ultrasound of renal vessels 2001

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Introduction

Sonography of the kidneys is well known as an excellentmodality and initial procedure in the examination ofpatients with renal pathology. Although able to providea vast array of information about the morphology of thekidney and the renal sinus, conventional US is not gen-erally helpful in providing evaluation of changes affect-ing the renal vessels. Such information can be obtainedthrough Doppler techniques. Modern commerciallyavailable machines are now able to outline with greatdetail both main renal vessels and intraparenchymalvasculature of the kidney using color and power Dop-pler techniques. By means of spectral analysis, DopplerUS can provide evaluation of flow characteristics, such

as direction and velocity, and give an estimate of intra-parenchymal flow resistances. Visualization of the renalvessels can be dramatically improved by using contrastenhancement. In clinical practice, the enhanced US sig-nal provided by contrast agents can reduce the numberof technically nondiagnostic cases as well as the numberof false-negative results. Furthermore, new technologi-cal advances can play an increasing role in renal vascu-lar imaging US. Three-dimensional volume-rendering

techniques can be used to recognize shape of vesselswith complex course and to analyze relationships amongthe different structures of the kidney. They can providepanoramic volumetric images of the large abdominalvessels, further outlining the relationships of renal ves-sels with the abdominal aorta. Blood flow morphologymaps can be also displayed within the background of theB-mode data. Combination with contrast agents pro-vides the potential to image the whole vascular renalnetwork and, through special measurement techniques,to estimate vascular volume and transit time, parame-ters which relate directly to tissue perfusion.

The purpose of this paper is to review the clinicalapplications of US and Doppler US techniques in thefield of renal vascular diseases.

Examination technique and normal findings

Anatomy

The renal arteries (RA) originate from the lateral sidesof the aorta, typically at the level of the superior borderof the second lumbar vertebra, directed slightly anteri-

Eur. Radiol. (2001) 11: 19021915DOI 10.1007/s003300101012 ULTRASOUND*

Alexander V. Zubarev Ultrasound of renal vessels

Published online: 30 August 2001 Springer-Verlag 2001

* Categorical Course ECR 2002

A. V. Zubarev ())Department of Radiology,Postgraduate Education and ResearchCenter, Government Medical Center,M. Timoshenko 21, 121359 Moscow,RussiaPhone: +7-95-7 6423 92Fax: +7-95-149 5827

Abstract Kidneys are known aswell-perfused organs and may un-dergo a variety amount of vascularpathological conditions such as re-

nal artery stenosis, renal veinthrombosis, arteriovenous fistula,and aneurysms. Sonography is usu-ally the first imaging method for re-nal vascular diseases. Modern USmachines are now able to outlinewith great detail both main renalvessels and intraparenchymal vas-culature of the kidney using colorand power Doppler techniques.

Knowledge about the use of differ-ent Doppler imaging modalities andtypical sonographic findings of themost frequently conditions affecting

renal vessels are of great impor-tance. This article reviews the clini-cal applications of US and DopplerUS techniques including basics andtechnological advances in the fieldof renal vascular diseases.

Keywords Kidney Renal vessels Ultrasound Doppler studies


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orly, approximately 11.5 cm below the superior mes-enteric artery origin. The right RA branches from theaorta, directs anteriorly for a few centimeters and thendescends obliquely, passing posteriorly to the inferiorvena cava, in a postero-lateral direction toward the kid-ney. The left RA has a more horizontal and typicallyshorter course. It runs from the aorta, posteriorly to the

left renal vein, into the renal hilum.Approximately 30% of patients with normally posi-

tioned kidney have multiple renal arteries, with one ormore accessory vessels [1, 2, 3]. Most of them arise closeto the main RA, although they can originate inferiorly,some distance away, supplying a portion of the lowerpole. Because of this variation in anatomy, accessoryrenal arteries are difficult to detect on Doppler studies,and most are missed.

The main RA divides at the hilum, either within oroutside of the kidney, into anterior and posteriorbranches that further divide into segmental and theninterlobar arteries. Renal arteries are terminal vessels

that do not communicate with each other. They are di-vided into four vascular regions: apical; anterior; poste-rior; and inferior [4]. Each segmental branch suppliesone of these regions. Approximately 90 % of the normalrenal blood flow goes to the cortex; only 10% suppliesthe medulla. The interlobar arteries further divide into anetwork of arcuate arteries that run at the corticomed-ullary junction and give off the cortical (interlobular)branches, which run radially to the renal periphery, andthe medullary branches, which supply the renal pyra-mids.

