Evaluation of Renal Artery Stenosis With Velocity Parameters of Doppler Sonography

Transcription

1 Article Evaluation of Renal Artery Stenosis With Velocity Parameters of Doppler Sonography Jian-Chu Li, MD, Lei Wang, MD, Yu-Xin Jiang, MD, Qing Dai, MD, Sheng Cai, MD, Ke Lv, MD, Zhen-Hong Qi, MD Objective. The purpose of this study was to evaluate the accuracy of velocity parameters for the diagnosis of renal artery stenosis (RAS) with color Doppler sonography and to determine the optimal threshold values for these parameters. Methods. The study group was composed of 187 renal arteries, which were examined by color Doppler sonography and angiography. Four Doppler parameters, including the peak systolic velocities (PSVs) in the renal and interlobar arteries, the renal-aortic ratio, and the renal-interlobar ratio (RIR), were measured. Receiver operating characteristic curve analysis was performed to determine the optimal parameter. The sensitivity, specificity, and negative and positive predictive values at various threshold values were calculated. Results. Doppler sonographic examination was technically successful in 96% of renal arteries (180/187). The RIR was determined to be the best parameter. With threshold values of RIR greater than 5, PSV greater than 150 cm/s in the renal artery, renal-aortic ratio greater than 2, and PSV less than 25 cm/s in the interlobar artery, the sensitivity values were 88%, 81%, 70%, and 74%, respectively. An RIR greater than 5 and PSV less than 15 cm/s in the interlobar artery provided the optimal combination of parameters, with sensitivity and specificity of 91% and 87%, respectively. Conclusions. The RIR is the best velocity parameter in the detection of RAS ( 50%), and its best cutoff is 5. Valuing influencing factors of PSV in the renal artery will help reduce misdiagnosis. The combination of RIR greater than 5 and PSV less than 15 cm/s in the interlobar artery provides the best diagnostic efficiency of RAS. Key words: Doppler sonography; hemodynamics; renal artery stenosis. Abbreviations PSV, peak systolic velocity; RAR, renal-aortic ratio; RAS, renal artery stenosis; RIR, renal-interlobar ratio; RSR, renal-segmental ratio Received October 6, 2005, from the Department of Ultrasonography, Peking Union Medical College Hospital, Chinese Academy of Medical Sciences and Peking Union Medical College, Beijing, China. Revision requested November 28, Revised manuscript accepted for publication January 10, Address correspondence to Yu-Xin Jiang, MD, Department of Ultrasonography, Peking Union Medical College Hospital, Chinese Academy of Medical Sciences and Peking Union Medical College, 1 Shuaifuyuan, Wangfujing, Beijing , China. jiangyx@pumch.ac.cn Renal artery stenosis (RAS) is the most common cause of secondary hypertension; however, the prognosis of RAS is optimistic because it can be cured by angioplasty, stent placement, or surgery. Therefore, early and accurate diagnosis of RAS is crucial. Although angiography is still the reference standard imaging modality for diagnosis of RAS, the related risks of the procedure are well known. As a noninvasive alternative, renal artery Doppler sonography gains popularity with the introduction of better machines and the accumulation of diagnostic experience. 1 4 Different Doppler parameters have been obtained from the main renal artery and intrarenal branches, so-called direct and indirect parameters. 1 Of all indirect parameters, acceleration and acceleration time were accepted as the most useful ones. 4 However, they were considered helpful only for diagnosis of severe RAS (diameter reduction 70% 80%), 2,5,6 and the reproducibility of their measurements was not good in some reports. 7, by the American Institute of Ultrasound in Medicine J Ultrasound Med 2006; 25: /06/$3.50

2 Velocity Parameters in Renal Artery Stenosis On the basis of the hemodynamic theory of arterial stenosis, as well as previous reports, 3 5,9,10 the renal artery peak systolic velocity (PSV) was considered one of the best Doppler parameters in the diagnosis of hemodynamically important RAS (diameter reduction 50%). However, the renal artery PSV among individuals varies considerably; thus the renal-aortic ratio (RAR), a ratio of the PSV in the renal artery to that in the aorta, was proposed to avoid the influence of interindividual variation. 