditions. While cardiac arrhythmia, aortic valve insufficiency, and tandem plaques may result in underestimation of the degree of stenosis, carotid arterial coiling or kinking, arteriovenous malformations, carotid arterial body tumors, and contralateral severe stenosis or occlusion may promote overestimation of luminal narrowing (15). Further disadvantages of color duplex flow US are limited frame rate, high angle dependency, limited spatial resolution along the beam direction, and "overwriting" of the vessel walls by the color overlay (the so-called blooming artifact), which can mask subtle lesions.

For these reasons, we evaluated B-flow imaging in the evaluation of ICAS. Advantages of this recently introduced technique are simultaneous imaging of tissue and blood-echo information, so that blooming artifacts are not possible. A high frame rate is possible, as well as high spatial and transverse resolution, so that imaging of complex flow phenomena becomes possible. A further advantage is that plaque contours or intraluminal structures can be imaged in more detail, as compared with that at color duplex flow US, so there arises the possibility of qualitative description of blood flow, as well as of plaque morphology. The absence of angle dependency in B-flow imaging enables exact planimetric evaluation of the stenosis, which promises high correlation between angiography and B-flow imaging (19). A limitation of B-flow imaging is that excessive pulsations of the vessel lead to movement of the surrounding structures, so that the vessel wall is sometimes ill defined. Further disadvantages are an inability to obtain signals after plaque calcification (a problem with all US techniques) and decreased sensitivity of B-flow imaging with increasing depth, because of the strong dependence of signal strength (19).

On the basis of our experience that high-grade stenosis produces a poststenotic region of higher gray-scale intensity (the jet stream) and the fact that pixel brightness or intensity, with almost no angle dependency, is determined by blood-echo strength and blood velocity (19), we evaluated B-flow imaging in the grading of ICAS, as compared with color duplex flow US. Our prospective pilot study revealed no correlation between the investigated B-flow and color duplex flow US parameters. Neither the gray-scale intensity nor the length and area of the jet stream yielded any hemodynamic information. Two reasons were suspected for the data mismatch but have been disproved with further statistical analysis: (a) the difficulty of clearly defining the points with maximum gray-scale intensity and the start and end points of the jet streams; however, interobserver variability was excellent (or at least almost excellent for the length of the jet stream) for all parameters; and (b) the quality of conditions for US assessment; however, no significant influence of this factor on B-flow parameters was identified. Scanning properties were fixed in all patients to exclude a possible influence on our results. The only exception concerned gain, but we calculated an additional gray-scale ratio to exclude possible bias. As our study population correlates well with our "standard" patient population with regard to age, sex, conditions for assessment of stenosis, and color duplex flow US parameters, we believe that our results are representative, although we included only a small number of patients in this pilot study. On the basis of these initial results, we will not use a larger patient series for the current objective. Objectives of further clinical B-flow studies will concern the accuracy of planimetric evaluation of ICAS, as compared with that of angiography and plaque morphology.

In conclusion, neither jet stream length nor area nor gray-scale intensity correlates with the PSV in the ICA, with the carotid ratio, or with the percentage grade of ICAS, so these B-flow parameters cannot be used in the evaluation of ICAS. Advantages of the method, such as angle independence, absence of blooming artifacts, and high spatial and transverse resolution, allow imaging of complex flow phenomena, as well as detailed examination of plaque morphology. Therefore, B-flow imaging has the potential to be used as an additional tool for this indication.


   FOOTNOTES
 
Abbreviations: CCA = common carotid artery, ECA = external carotid artery, EDV = end-diastolic velocity, ICA = internal carotid artery, ICAS = ICA stenosis, PSV = peak systolic velocity

Author contributions: Guarantors of integrity of entire study, R.A.B., E.M., J.L.; study concepts, R.A.B., M.R., J.L.; study design, R.A.B., M.R.; literature research, R.A.B., M.R.; clinical studies, R.A.B., M.R., I.K., R.A.; data acquisition, R.A.B., M.R., I.K., R.A.; data analysis/interpretation, R.A.B., M.R., I.K., E.M.; statistical analysis, R.A.B., M.R.; manuscript preparation, R.A.B., E.M., J.L.; manuscript definition of intellectual content, all authors; manuscript editing, R.A.B.; manuscript revision/review, E.M., J.L.; manuscript final version approval, all authors.