The renal veins approximately follow the arteriesand join at the hilum to form the main renal vein. In-trarenal veins have collaterals, which unite with eachother. The right renal vein runs in a postero-anteriordirection, with a relatively short course to enter the venacava. The left vein is more horizontal and crosses ante-riorly to the aorta and posteriorly to the superior mes-enteric artery to enter the vena cava, at the level of thefirst lumbar vertebra. It is worth remembering that theleft gonadal vein, adrenal vein, and the lumbar veinsusually enter the left renal vein.

Transducer position

Main renal arteries

The first requirement is to choose a good scan plane.This is a key point. The main renal arteries can be im-aged in an abdominal transverse section, by placing thetransducer in a midpoint between the xiphoid processand the umbilicus (Fig. 1). By applying compressionwith the transducer, the bowel loops can be displaced, tosee the aorta in a transverse section, together with theorigin of both renal arteries. With the axial approach,

the arteries often may be followed to the renal hilum,especially on the right side where the liver can be usedas an acoustic window (Fig. 2). In patients with abun-dant bowel gas obscuring visualization of the aorta, an-gling the probe cranially or caudally can help to over-come the problem and allow identification of the ves-sels.

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Fig.1 Anatomy of the main renal vessels. 1 Transverse mid ab-dominal section; 2 Oblique longitudinal approach; Ao aorta in atransverse section; IVC inferior vena cava in transverse section;RRVright renal vein; RRA right renal artery; LRVleft renal vein;LRA left renal artery

Fig.2 Power Doppler US image of the both normal renal arteries.Abdominal transverse section


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An alternative method of imaging the originsof RA isthe use of an oblique longitudinal approach with the pa-tient in a 45 right anterior oblique position (Fig. 1). Thetransducer should be placed in the right subcostal posi-tion to obtain longitudinal view of the right kidney. Thenthe probe should be moved in a medial direction follow-

ingthecourseoftherightrenalveinfromthehilumtotheinferior vena cava. The transducer is angled until theaorta, togetherwith the originsof both RA, appears. Thedisadvantage of this approach is that only the origins ofthe vessels are shown. Nevertheless, this projection isbest for determining whether accessory arteries, aplasia,or hypoplasia (Fig. 3) of the renal arteries are present.

The left renal artery and vein also can be seen fromthe left flank, using the kidney itself as an acoustic win-

dow to obtain coronal scan planes of the renal hilum andthe aorta with the origins of the left renal vessels(Fig. 4).

Normal findings

When the origins of renal arteries are imaged with colorDoppler in the transverse position, the first segment ofthe right renal artery has flow directed toward thetransducer, then represented in red color. Color changeis detected shortly after the origin, where the directionof flow goes posteriorly. Most of the course of the ves-sels is then displayed in blue.

If the origins of renal arteries are imaged in the ob-lique longitudinal section, right RA passes directly to-ward the transducer from the aorta and is colored red,whereas left RA courses away from the transducer andis blue (Fig. 5).

On color Doppler examination, flow within the renalvein is opposite in color to that within the renal artery.Power Doppler can delineate a better image of theproximal RA without the absence of flow in the arterialsegments that run horizontal to the US beam, but withloss of directional and velocity information. Power

Doppler provides superb visualization of the entire re-nal vascular tree from the main RA to the arcuate ar-teries and beyond. The segmental renal branches liewithin the echogenic renal hilum. Interlobar arteries canbe visualized lateral to the renal pyramids, and the arc-uate vessels run behind the renal pyramids parallel tothe renal cortex. With power Doppler several furthergenerations of vessels (the interlobular arteries) areseen radiating to the renal capsule (Fig. 6).

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a

b

Fig.3 a Color Doppler US image in oblique longitudinal section.Hypoplasia of the right renal artery. b Same patient. Correlativeconventional angiography

Fig.4 Color Doppler imaging of the left renal artery and vein fromthe left flank using the kidney as an acoustic window


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Pulsed Doppler

The longitudinal position can be recommended formeasuring the Doppler spectrum of both renal arteriesdue to optimal angle of both RA in this position. TheDoppler signal in the RA is a low-resistance signal,similar to that found in all the parenchymal organs ofthe body. The important features of this signal are therapid systolic rise and the continuous high-velocity flowthroughout diastole (Fig. 7). A small spike occurs at theend of the systolic rise (the so-called early systolic

peak). It is an important feature to identify when mea-suring the systolic acceleration. This feature is seen onlyin the main renal artery and its major branches. Thepeak systolic velocity (PSV) in the main renal artery andits major branches should be less then 100 cm/s [5]. Thevelocity slowly decreases in the intrarenal arteries asthey branch into the kidney. The diastolic velocity is alittle less than half of the systolic velocity. This valuenormally is expressed as a ratio of end diastole to peaksystolic flow, the most commonly used ratio being theresistance index or Pourcelot resistive index (RI), whichmeasures less then 0.7 in the normal kidney [5]. The re-nal/aortic ratio is used to normalize measurements ofthe velocity within the renal artery. A summary of themost common parameters used to evaluate the flowpatterns within the renal arteries is shown in Table 1.