8,10 12 Nevertheless, the diagnostic efficiency ranged from series to series, with wide threshold values from 2 to 3.5. Recently, de Oliveira et al 11 pointed out that the renal-segmental ratio (RSR), a ratio of the PSV in the renal artery to that in the segmental artery, was the optimal Doppler parameter in the diagnosis of RAS of 50% or greater compared with other conventional Doppler parameters. The purpose of our study was to evaluate the accuracy of velocity parameters and influencing factors of renal artery PSV and to determine the optimal threshold values for these parameters in the detection of RAS of 50% or greater. Materials and Methods From July 1996 to May 2004, 91 consecutive patients were prospectively evaluated by color Doppler sonography and angiography. There were 41 men and 50 women with a mean age of 42 years (range, 9 82 years). The study group was composed of 187 renal arteries, including 180 main renal arteries, of which 2 patients had only 1 kidney, and 7 accessory renal arteries. The selection criteria for renal angiography were established on the basis of their clinical manifestations. Of the 91 patients, 62 were thought to have RAS hypertension caused by malignant hypertension or unexplained deterioration of renal function; 24 had clearly diagnosed or suspected Takayasu arteritis who were simultaneously considered potentially to have RAS hypertension; and 5 were thought to have Takayasu arteritis. Patients included in this study had both an aortography result obtained within 3 months of Doppler sonography and Doppler sonography that was performed before angiography. The study was approved by the hospital Ethics Committee, and all patients signed an informed consent form before participation in the study. Both of these examinations were blinded to the angiographic findings at the time of Doppler sonographic examination. Color Doppler sonographic studies were performed with a variety of equipment (LOGIQ 9 and LOGIQ 700, GE Healthcare, Milwaukee, WI; and HDI 5000, Philips Medical Systems, Bothell, WA). Either a 3.5- or 5.0-MHz curvilinear array transducer was used. The angle of insonation was set at 60 or less during the study of the aorta and renal arteries. The sample gate was placed in the center of the arterial lumen, and the width of the gate was set as 2 to 5 mm. The entire scanning was performed as follows. First, the PSV in the abdominal aorta was recorded at the level of 1 cm below the origin of the superior mesenteric artery. Second, Doppler traces were obtained from proximal, middle, and distal segments of each renal artery at the possibly smallest Doppler angle. Finally, Doppler spectra were elicited at the upper, middle, and lower pole interlobar arteries along the pyramids, and the one with most marked slope was selected for recording the PSV. The criteria for determining technical success of color Doppler sonography for scanning the extrarenal renal artery were established as follows. Continuous color signals were displayed in the lumen of the extrarenal renal artery, and favorable Doppler spectra were detected in each segment unless the renal artery was occluded. On the basis of published standards, 6,8,11,13 the renal artery lesions were graded according to the percentage of lumen reduction in diameter: mild (0% 49%), moderate (50% 69%), severe (70% 99%), and total occlusion. Receiver operating characteristic curves were performed to determine the optimal parameter. To analyze the differences between 2 parameters, if equal variance was assumed, a t test was used; otherwise, a Kruskal-Wallis test was adopted. The values were expressed as means ± SD. Statistical analysis was performed by SPSS 10.0 software (SPSS Inc, Chicago, IL). Results Results of Angiography Among the 180 main and 7 accessory renal arteries shown on angiograms, there were 93 renal arteries with RAS and 13 renal arteries with occlusion. Of the 93 stenosed renal arteries (diameter reduction, 50% 99%), 42 were caused by atherosclerosis, 30 by Takayasu arteritis, and 21 by fibromuscular dysplasia. Sixty-four stenosed lesions were found in the proximal segment of 736 J Ultrasound Med 2006; 25:

3 Li et al the renal artery, 10 in the middle segment, 5 in the distal segment, 7 in both proximal and middle segments, and 7 in a diffused or segmental region. Of the 13 renal occlusive arteries, 6 were caused by Takayasu arteritis, and 7 were caused by atherosclerosis. Results of Doppler Sonography Among the 187 renal arteries shown on angiograms, 1 main and 6 accessory renal arteries were not detected, whereas the other 180 arteries were examined successfully by color Doppler sonography. All 7 renal arteries that failed to be visualized were normal on angiograms. The technical success rates in this study were 96% (180/187) in detecting both main and accessory renal arteries and 99% (179/180) in detecting only main renal arteries. One hundred