   REFERENCES
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
RESULTS
 DISCUSSION
 REFERENCES
 

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  3. North American Symptomatic Carotid Endarterectomy Trial Collaborators. Benefit of carotid endarterectomy in patients with symptomatic moderate or severe stenosis. N Engl J Med 1998; 339:1415-1425.[Abstract/Free Full Text]
  4. Asymptomatic Carotid Atherosclerosis Study Collaborators. Endarterectomy for asymptomatic carotid artery stenosis. JAMA 1995; 273:1421-1428.[Abstract]
  5. Staikov IN, Arnold M, Mattle HP, et al. Comparison of the ECST, CC and NASCET grading methods and ultrasound for assessing carotid stenosis. J Neurol 2000; 247:681-686.[CrossRef][Medline]
  6. Hood DB, Mattos MA, Mansour A, et al. Prospective evaluation of new duplex cirteria to identify 70% internal carotid artery stenosis. J Vasc Surg 1996; 23:254-261; discussion 261–262.[CrossRef][Medline]
  7. Hunink MG, Polak JF, Barlan MM, O’Leary DH. Detection and quantification of carotid artery stenosis: efficacy of various Doppler velocity parameters. AJR Am J Roentgenol 1993; 160:619-625.[Abstract/Free Full Text]
  8. Padayachee TS, Cox TCS, Modaresi KB, Colchester ACF, Taylor PR. The measurement of internal carotid artery stenosis: comparison of duplex with digital subtraction angiography. Eur J Vasc Endovasc Surg 1997; 13:180-185.[CrossRef][Medline]
  9. Bell PR. Carotid endarterectomy: preoperative angiography is outdated (letter). BMJ 1995; 310:1136.[Free Full Text]
  10. Curley PJ, Norrie L, Nocholson A, Galloway JM, Wilkinson AR. Accuracy of carotid duplex is laboratory specific and must be determined by internal audit. Eur J Vasc Endovasc Surg 1998; 15:511-514.[CrossRef][Medline]
  11. Carpenter JP, Lexa FJ, Davis JT. Determination of sixty percent or greater carotid artery stenosis by duplex Doppler ultrasonography. J Vasc Surg 1995; 22:697-703; discussion 703–705.[CrossRef][Medline]
  12. Carpenter JP, Lexa FJ, Davis JT. Determination of duplex doppler ultrasound criteria appropriate to the North American Symptomatic Carotid Endarterectomy Trial. Stroke 1996; 27:695-699.[Abstract/Free Full Text]
  13. Chen JC, Salvian AJ, Taylor DC, Teal PA, Marotta TR, Hsiang YN. Predictive ability of duplex ultrasonography for internal carotid artery stenosis of 70%–99%: a comparative study. Ann Vasc Surg 1998; 12:244-247.[CrossRef][Medline]
  14. Moneta GL, Edwards JM, Chitwood RW, et al. Correlation of North American symptomatic carotid endarterectomy trial (NASCET) angiographic definition of 70% to 99% internal carotid artery stenosis with duplex scanning. J Vasc Surg 1993; 17:152-157; discussion 157–159.[CrossRef][Medline]
  15. Nicolaides AN, Shifrin EG, Bradbury A, et al. Angiography and duplex grading of internal carotid stenosis: can we overcome the confusion? J Endovasc Surg 1996; 3:158-165.[CrossRef][Medline]
  16. Winkelaar GB, Chen JC, Salvian AJ, Taylor DC, Teal PA, Hsiang YN. New duplex ultrasound scan criteria for managing symptomatic 50% or greater carotid stenosis. J Vasc Surg 1999; 29:986-994.[CrossRef][Medline]
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  18. GE Ultrasound Europe. B-Flow: a new way of visualizing blood flow—ultrasound technology update Solingen, Germany: GE Ultrasound Europe, 1999.
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