The RI values, as measured in healthy subjects, showa significant dependence on age and the area sampled.The values in the main RA are higher in the hilar region(0.65 0.17) than in the more distal small arteries, andthey are lowest in the interlobar arteries (0.54 0.20).The RI values are higher in elderly patients. The age-dependent RI values are shown in the Table 2.

In clinical practice the value of RI 0.7 is used to dis-

criminate between normal and pathologic resistances toflow. Although it is nonspecific, an elevated RI ofgreater than 0.7 suggests renal parenchymal diseaseprocesses or postrenal obstruction [5].

The renal vein Doppler shift signal is continuous inthe smaller veins and with slightly phasic changes withrespiration in the main veins. The left renal vein showslittle or no phasic swing during the cardiac cycle,whereas the right one shows a variable amount of pul-

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Fig.5 Color Doppler imaging of the both renal arteries and theaorta. Longitudinal section. Right renal artery shown as red, leftrenal artery shown as blue

Fig.6 Power Doppler US image of the right kidney with renalvessels. Good visualization of the entire renal vascular tree

Fig.7 Spectral Doppler US image from the right renal artery innormal subjects. Note small spike that occurs at the end of systolicrise. This feature is seen only in the normal main renal artery


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satility, reflecting changes in right atrial pressure. Thiscan be related to its shorter course, and to the commonabsence of valvular structures within it (Fig. 8).

The rate of technically inadequate color Doppler USexaminations of the main RA varies between 9% [8]and 23.5% [9]. Some optimistic reports, such as thosefrom Robertson et al. [10], who carefully controlled theduration of the examination limiting attempts to 20 min(allowing a longer time was impractical in that clinicalsetting) found an identification rate of 95.5% of rightand 82% of left renal arteries; however, others, such asBerland et al. [11], are less optimistic. From their studywith clinical comparisons with angiography, duplexscanning showed unsatisfactory results: Only 58% ofthe main RA and no accessory RA were found [11]. Inthis way, contrast agents should have application incases where the Doppler signal is difficult to obtain, ei-ther because of a weak signal or because of signal at-tenuation by overlying tissue [12]. The enhanced signal

provided by the contrast agent should reduce the num-ber of technically nondiagnostic cases and the numberof false-negative results.

Renal artery stenosis

Renal artery stenosis (RAS) is a relatively rare but im-portant cause of renovascular hypertension. Both arte-

riosclerosis, and less commonly fibromuscular dysplasia,can lead to renovascular hypertension. Atheromatouslesions involve the proximal renal artery, whereas fi-bromuscular dysplasia involves the distal main renal ar-tery and segmental renal arteries. Detection of RAS isimportant since it is a potentially curable cause of hy-pertension, by the use of radiologic techniques or sur-gery, and is a direct contraindication to the use of an-giotensin converting enzyme inhibitors during medicaltherapy of hypertension. Conventional angiography iscurrently the gold standard in the detection of renal ar-tery stenosis. Unfortunately, it is an invasive test and isnot suitable as a screening procedure. Magnetic reso-nance angiography is a very promising modality in thisfield and, albeit expensive, is non-invasive but cannot beused as a screening, widely available test [13]. Contrast-enhanced helical CT is a promising test also, especiallywhen multidetector equipment is used. The need of io-dinated contrast medium, however, does not allow itswidespread use as a screening test, given possible neph-rotoxicity. Doppler US is a non-invasive, widespreadand relatively inexpensive diagnostic modality. It is notsurprising, therefore, that it has been extensively inves-tigated as a screening test for renal artery stenosis(Fig. 9) [14, 15].

Duplex US criteria of RAS can be divided into twogroups based on direct findings obtained at the level ofthe stenosis (proximal criteria), or on flow changes ob-served in the renal vasculature, distal to the site ofstenosis (distal criteria).