sixtyseven of the 180 main renal arteries were subjected to statistical analysis; the 13 occlusive arteries were excluded. Compared with the reference standard of angiographic results, the statistical results are shown in Table 1. When parameters such as the renal artery PSV, RAR, and renal-interlobar ratio (RIR), taken from 3 groups of mild, moderate, and severe stenosis, were compared with one another, statistically significant differences were found (P <.005). No difference was found between the mild and moderate stenosis groups in the interlobar artery PSV (P =.523). However, a remarkably statistically significant difference was found between the mild and severe stenosis groups and between the moderate and severe stenosis groups in the interlobar artery PSV (P <.001). Receiver operating characteristic curves indicated that the RIR was the optimal velocity parameter in the diagnosis of RAS of 50% or greater (Figure 1). The area under the receiver operating characteristic curve for the RIR (0.927) was statistically significantly greater than that for the renal artery PSV (0.884) (P <.0001), that for the RAR (0.876) (P <.0001), and that for the interlobar artery PSV (0.224) (P <.0001). Considering the better sensitivity and specificity, the best threshold values of the RIR, RAR, renal artery PSV, and interlobar artery PSV were determined to be 5, 2, 150 cm/s, and 25 cm/s, respectively. The diagnostic efficiency of the RIR at various threshold values in the diagnosis of RAS of 50% or greater is shown in Table 2. The results of color Doppler sonography were defined as positive if findings for 1 of the 2 combined parameters were positive. The efficiency of the combination of these parameters in the diagnosis of RAS of 50% or greater is shown in Table 3. Compared with use of a single parameter, the sensitivity of the combination of RAR greater than 2 and renal artery PSV greater than 150 cm/s was 1% higher, whereas the specificity was 7% lower. The sensitivity of the combination of RIR greater than 5 and renal artery PSV greater than 150 cm/s was 1% higher, whereas the specificity was 5% lower. The sensitivity of the combination of RIR greater than 5 and interlobar artery PSV less than 15 cm/s was 3% higher, whereas the specificity was 11% lower. When renal artery PSV greater than 150 cm/s was applied to diagnose RAS of 50% or greater, there were 18 renal arteries with false-negative findings and 5 with false-positive findings. Of the 18 renal arteries with false-negative findings, 6 had moderate stenoses, and the other 12 had severe stenoses, of which 1 was segmental and 11 were single. Among these single severely stenosed renal arteries, there were 2 severe stenoses (diameter reduction 95%) without other conditions (Figure 2), 3 main RAS with an ipsilateral patent accessory renal artery (Figure 3), 2 with renal dysfunction, 1 with occlusion of the infrarenal abdominal aorta, 1 with severe stenosis of the bilateral iliac common arteries, 1 with renal atrophy, and 1 with stenosis of the intrarenal artery. Five renal arteries with falsepositive findings appeared in young patients (15 22 years old), in which the contralateral Table 1. Statistical Results of Velocity Parameters Between Stenotic Groups Stenotic Degree, % P by Stenotic Degree Parameter 0 49 (n = 74) (n = 16) (n = 77) 0 49 vs vs vs Renal artery PSV, cm/s ± ± ± <.005 <.005 <.005 RAR 1.15 ± ± ± 2.34 <.005 <.005 <.005 RIR 3.37 ± ± ± <.005 <.005 <.005 Interlobar artery PSV, cm/s ± ± ± <.001 <.001 The Kruskal-Wallis test was used here; n indicates the number of patients in each group; the total number analyzed was 167. J Ultrasound Med 2006; 25:

4 Velocity Parameters in Renal Artery Stenosis Discussion Figure 1. Receiver operating characteristic curves for the renal artery PSV (RAPSV), PSV in the interlobar artery (IAPSV), RAR, and RIR in the detection of RAS (diameter reduction 50%). renal arteries had nonatherosclerotic stenosis (3 Takayasu arteritis and 2 fibromuscular dysplasia). Additionally, 1 of them with Takayasu arteritis had occlusions in the left subclavian and left common carotid arteries combined with stenosis in the right subclavian artery. Ten of 13 complete occluded renal arteries visualized on angiograms were correctly diagnosed on the basis of either color signals or Doppler spectra within the course of the renal artery recognized on gray scale sonography. Of the remaining 3 arteries, 1 had a misdiagnosis of severe stenosis because of mistakenly recognizing the collateral branch as the renal artery, and the other 2 could not be clearly distinguished from complete occlusion or severe stenosis. Table 2. Detection of RAS* Using the RIR at Various Threshold Values RIR Threshold Sensitivity, % Specificity, % PPV, % NPV, % > > > The total number analyzed was 167. NPV indicates negative predictive value; and PPV, positive predictive value. *Renal artery stenosis of 50% or greater. One novel finding in our study is that the RIR was potentially the best velocity parameter in the diagnosis of RAS of 50% or greater. Both the RIR and RAR reflect hemodynamic changes in flow blood velocity and are not affected by either upstream or downstream stenosis because they are directly related to the changes in crosssectional area but not to flow volume. 