The proximal criteria are direct signs obtained at thesite of the stenosis. The first, most important sign is theincrease in peak systolic velocities (PSV). Velocitieshigher than 1.5 m/s are significant and should be used as

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Table 1 Normal renal Doppler indices. S peak systolic velocity; Dend diastolic velocity. (From [6])

Index Formula Normal value

S: main renal artery 60100 sm/s(< 180 sm/s)

Resistance index SD/S 0.560.7 (< 0.7)

Pulsatility index SD/mean 0.70.14

Renal/aortic ratio Renal systolic velocity/aortic systolic velocity

< 3.0

Systolic rise time Time to early systolicpeak

0.11 0.06

Systolic acceleration 11 8 m/s2

Table 2 Normal resistive index values in the interlobar arteriesaccording to patient age. (From [7])

Age (years) Mean Mean 2 SD

< 20 0.567 0.5230.6112130 0.573 0.5280.618

3140 0.588 0.5460.6304150 0.618 0.5610.6755160 0.688 0.6030.7336170 0.732 0.6490.8157180 0.781 0.7070.855> 80 0.832

Fig.8 Spectral Doppler US image of the left renal vein in normalsubjects. Spectral trace shows little swing within the cardiac cycle


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a criterion for RAS 50%, and > 1.8 m/s for stenosis60 % (Fig. 10). Comparison of the velocities in the aortawith those in the renal arteries, the so-called renal/aorticratio (RAR), is also helpful. A PSV in the renal arterythree times higher than the aortic PSV indicates the

presence of RAS [16]. The use of the ratio (RAR) in-stead of the absolute peak systolic value is preferablesince hypertension itself can cause an increase of peaksystolic flow velocities within all vessels of the hyper-tensive patient. The second criterion is the presence ofpoststenotic turbulences; these are seen as a widening ofthe Doppler trace at spectral analysis of signals at thestenosis and as a mosaic color pattern on the colorDoppler image (Fig. 11).

Distal criteria analyze the flow changes induced bythe stenosis at the level of intrarenal vessels. Patientswith severe renal artery stenosis commonly present with

pathologic intrarenal signals, the so-called tardus-par-vus waveform, first described by Handa et al. [17]. Tar-dus means slow and late and parvus means small andlittle. Tardus refers to the fact that systolic accelerationof the waveform is slowed, with consequent increase intime to reach the systolic peak. Parvus refers to the factthat the systolic peak is of low height, indicating slowedvelocity (Fig. 12). Poststenotic systolic peak are round-ed with lengthened systolic rise time (or slow systolic

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a

b

Fig.9 a Power Doppler US image of the right renal artery withechogenic plaque inside the vessel close to the origin. b Same pa-tient. Correlative conventional angiography with the plaque in theorigin of the right RA

Fig.10 Spectral Doppler waveform from the stenotic area in theright RA. Increased peak systolic velocities are seen

Fig.11 Color Doppler imaging of the right RAS. Mosaic flow isseen within the stenotic area


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acceleration time the time in seconds from the onset ofthe systole to peak systole), slower than 0.07 s. The ac-celeration index (the slope of systolic upstroke) is de-creased < 3m/s2 [18].

Although this phenomenon has been postulated tobe secondary to decreased perfusion pressure [19], theexplanation actually is more complicated, as systolic

acceleration also relates to peripheral resistance, vesselcompliance, and other variables, as well as to upstreamstenosis. For example, it has been shown that increasingvessel compliance accentuates the decrease in systolicacceleration independent of the transstenotic pressuredrop [20, 21].

Although generally confirming the efficiency of tar-dus parvus, Stavros et al. pointed out that simple patternrecognition of Doppler tracings from segmental renal

arteries was more valuable than calculating the acceler-ation index and acceleration time, with loss of the nor-mally seen early systolic peak indicating RAS [22]. Per-sistence of the early systolic peak could be observedonly in patients with mild stenosis. In their study an ac-celeration index less than 3.0 m/s2 had an accuracy of85%, a sensitivity of 89%, and specificity of 83% for

detecting RAS. An acceleration time of 0.07 s was89% accurate, 78% sensitive, and 94% specific. Thetardus-parvus waveform was 96% accurate, 95% sensi-tive, and 97 % specific for RAS [22].

Sensitivity of the technique may be improved by theadministration of Captopril. Rene et al. [23] studied 62renal arteries in 31 hypertensive patients who under-went Doppler scanning before and 1 h after administra-tion of Captopril prior to angiography. They concludedthat abnormalities of the tardus-parvus phenomenon ofRAS in distal vessels can be made more apparent afteradministration of Captopril with an improvement ofsensitivity.