14 Moreover, they could reduce or even eliminate interindividual variations in the renal artery PSV. Nevertheless, practically, the PSV in the abdominal aorta also had interindividual variation and was affected by other factors, such as heart diseases and lower extremity arterial diseases. Moreover, it might still remain unaffected even if the severity of RAS varies. On the contrary, the renal artery PSV increased and the interlobar artery PSV decreased with the increasing severity of RAS. This implies that the RIR is better than the RAR in diagnosis of RAS because more substantial changes could be displayed in the RIR (Figure 4). de Oliveira et al 11 also reported that the RSR was the best parameter to diagnose RAS of 50% or greater compared with other conventional Doppler parameters. However, for a middle or distal segment of RAS, the jet flow from the stenotic site would not completely disappear in the segmental renal artery but could possibly disappear in the interlobar renal artery and thus increase the chance that a decreased PSV in the interlobar renal artery is obtained (Figure 5). In this case, the RIR might be better than the RSR, even though further evidence is needed. The renal artery PSV played a very important role in the diagnosis of RAS because it not only could predict the degree of RAS but also was applied to calculate other important velocity ratio parameters. However, great variability in the correct measurement of the renal artery PSV, especially in tortuous and deeply located vessels, has been reported. 15 Therefore, proper examination skills should be adopted to gain the true PSV in the renal artery. These examination skills are summarized as follows. First, the sample line should be parallel with the direction of jet stream instead of the vessel wall because in most cases the vessel wall could not be recognized clearly on either gray scale sonography or color Doppler imaging. Second, the sample gate should be placed at the most seriously stenotic site under the guidance of favorable color Doppler imaging. 738 J Ultrasound Med 2006; 25:

5 Li et al Table 3. Effectiveness of Combinations of Velocity Parameters for Detection of RAS* Threshold Sensitivity, % Specificity, % PPV, % NPV, % RAR >2/renal artery PSV >150 cm/s RIR >5/renal artery PSV >150 cm/s RIR >5/interlobar artery PSV <15 cm/s The total number analyzed was 167. NPV indicates negative predictive value; and PPV, positive predictive value. *Renal artery stenosis of 50% or greater. Third, usually repeated sampling around the turbulent signals at the site that was suspected of having a stenosis is effective. Finally, the PSV in the proximal renal artery should be recorded by using coronal scanning for bilateral renal arteries or by right intercostal or subcostal transverse scanning for the right renal artery but not by transverse scanning of the mesogastric-epigastric area. The last scanning skill has 2 obvious disadvantages. One is that keeping a Doppler angle smaller than 60 on patients with the horizontal course of the proximal segment of the renal arteries is usually difficult to carry out. The other is that the renal artery PSV recorded is higher than the true value at the angle of 60 less on patients with deeply located renal arteries, overlying bowel gas, or both. In this study, the best threshold value of the RAR was 2, which was lower than most published data. 1,4,8,10,12,16 We consider this value a major improvement because it was obtained after great efforts to record the true PSV in the renal artery as best as possible with the smallest possible Doppler angle. Our finding was also in accordance with that of van der Hulst et al, 7 who reported a lower threshold value of the RAR of 1.2 with sensitivity of 94% and specificity of 93% in the detection of hemodynamically important RAS. In their study, the low threshold value was mainly related to the application of an endovascular flow wire to ensure accurate Dopplerbased