Many studies have shown that analysis of distal signsobserved at Doppler US can be a useful fist approach topatients with suspected RAS [22, 24, 25]. Other reportsfound recognition of tardus-parvus effect unsatisfactory[26]. Many factors influence systolic acceleration andmay make the test non-specific. Extrarenal factors, suchas aortic/mitral valvular disease, left ventricular dys-function, or even cardiovascular medications, might af-fect systolic acceleration as well. Numerous factors,such as age, hypertension, and diabetes, affect vesselcompliance. Such variables may explain why some au-thors have not been able to reproduce these results [26].

Additional distal criteria have been developed: Astudy by Schwerk et al. demonstrated that differences inresistive index (RI) could help in diagnosing RAS [27].Decreasing PSV in RAS results in lowering of resistiveindex (RI) values. Right-to-left difference DRI betweenkidneys greater than 0.05 had a sensitivity of 100% forRAS greater than 60%. Halpern et al. suggested thatpatients should be screened by distal measurements ofearly systolic acceleration, because determining RARwhen segmental samplings are normal would be super-fluous [28].

Using combined criteria (Table 3) pertaining to thestenosis site and downstream patterns the sensitivity ofDoppler US varies from 64 to 89 %, and specificity from82 to 99 % [9, 15, 25].

In renal transplants, which are easier to explore be-cause of superficial location, as well as of knowledge ofthe course of the renal artery from the surgical report,the results were more convincing. Although Dopplertechniques are well suited as a screening test in trans-planted kidneys, their use for suspected renal arterystenosis in native kidneys has some problems. The mainproblem is that it is difficult to visualize the whole ofboth renal arteries in all patients [11]. Moreover, acces-

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a

b

Fig.12 a Normal renal artery waveform. The acceleration index(AI) is the slope of the line (m/s2) connecting the two points rep-resenting onset of systole and the early systolic peak complex. The

acceleration time (AT) is the time in seconds between those twopoints. b A tardus-parvus renal artery waveform. Rounding andflattening of the systolic peak and prolongation of the AT. The AIis more horizontal


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sory renal arteries are usually missed on Doppler stud-ies, and stenosis of these may also cause hypertension.Also, Doppler studies may be insensitive in patientswith mild stenosis; thus, the role of Doppler sonographyas a screening test in hypertensive patients remainscontroversial. At present, its use cannot be separated

from a careful clinical evaluation of the patient popula-tion. Given its technical difficulties, it has to be em-ployed in patients well selected based on clinical criteriaof high probability of having RAS.

Ultrasound contrast agents have recently added newpossibilities to color and duplex Doppler in the detectionof RAS [30, 31]. They have been shown to increase thepercentage of diagnostic examinations in analyses ofmain renal arteries [30]. With echo enhancers a renewalof interest in Doppler studiesof the main renalarteries isoccurring. Ultrasound contrast agents increase the in-tensity of the Doppler signals, thusproducing more rapidand complete visualization of the intrarenal and extra-renal arteries [29]. Contrast agents have application incases where the Doppler signal is difficult to obtain, ei-ther because of signal attenuation by overlying tissue orbecause of a weak signal [12]. Missouris et al. [32]suggestthatrenal duplex scanning using contrast enhancement isa promising new non-invasive technique in screeningpatients with suspected RAS. Contrast enhancementproduces more reproducible spectral waveforms, im-proves accuracy, and halves the examination time [32]. Arecent study by Claudon et al. [33] showed that the num-ber of examinations with successful results increasedfollowing enhanced Doppler US examination comparedwith nonenhanced Doppler US, including patients withobesity or renal dysfunction. Moreover, the agreement

between US data and angiography in RAS was higherwith enhanced Doppler US. They have shown that con-trast media decrease examination time but do not causean increase in sensitivity [33].

Contrast media for US do not undergo renal filtra-tion or tubular excretion and can be, on the whole, con-sidered as purely vascular tracers. A new interestingapplication of these agents in suspected RAS is quanti-fication of the renal enhancement of color or power

Doppler signals over time after intravenous injection ofa bolus of contrast medium. This technique producestimeintensity curves, which, in renal artery stenosis,have area under the curve larger than in normal kidneys[34]. In severe stenosis, furthermore, there is also a de-lay in the wash-in phase of the curve. At present, how-ever, only preliminary results have been presented in

the literature, and further studies are needed before theintroduction of this technique in clinical practice.

Renal vein thrombosis

Renal vein thrombosis may occur in up to 40 % in dehy-drated or septic infants [35]. In native kidneys, renal veinthrombosis starts in small intrarenal veins in situations offaulty coagulation mechanism and slowed flow [36]. Thepost-glomerular circulation, because of slow flow, is par-ticularly prone to thrombosis. In adults it most common-ly appears in association with renal disease including

glomerulonephritis, systemic lupus erythematosus, di-abetus mellitus, nephrotic syndrome, in severe hypo-volemic shock, and following kidney transplantation.The US features are non-specific, and only renal en-largement, with decreased echogenicity in the earlystages followed by an increase in cortical reflectivity canbe detected [37]. Doppler US cannot accurately diagnosethromboses of intraparenchymal veins. The presence ofvenous Doppler signals within the kidney or renal vein,in fact, does not exclude the diagnosis of a thrombosisinvolving only one of the many intraparenchymal veins.In fact, although in acute renal vein thrombosis, thewhole kidney is underperfused, and although only arte-rial signals can be seen at the renal hilum, it must be re-membered that venous collaterals develop rapidly, andwhen this happens venous signals are re-established.Then, the presence of parenchymal venous flow does notexclude RVT, as collateral flow develops very quickly,particularly in children (Fig. 13) [38]. Color Doppler canbe accurate for diagnosing chronic renal vein thrombosiswhen the main renal vein can be directly visualized, andflow signals cannot be detected in it [38].

Renal vein thrombosis can be caused also from tu-mor involvement in patients with renal cell carcinoma(RCC). Color Doppler sonography is accurate in dem-onstrating tumor thrombus in the renal veins. The USvascular features include distension of the renal vein,

full of echogenic material (Fig. 14). The presence of ar-terial Doppler signals within the thrombus allows un-equivocal demonstration of tumor involvement of thevessel. In patients with renal transplant, with completethrombosis of the veins of the allograft, reduction of di-astolic flow in RA and reversal of flow in diastole withdistended renal vein and absence of flow signals fromthe renal vein have been reported as pathognomonicsigns of renal vein thrombosis [39].

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Table 3 Criteria for RAS. (From [29]). PSVpeak systolic velocity;RAR renal/aortic ratio; ATacceleration time; AI acceleration; RIresistive index

Main RAPSV in stenotic area > 1.8 m/sRAR > 3.0

Intrarenal arteries

ESP AbsentAT > 0.07 sAI < 3 m/sRI > 0.8DRI (leftright) > 5 %


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In very lean, but otherwise healthy subjects, the leftrenal vein can become compressed between the aortaand the superior mesenteric artery, resulting in the so-

called nutcracker syndrome (renal vein entrapmentsyndrome) [40]. Kim et al. proposed that a cut-off valueof greater than 5.0 for the ratio of antero-posterior di-ameter and the ratio of peak velocity (both the AP di-ameter and PV being measured at hilar and aorto-mes-enteric sites of the left renal vein) be used as a criterionin diagnosing nutcracker syndrome [41].

Aneurysms

Aneurysms due to atherosclerosis usually occur in theinfrarenal aorta and common iliac arteries; however,they are also found in the renal arteries. Most renal ar-tery aneurysms have been found in persons 5070 yearsof age. Renal artery aneurysms can cause rupture,thrombosis, embolization, and dissection [42]. ColorDoppler US provides an effective, non-invasive meansof diagnosing renal artery aneurysm. Aneurysms maybe identified along the course of the main renal artery asan outpouching containing color flow. Slow velocitiesand a whirling pattern of flow can usually be observedon Doppler studies.

Color Doppler US can provide a quick, easy way indemonstration of aneurysmal dilatations of the vascularwall in the abdominal aorta. Most fusiform and saccularaneurysms of the aorta arise below the level of renal ar-teries, but it is nevertheless important to determinewhether the renal vessels are involved, since this alterspatient management. Aortic dissection may extend intoa renal artery, thus interrupting renal blood flow. Scan-ning along the longitudinal approach is the best way todemonstrate the relationships between the aneurysmand RA (Fig. 15).

Arteriovenous fistulas

Both congenital and postbiopsy renal AV fistulae can bediagnosed on color Doppler examination. The most fre-quent cause of AV fistula in both the native and trans-planted kidney is complication of percutaneous biopsy[43]. Small fistulas are not visible by conventional US,

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Fig.13 Chronic thrombosis of the right renal vein. Absence offlow in the main right renal vein. Collateral flow is clearly seen

Fig.14 Power Doppler US image of the thrombosed right renalvein

Fig.15 Color Doppler imaging of the abdominal infrarenal aneu-rysm. Aneurysm arises below the origins of both renal arteries


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and can be detected by color Doppler only. The AV fis-tula often appears on color Doppler US as a non-specificmosaic pattern with color aliasing, reflecting rapid flowrate and fine movements of tissues surrounding it [44].Two adjacent vessels can be usually recognized in whichwaveform analysis shows decreased RI and increasedpeak flow velocities in the afferent artery, and pulsatileflow pattern in the efferent vein (Fig. 16). Steal phe-nomena can case hypoperfusion of renal tissues sur-rounding the fistula [43]. Large AV fistulas are seen ascystic or complex structures in which color Dopplerdemonstrates vortices of high-velocity, low-impedanceflow. Although rare, AV fistulas are potentially lethalpathologic conditions. Failure to recognize properly anAV fistula, in fact, puts the patient at high risk of hemor-rhagic complications during a renal biopsy.

Renal allografts

The examination technique for renal allografts is mucheasier than that for native kidneys, due to the superficiallocation of the transplanted organ. The examination ofthe transplanted kidney should be performed using su-

perficial 7.5-MHz probes to study cortical perfusion andabdominal convex 3.5-MHz probes for direct visualiza-tion of the deeper structures such as transplanted arteryand vein.

The allograft vascular imaging protocol should in-clude visualization of the anastomotic region to excludevascular stenosis, anastomotic aneurysms, or false an-eurysms. Also accurate detection of the course and flowin the transplanted artery and vein should be performed

due to possible stenosis and occlusions. Using superfi-cial probes and Power Doppler mode, flow within thecortical region of the whole kidney should be studied.Intrarenal resistance indices (RI, PI) should be sampled.

Arteriovenous fistula and false aneurysms could befound after biopsy in transplanted kidney

In normal transplants the valuesof RI is less than 0.71,and shows a slight decrease toward the periphery. A re-duced diastolic flow velocity associated with an increasein intrarenal resistance index can be detected in acuteand chronic rejection reactions, urinary obstruction, ar-teriosclerosis of the vasculature, and acute tubular ne-crosis [45, 46, 47]. The significance of the changes in RI

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Fig.16 Arteriovenous postbiopsy fistula of the kidney. SpectralDoppler waveform shows low-resistance afferent artery spectrum.(Courtesy of L. E. Derchi, Genova)

a

b

Fig.17a, b Patient 12 years after transplantation of the kidney.Renal allograft dysfunction. Chronic rejection. a Power Doppler ofthe allograft. Marked decreasing of the cortical vascularity. b A 3DPower Doppler angiography shows reduced density and tortuosityof the interlobular vessels


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and the diagnostic predictive value of these indices inassessing the etiology of the transplant-related compli-cations remain controversial [48, 49]. Using superficialprobes a tiny blush through the whole cortical region upto the capsule should be seen in a normally functioningrenal allograft [50]. According to Martinoli et al. [60],loss of visualization of interlobular vessels at Power

Doppler US could be regarded as an additional hallmarkof renal transplant dysfunction. But abnormalities of theinterlobular vascular pattern in some patients withchronic rejection are probably nonspecific in identifyingthe nature of renal transplant dysfunction (Fig. 17).

An anastomotic stenosis could be assumed when theregistered maximum systolic velocities exceed150200 sm/s, shows a local increase of more than 50 %,or if the velocity related to the iliac artery increases be-yond a factor of 2.63.0 [51, 52, 53]. In a study of 109transplanted kidneys, a peak systolic velocity 2.5sm/sin the transplanted renal artery had a sensitivity of100% and specificity 95% for the detection of RAS,

although the use of measurements of RI, accelerationindex, and time in intrarenal vessels were less useful asdiscriminating diagnostic tests between normal andstenosed group of patients with transplants [54].

Occlusions of the transplanted or segmental arteriesshow complete or regionally limited flow in the intrare-nal vasculature [45, 55]. Absence of flow with pulsedand color Doppler is a valuable additional sign con-firming the diagnosis [56].

Arteriovenous fistulas can develop after percutane-ous biopsies of kidney transplants. They are recognizedby focal massively disturbed flow, with high intrarenalflow velocities [43].

Renal venous thrombosis can be confirmed by ab-sence of venous flow intrarenally and in the renal vein,as well as on the basis of high-impedance arterial Dop-pler signals, which present reversed flow during the di-astole [57].

Technical advances

Modern commercially available machines are now ableto outline with great detail the intraparenchymal vascu-lature of the kidney. Studies of cortical perfusion arepossible using relatively high-frequency (at least 5 MHz)high-resolution transducers and preferably using Power

Doppler. The use of Power Doppler with contrast agentsnot only facilitates this study but enables perfusion stud-ies similar to those created by isotope studies to beperformed [58]. Signals from interlobular vessels arevisible in the whole cortex, including the most peripheralregion close to the capsule. Harmonic imaging with con-trast agent injections can be used to map regional differ-ences in flow as well as quantitative measurements of acontrast agent's transit time and has the potential to as-

sess kidney abnormalities associated with renal bloodflow [59]. Studies of cortical perfusion are new, andtherefore only preliminary experiences have been re-

ported in the literature [56, 60, 61]; however, the first re-sults are very promising in establishing a patholog-icsonographic correlation in acute renal parenchymalinflammation [62] and in renal allograft evaluation [56,60, 61]. It must be remembered that care should be takenin diagnoses of perfusion defects, since absence of de-tectable flow at the interlobular level does not alwayscorrespond to cortical areas that lack perfusion on other,more reliable techniques, such as MR [60].

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Fig.18 Ultrasound contrast study of renal perfusion. Flash Echoimaging. (Provided by Toshiba Medical Systems Europe)

Fig.19 Three-dimensional US angiography of the both renal ar-teries and the entire aorta


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In clinical practice, the enhanced US signal providedby contrast agents can reduce the number of technicallynondiagnostic cases, as well as the number of false-neg-ative results.

With additional use of contrast agents and phase in-version harmonic imaging the whole course of the renalarteries could be imaged without motion or aliasing ar-tifacts. Harmonic ultrasound with contrast agents addedblood flow morphology maps within the background of

the B-mode volumetric data. Combination with contrastagents provides the potential to image the whole vascu-lar renal network and, through special measurementtechnique (Flash Echo), to estimate vascular volumeand transit time, parameters which relate directly to tis-sue perfusion (Fig. 18).

One of the promising and rapidly developing tech-niques is three-dimensional US angiography [63, 64, 65,66, 67]. Three-dimensional US can overcome somedrawbacks of 2D US and can provide the angiogram-likeimages of both, renal arteries and the entire aorta(Fig. 19). Careful freehand scanning with a smooth, lin-ear translation, or sector sweep, can acquire 3D data setsin a single breath-hold. Different approaches can be usedto acquire 3D data sets of renal vessel: anterior or ante-

1913

a

b

Fig.20 a A 3D US angiography of the right kidney. Maximum in-tensity projection image demonstrates accessory renal artery. b A3D volume-rendering image of the left kidney. Accessory left renalartery

a

b

Fig.21 a A 3D US angiography of the right renal arteries andaorta. Accessory renal artery is clearly seen. b Same patient: cor-relative conventional angiography


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rolateral, for evaluation of both RA and entire aorta; andcoronal for visualization of renal vasculature, main RA,and entire aorta. After acquisition of 3D data, postpro-cessing is performed using maximum and minimum in-tensity projections (MIP or MinIP) to obtain angiogram-like 3D images for further analysis (Fig. 20). We believethat 3D US angiography has the potential to become ex-

cellent for screening in evaluation of accessory renal ar-teries especially in children, young adults, patients withrenal failure, allergy to iodinated contrast agents, fear ofionizing radiation, or arterial catheterization. Magneticresonance angiography, with its high cost and less avail-ability, should be reserved for problem cases.

It is known that accessory renal arteries are foundfrequently. Previous researchers have reported poorability of color Doppler US to depict accessory renalarteries [15]. Contrast agents and harmonic imagingmay have a role to play in future and increase the sen-sitivity and specificity of color Doppler US in detection

of accessory renal arteries [68]. Three-dimensional USangiography can enhance the possibility of 2D US inevaluation of accessory renal arteries. An angiogram-like image of 3D US angiography is becoming an excel-lent alternative to conventional angiography (Fig. 21).This method is promising as the primary study for ac-cessory RA and for determining the relationships of the

origins of the RA to abdominal aneurysms, and can bethe preferred technique for patients with a contraindi-cation to conventional angiography.

Further developments of computing power, wide-spread diffusion of state-of-the-art US machines, ma-trix-array transducers, harmonic imaging techniquesand reconstruction algorithms for surface and volumerendering will lead real-time 3D US scans to becomeroutine in clinical practice.

Acknowledgements I am grateful for all the help provided by L.Derchi during preparation of this article.

1914

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