velocity measurements. The renal artery PSV of 150 cm/s or greater in the detection of RAS of 50% or greater only yielded sensitivity of 81%. This partially was related to 32% (29/91) of patients with Takayasu arteritis, some of whom had an irregular hemodynamic change caused by the involved multilevel artery stenoses or occlusions. On the basis of our analysis and in combination with previous reports, 6 8,17 20 the false-negative causes of the renal artery PSV are as follows: (1) moderate RAS, highly severe RAS, or diffuse or segmental RAS; (2) RAS with ipsilateral renal atrophy; (3) patients with an ipsilateral patent accessory artery or abundant Figure 2. Severe restenosis of a stent of left renal artery in a 14-year-old patient. A, The renal artery PSV of 77 cm/s at the stenotic site does not increase; the RAR of 0.6 is normal; and the RIR of 5.5 is higher. B, Subsequent angiogram shows severe restenosis (>95% diameter narrowing) of the stent of the left renal artery (arrow). A B J Ultrasound Med 2006; 25:

6 Velocity Parameters in Renal Artery Stenosis Figure 3. Severe stenosis of the right main renal artery in a 29-year-old man. A, Angiogram shows severe stenosis of the middle segment of right main renal artery (short arrow) and an ipsilateral patent accessory renal artery (long arrows). B, The renal artery PSV of 111 cm/s at the stenotic site does not elevate obviously. C, The stenotic site (arrow) is shown on color Doppler imaging. AO indicates abdominal aorta; GB, gallbladder; IVC, inferior vena cava; LI, liver; RK, right kidney; RRA, right renal artery. A Figure 4. Severe stenosis of the middle segment of the right renal artery in a 58-year-old woman. A, The renal artery PSV at the stenotic site is 438 cm/s, and the RAR is 5.1 (438/86). B, Spectral analysis of the ipsilateral interlobar artery shows typical abnormal changes: PSV measures 14 cm/s; systolic acceleration time measures 0.18 seconds, and resistive index measures The RIR is 31.3 (438/14). C, The middle segment of the right renal artery is confirmed to have severe stenosis (arrow) by renal angiography. A B B C C 740 J Ultrasound Med 2006; 25:

7 Li et al Figure 5. Changes in intrarenal artery waveforms in 55-year-old man with severe stenosis of the middle segment of left renal artery. A, Spectral analysis of the ipsilateral segmental artery downstream of the stenosis shows the turbulent and chaotic Doppler flow pattern with a sting outline (arrows) but without a prolonged acceleration time (0.04 seconds). These indicate that the blood flow at the level of the segmental arteries is affected by the jet stream from the stenosis. B, Spectral analysis of the ipsilateral interlobar artery shows the laminar flow with a smooth outline and a prolonged acceleration time (0.13 seconds). These suggest that the blood flow at the level of the interlobar arteries is not affected by the jet stream from the stenosis. C, Subsequent angiogram reveals severe stenosis of the middle segment of the left renal artery. A B C collateral vessels; (4) RAS combined with stenoses of distal primary branches or intrarenal arteries; and (5) patients with infrarenal aorta stenosis or severe stenosis or occlusion of the bilateral iliac artery. False-positive findings occur more often in young patients and in patients with hyperthyroidism 21 or a tortuous renal artery. 15 Severe stenoses or occlusions of multiple branches of the brachiocephalic trunk also increase the chances of a false-positive finding. There was such a patient with Takayasu arteritis in our study, in which the elevated PSV in the renal artery was caused by the excessive cardiac output into the abdominal aorta and renal arteries. Furthermore, when blood flow in the renal artery is affected by the jet from a stenosis of the suprarenal abdominal aorta, the renal artery PSV may elevate and produce a false-positive result. House et al 12 reported that the combination of an RAR greater than 3 and renal artery PSV greater than 180 cm/s enhanced sensitivity 5% higher than any of the individual parameters in the detection of RAS of 60% or greater. In our study, we found that the RIR and the interlobar artery PSV provided the best combination of parameters in the diagnosis of RAS of 50% or greater mainly because they reflected opposite velocity changes between the stenotic site and downstream circulation, and combinations of 2 each of the RAR, RIR, and renal artery PSV also produced mildly higher sensitivity than any of the individual parameters. The reason could be that all 3 parameters are related to the changes in the renal artery PSV at the stenotic site, and the influencing factors of renal artery PSV may affect the RIR and the RAR. The technical success rate in our study was 99% (179/180) in the main renal artery, which was higher than most results published before. 1,12,15,17 There are 2 possible reasons. First, all the patients included in our study were Chinese and J Ultrasound Med 2006; 25:

8 Velocity Parameters in Renal Artery Stenosis were relatively thinner overall than many Western people; and second, most patients had unilateral or bilateral main RAS, and therefore, the renal artery should have been more easily identified as a result of the mottled blood flow signal in the stenotic site. However, our study findings might not be generalized to all populations, especially those including overweight patients; although several methods were used to obtain a reliable renal artery PSV measurement, it might still be difficult when performed in overweight people. In conclusion, the RIR is the best of the 4 velocity parameters in detection of RAS of 50% or greater. Its optimal threshold value is 5. Valuing influencing factors of the PSV in the renal artery will help reduce misdiagnosis. The combination of an RIR greater than 5 and interlobar artery PSV less than 15 cm/s provides the best diagnostic efficiency of RAS. References 1. Conkbayir I, Yucesoy C, Edguer T, Yanik B, Yasar Ayaz U, Hekimoglu B. Doppler sonography in renal artery stenosis: an evaluation of intrarenal and extrarenal imaging parameters. Clin Imaging 2003; 27: Ripolles T, Aliaga R, Morote V, et al. Utility of intrarenal Doppler ultrasound in the diagnosis of renal artery stenosis. Eur J Radiol 2001; 40: Olin JW, Piedmonte MR, Young JR, DeAnna S, Grubb M, Childs MB. The utility of duplex ultrasound scanning of renal arteries for diagnosing significant renal artery stenosis. Ann Intern Med 1995; 122: Mitty HA, Shapiro RS, Parsons RB, Silberzweig JE. Renovascular hypertension. Radiol Clin North Am 1996; 34: Patriquin HB, Lafortune M, Jequier JC, et al. Stenosis of the renal artery: assessment of slowed systole in the downstream circulation with Doppler sonography. Radiology 1992; 184: Hoffmann U, Edward JM, Carter S, et al. Role of duplex scanning for the detection of atherosclerotic renal artery disease. Kidney Int 1991; 39: de Oliveira SIRS, Widman A, Molnar LJ, Fukushima JT, Praxedes JN, Cerri GG. Color Doppler ultrasound: a new index improves the diagnosis of renal artery stenosis. Ultrasound Med Biol 2000; 26: House MK, Dowling RJ, King P, Gibson R. Using Doppler sonography to reveal renal artery stenosis: an evaluation of optimal imaging parameters. AJR Am J Roentgenol 1999; 173: Hélénon O, el Rody F, Correas JM, et al. Color Doppler US of renovascular disease in native kidneys. Radiographics 1995; 15: Legemate DA, Teeuwen C, Hoeneveld H, Ackerstaff RG, Eikelboom BC. Spectral analysis criteria in duplex scanning of aortoiliac and femoropopliteal arterial disease. Ultrasound Med Biol 1991; 17: Spies KP, Fobbe F, El-Bedewi M, Wolf KJ, Distler A, Schulte KL. Color-coded duplex sonography for noninvasive diagnosis and grading of renal artery stenosis. Am J Hypertens 1995; 8: Nchimi A, Biquet JF, Brisbois D, et al. Duplex ultrasound as first-line screening test for patients suspected of renal artery stenosis: prospective evaluation in high-risk group. Eur Radiol 2003; 13: Rabbia C, Valpreda S. Duplex scan sonography of renal artery stenosis. Int Angiol 2003; 22: Motew SJ, Cherr GS, Craven TE, et al. Renal duplex sonography: main renal artery versus hilar analysis. J Vasc Surg 2000; 32: Zierler RE. Is Duplex scanning the best screening test for renal artery stenosis? Semin Vasc Surg 2001; 14: Bude RO, Larson RG, Nichols WW, Rubin JM. Stenosis of the main artery supplying an organ: effect of end-organ vascular resistance on the poststenotic peak systolic velocity in an in vitro hydraulic model at Doppler US. Radiology 1999; 1: Li JC, Zhang JX, Zhou MK, et al. Evaluation of renal artery stenosis with color Doppler flow imaging. Chin J Ultrasonogr 1996; 5: Kliewer MA, Tupler RH, Carroll BA, et al. Renal artery stenosis: analysis of Doppler waveform parameters and tardusparvus pattern. Radiology 1993; 189: van der Hulst VP, van Baalen J, Kool LS, et al. Renal artery stenosis: endovascular flow wire study for validation of Doppler US. Radiology 1996; 200: Desberg AL, Paushter DM, Lammert GK, et al. Renal artery stenosis: evaluation with color Doppler flow imaging. Radiology 1990; 177: Miralles M, Cairols M, Cotillas J, Gimenez A, Santiso A. Value of Doppler parameters in the diagnosis of renal artery stenosis. J Vasc Surg 1996; 23: J Ultrasound Med 2006; 25: