The Kidneys

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Chapter 9

image The Kidneys

Renal Vascular Doppler Ultrasound

Ultrasound is the imaging modality of choice for evaluation of the kidneys, especially in patients with borderline renal function, the incidence of which is increasing. In comparison with other modalities, ultrasound has the distinct advantage of providing clinically diagnostic information without the need for ionising radiation or contrast agents. The combination of spectral, colour, and/or power Doppler is extremely helpful in renal vasculature evaluation.

CLINICAL CONSIDERATIONS

Common clinical indications for renal ultrasound include renal insufficiency and renal failure. Specifically, the request to exclude renal obstruction as the aetiology of acute renal failure leads the list. Doppler ultrasound is not routinely performed to evaluate acute renal failure but may be prompted by certain clinical indicators (Box 9-1) or greyscale findings. Colour and spectral Doppler is more commonly used in the native kidneys for evaluation of unexplained or uncontrolled hypertension caused by renal artery stenosis (RAS) or for determination of vessel patency. In hypertensive patients, some authors suggest Doppler should be reserved for those patients with a strong clinical suspicion for RAS who are likely to benefit from intervention.1 As will be shown, there are many other vascular abnormalities than can be demonstrated by Doppler, and these can present with a wide variety of symptoms or signs.

BOX 9-1   CLINICAL CRITERIA USED TO SELECT WHO SHOULD BE EVALUATED FOR RENAL ARTERY STENOSIS (RAS)

TECHNICAL CONSIDERATIONS

Renal Doppler ultrasound can be one of the most challenging vascular ultrasound examinations. Extensive training and experience in performance and interpretation of the study will reduce examination time, improve study quality, and optimise diagnostic accuracy. A dedicated quality control programme is an important mechanism to assess accuracy and to review cases in which an incorrect diagnosis was made. The goal of this review process should be to improve the quality of future studies.

One common challenge in renal vascular ultrasound is direct visualisation of the proximal renal arteries. Overlying bowel gas may completely obscure the renal vascular origins and result in a non-diagnostic study. To improve the likelihood of a diagnostic study, we request that patients be fasting for at least 6 to 8 hours, when possible, to decrease bowel gas. Another limitation is the deep location of the native renal vessels, especially in obese patients, and utilisation of appropriate sonographic windows may be helpful. Graded transducer pressure can bring the transducer closer to the arteries and simultaneously displace overlying gas. However, the renal arteries occasionally may not be directly visualised despite optimal technique; in some of these technically difficult cases, the identification of segmental arterial waveform abnormalities may still allow successful diagnosis of RAS.

Main Renal Artery Evaluation

Doppler evaluation of the renal arteries should not occur without a thorough greyscale examination of the kidneys. Greyscale imaging can provide useful information about renal size and cortical thickness and should be part of the initial series of images. For Doppler image acquisition, a preliminary scan of the abdominal aorta is performed with colour Doppler in the transverse plane beginning at the level of the superior mesenteric artery (SMA) to locate the main renal arteries, which typically originate within 2 cm of the SMA (Fig. 9-1). Transverse images may be obtained from a midline approach with the patient supine or rolled into the left lateral decubitus position. The imager can localise the right renal artery passing posterior to the IVC then rotate the transducer while maintaining the artery in view. Also, the transducer can be placed longitudinally lateral to the rectus muscle resulting in a ‘banana peel’ image (Fig. 9-2), in which the aorta is the banana and the renal artery is the banana skin on each side, being peeled off the banana. If the main renal arteries cannot be demonstrated from the midline approach, a right or left lateral approach is used to follow each artery centrally from the renal hilum. Regardless of the technique used to identify the renal artery origin, the entire main renal artery should be visualised sonographically. Lack of visibility of even a 10 mm segment of main renal artery will limit the sensitivity of the direct method for RAS. This is especially relevant in younger patients in whom fibromuscular dysplasia is a concern; stenosis in these patients may not be near the renal artery origins (described later). As part of the direct Doppler evaluation, the peak systolic velocity with angle correction should be measured at the renal artery origin, mid and distal artery, and at any region of turbulent disorganised flow with aliasing on colour Doppler.

Accessory renal arteries occur commonly (approximately 30% of kidneys), but are not always demonstrated sonographically. In fact, studies suggest that only 21–41% of accessory renal arteries are visualised by Doppler evaluation.2,3 This low success rate has prompted some individuals to argue that sonographic evaluation for RAS is not sensitive enough as a screening study. However, Bude et al. found that less than 1% of accessory renal arteries were the only stenotic artery,4 which essentially negates the significance of not visualising an accessory renal artery.

As mentioned earlier, the deep location of main renal arteries often limits their direct evaluation, and it will drive the choice of transducer. Lower-frequency transducers will have better sonographic depth penetration, with a trade-off of decreased spatial resolution. As a general rule, the highest-frequency transducer that allows good demonstration of the artery and arterial waveform is preferable.

Greyscale visualisation of the renal arteries should be optimised prior to colour and spectral Doppler evaluation. Doppler gain should be adjusted for flow detection by increasing the gain to a level just below the appearance of colour artifact in adjacent structures. Pulse repetition frequency, or velocity scale, is the frequency of sampling, and under-sampling may underestimate peak velocities. In newer systems, built-in software can automatically optimise these parameters, with manual adjustment occasionally required by the sonographer. For spectral Doppler, the Doppler gate should be set to include the entire arterial lumen and angled to the direction of flow. The angle of insonation should be maintained at 60 degrees or less. As angulation increases to 80–90 degrees, the confidence in the measured velocity decreases, as the cosine of the angle of insonation approaches zero. This yields large differences in measured velocity for a small variation in the relative angle of flow.

Segmental Intrarenal Artery Evaluation

When the main renal artery is not well seen in its entirety, evaluation of the segmental intrarenal arteries may allow a non-diagnostic direct examination to become diagnostic for RAS.5,6 We always examine the segmental arteries even when the main renal arteries are well seen, because the segmental artery waveform morphology may be useful in detecting concomitant renal parenchymal disease.

A posterior flank approach reduces the distance from the transducer to the segmental arteries. Of note, the liver and spleen should not be used as a window to improve visualisation of the kidney, as the vessels are closer to a zero degree angle with the transducer positioned using a more posterior approach. The upper, interpolar, and lower pole segmental arteries are individually studied. A heel–toe technique is commonly applied in which one edge of the transducer is angled into the skin to align the targeted segmental artery flow as close to the transducer angle of insonation (theta less than 20 degrees) as possible to enhance the signal quality. This can enhance the definition of the early systolic peaks. Electronic beam steering can also be used to better align angulation of the insonating beam to enhance waveform morphology.

Characteristics of the spectral Doppler tracing in normal segmental intrarenal arteries should include rapid upstroke to an early systolic peak with gentle decrease in flow velocity during late systole and diastole (Fig. 9-3). Persistent antegrade flow throughout the cardiac cycle should be present without return to baseline. The resistive index (RI), calculated as:

image

is a common parameter for characterisation of arterial flow. The RI is inversely proportionate to the relative amount of diastolic flow. For instance, an end diastolic velocity that is 20% of the peak flow will result in RI of 0.80. The upper limit of RI in normal adults has been reported as less than 0.70,7 but concern for pathology is not often raised until the RI is 0.75–0.80, or higher. Furthermore, the RI may be affected by other factors such as heart rate, Valsalva, and arterial compliance. In fact, RIs greater than 0.70 are common in elderly patients.8 The impact of systemic vascular disease in chronic renal dysfunction is significant. It has been recently suggested that the renal RI measurement does not distinguish local from systemic vascular damage. A new potential ultrasound measurement, the difference of RIs between the spleen and kidney, may allow more specific evaluation of renal parenchymal damage.9 However, this study has not yet been further validated or widely applied in practice.

Anatomy of the Native Kidneys

ARTERIAL ANATOMY

The renal arteries typically arise from the abdominal aorta caudal to the level of the SMA. The right renal artery usually originates from the anterolateral aspect of the aorta, while the left renal artery usually originates from the posterolateral aspect. As noted earlier, approximately 30% of patients will have more than one renal artery.10 Accessory renal arteries usually arise from the aorta caudal to the main renal artery to supply the renal lower pole, but occasionally will course cranially to supply the upper pole. Rarely, accessory arteries may arise from an iliac artery or even the SMA. Renal anomalies such as horseshoe or pelvic kidney almost always have multiple renal arteries, which may arise from the aorta or iliac arteries.

The main renal artery divides into dorsal and ventral rami that course posterior and anterior to the renal pelvis. The anterior and superior aspects of the kidney are typically supplied by the larger ventral division. The posterior and inferior portions of the kidney are supplied by the smaller dorsal division. The junction of these ventral and dorsal divisions creates a relatively avascular plane (Brodel’s line), which is the preferred track of percutaneous nephrostomy placement, and should be considered when performing a renal biopsy.

The branching pattern of the renal arteries progresses symmetrically to the renal cortex (Fig. 9-4). Segmental branches arise from the dorsal and ventral rami and run along the infundibulae before dividing into interlobar arteries. These interlobar arteries course between the pyramids, and then branch into arcuate arteries, which run along the bases of medullary pyramids. Within the cortex, small interlobular arteries course outward toward the surface of the kidney.

VENOUS ANATOMY

The renal venous anatomy parallels the arterial anatomy. Normal venous flow on spectral Doppler has a relatively low velocity. Its waveform is driven by right atrial activity. Accessory left renal veins are less frequent than accessory renal arteries; however, accessory right renal veins are quite common. Left venous anomalies may be seen in approximately 11% of patients.11 Variants most commonly include the retroaortic and circumaortic renal veins (Fig. 9-5), and these may be clinically relevant even beyond filter placement. In a recent study by Karazincir et al., the incidence of retroaortic left renal vein was found to be significantly higher in patients with varicocele, compared with controls12 (see Fig. 9-24 in the varicocele section).

The left renal vein receives drainage from the inferior phrenic, capsular, ureteric, adrenal and gonadal veins and flows across midline into the normal IVC. In patients with a left-sided IVC, the left common iliac vein continues cranially as the left IVC and drains into the inferior aspect of the left renal vein. The right renal vein is shorter than the left and courses obliquely into the IVC. The right renal vein receives capsular and ureteric veins; however, the right inferior phrenic and gonadal veins enter directly into the IVC. Valves may be present within the renal veins, but their reported incidence varies greatly. Renal vein varices may be secondary to renal vein thrombosis or portal hypertension, or they may be idiopathic. Like varicoceles, renal varices are more common on the left than the right. In cases of left renal vein thrombosis, the varices may extend through the inferior phrenic, adrenal, gonadal, and ureteric veins. On the right, the only common branch is the ureteric vein.

Renal Failure and Obstruction

Doppler can play a supportive role in the diagnosis or exclusion of renal obstruction in patients with acute renal failure. Identification of a dilated renal collecting system is fairly easy with ultrasound. The difficulty, however (in the absence of prior examinations) is the differentiation of an acutely obstructed high-pressure system versus that of a low-pressure, chronically dilated system. It has been suggested that elevated resistive index may help differentiate between severe acute urinary obstruction and chronic dilatation.13–15 The RI of the obstructed kidney may be elevated relative to the normal contralateral kidney. An RI difference of greater than 0.10 between the non-obstructed and obstructed kidney is the suggested threshold for diagnosis of acute obstructed uropathy. However, intra-renal autoregulatory hormonal systems counteract the mechanical effect of the high-pressure collecting system pressing upon the parenchyma. This rapidly modifies the resistance to flow, reducing sensitivity of the test. In the setting of partial obstruction or less severe obstruction, this finding also lacks sensitivity.16,17 As an aside, in cases of chronic renal disease without obstruction, elevated RI > 0.80 has been shown to be associated with worsening renal function and mortality.18

Another Doppler tool to assist in the evaluation of urinary obstruction can be performed within the bladder. In cases with suspected renal obstruction, sonographic evaluation for a ureteral jet should be a component of the renal ultrasound examination (Fig. 9-6). Although entry of urine into the urinary bladder is not synchronous, demonstration of three or more ureteral jets by Doppler on one side without a single pulse of flow from the contralateral side implies obstruction of the non-pulsing ureter.

Nephrolithiasis

Colour Doppler evaluation should be a routine component of the renal ultrasound examination when nephrolithiasis is suspected. Greyscale alone has poor sensitivity for small renal stones. Using colour Doppler, twinkle artifact19 can increase confidence in the presence of renal, ureteral or bladder stones (Fig. 9-7). The irregular surface of the calculus causes a Doppler shift which manifests as a noisy colour and spectral signal. This helps confirm that a bright echogenic focus within the renal hilum is indeed a calculus, rather than bright echogenic renal hilar adipose tissue. This technique should be used to demonstrate the presence of the stone, not to measure size of the stone. This is an especially useful adjunct to evaluate for distal ureteral stone with endovaginal technique when a dilated upper renal collecting system is identified in pregnancy (Fig. 9-8).

Renal Tumours

Greyscale ultrasound is the primary sonographic technique for detection of a renal tumour, but colour Doppler can provide additional information for surgical planning. Colour Doppler may be useful in intraoperative ultrasound examinations when trying to precisely identify the depth of tumour invasion (Fig. 9-9). Colour Doppler may also be helpful to confirm the extent of renal cell tumour invasion into the inferior vena cava (Fig. 9-10). This may be helpful for surgical planning by clearly delineating the cranial extent of tumour. Colour Doppler can help confirm whether tumour involves the intrahepatic portion of the IVC, which would significantly increase the complexity of surgical removal. The presence of arterial signal within the thrombus confirms it as tumour thrombus (versus bland thrombus).

Renal Vascular Abnormalities

RAS/HYPERTENSION

Renal vascular disease is an uncommon cause of hypertension; however, it is potentially curable, and is most commonly considered in young adult patients with abrupt onset of hypertension and uncontrolled or rapidly accelerating hypertension. Stenotic renovascular disease has two primary aetiologies, atherosclerosis and fibromuscular dysplasia. In older patients the underlying aetiology is most frequently renal ostial atherosclerotic disease. The second most common aetiology is fibromuscular dysplasia, which is an uncommon disorder seen most frequently in younger women, and unlike atherosclerotic lesions, generally responds well to angioplasty.20,21 The role of imaging is influenced by the potential benefit of intervention.1 Although renal artery stenosis can be identified with imaging, recent studies suggest that simply the identification of stenosis does not justify invasive treatment by stenting. Two recent randomised trials, including the ASTRAL study, have shown no benefit to revascularisation of atherosclerotic lesions, with regards to either blood pressure or renal function.22,23 The ASTRAL study found that there was no significant difference in two randomised cohorts of patients between medical management versus stent therapy for the treatment of RAS. The long-standing ischaemic insult to the renal parenchyma with associated cholesterol crystal embolisation causes significant damage that does not benefit from reestablishment of more normal flow through the main renal artery. Evaluation of renal perfusion in the presence of stenosis may be more useful to select patients who will benefit from intervention. Some authors have suggested Doppler measurement of resistive index, with high resistance behind a stenosis indicating significant parenchymal damage. A patient with a low-resistance tardus-parvus waveform may, however, still benefit. Recent advances in MR perfusion technology allow measurement of perfusion/diffusion and tissue oxygenation, making it more accurate in this prediction.

The results of the ASTRAL trial have stirred much controversy in the literature. Recent reviews suggest there still may be a role for imaging and stenting in patients with acute-onset hypertension or rapidly accelerating hypertension. This is suggested for those patients in whom parenchymal renal damage has not yet advanced to the point of irreversible hypertension.

Although CT and MR angiography are probably used more frequently for evaluation of RAS in most institutions, ultrasound is an ideal initial screening study because of the lack of ionising radiation or contrast administration. In ultrasound programmes experienced with renal vascular ultrasound, recent studies have shown the sensitivity and specificity of Doppler ultrasound for RAS is up to 95% and 90%, respectively.24

There are two main methods for sonographic detection of RAS: direct demonstration of RAS and indirect assessment of the downstream effect of the stenosis on the segmental renal arteries25 (Box 9-2). Greyscale findings are first considered and a renal length disparity greater than 2 cm with cortical thinning should raise concern for the diagnosis. Visualisation of turbulent flow within the renal artery on colour Doppler suggests an area of stenosis.26 Increased velocities in the focal area of stenosis may appear as colour aliasing if the velocity is beyond the scale limits and wraps into the other end of the colour scale (Fig. 9-11), and should lead to close evaluation with spectral Doppler in this area. Spectral Doppler confirmation of RAS relies primarily on demonstration of elevated PSV and demonstration of disturbed flow distal to the lesion.

BOX 9-2   SUMMARY OF DOPPLER PARAMETERS FOR DIAGNOSIS OF RAS

Varying criteria have been suggested for the direct spectral Doppler diagnosis of RAS, which has resulted in controversy. A PSV of greater than 200 cm/s (Fig. 9-12) has been suggested for Doppler diagnosis of 60% diameter reduction of the renal artery.27,28 In a recent meta-analysis, PSV was the best critical factor in diagnosing RAS, with sensitivity and specificity of 85% and 92%, respectively.29 The ratio of the renal artery PSV to the aortic PSV (RA/Ao) is another criterion suggested for diagnosis of RAS. An RA/Ao PSV ratio of greater than 3.5:1 (Fig. 9-13) suggests significant RAS, yielding 91% sensitivity and 91% specificity.30 An elevated ratio of peak renal artery systolic velocity to distal renal artery systolic velocity has also been suggested as a criterion for diagnosis of RAS.31,32 One study of 187 renal arteries with angiographic correlation also showed an absolute renal interlobar PSV of less than 15 cm/s resulted in 87% and 91% sensitivity and specificity, respectively, for Doppler diagnosis of 50% stenosis.32 Jian-Chu et al. have recently studied the impact of atherosclerosis and age on Doppler sonographic parameters for the diagnosis of RAS and suggest that use of the renal-aortic ratio and renal-interlobar ratio diagnostic thresholds differ in patients older versus younger than 46 years. Other sonographic criteria were not substantially affected by patient age in their study.33

The indirect method of RAS assessment of segmental renal artery waveforms becomes important when the entire length of the main renal artery cannot be directly seen with ultrasound. Stavros et al.5,34 have suggested that normal intrarenal waveform morphology with early systolic peak in the upper, interpolar, and lower pole segmental renal arteries may be used to adequately exclude significant RAS. This normal compliance peak (Fig. 9-14) is not present when there is flow-limiting stenosis in the proximal artery, although others have found this sign less sensitive.35

Another criterion of RAS is prolonged acceleration time of greater than 0.07 second. Acceleration time is the time interval from onset of systole to the early systolic peak. In blunted waveforms, this early peak may be absent, and the measurement should extend from onset of systole to the first point of deflection.36 The presence of the tardus–parvus waveform morphology is helpful in the diagnosis of severe RAS; however, its absence does not exclude RAS.37 The tardus–parvus spectral waveform is defined by the slow upstroke (the tardus) and spectral broadening with blunting of the systolic peak (the parvus) (Fig. 9-15). It is important to note that in patients with atherosclerotic disease, vessel compliance may be diminished, making the tardus–parvus waveform morphology less obvious.38

A less common criterion suggested for diagnosis of RAS is excessive difference in RI of the two kidneys (≥ 0.07).5,39 For lesser degrees of stenosis, the abnormal kidney will show lower RIs beyond the point of stenosis due to post-stenotic dilatation. However, as the stenosis becomes more flow limiting, there may not be persistence of flow throughout the cardiac cycle, resulting in a high RI.

Another potential application of Doppler is for follow-up to assess for restenosis in patients after stent placement for RAS (Fig. 9-16). This may be ordered due to worsening renal function or even when clinical signs of restenosis are absent. In a study of 64 stented patients without clinical suspicion of restenosis, 22 patients had elevated PSV of greater than 200 cm/s and had significantly worsened rate of renal function decline than those patients with PSV of less than 200 cm/s.40 In 6 of 11 of these patients with this criterion who underwent angiography despite the lack of clinical suspicion, all had restenosis of greater than 70% at angiography.

RENAL ARTERY THROMBOSIS

Complete thrombosis of the native renal artery is uncommon except in the setting of stents or bypass grafts for abdominal aortic aneurysm repair, or in the setting of trauma. In the setting of trauma, there may be avulsion of the renal artery with or without extravasation. Traumatic dissection of the artery can also result in arterial occlusion, and can have similar Doppler findings as renal artery avulsion. Asymmetric lack of flow may involve the entire kidney or a segment, dependent on the level of the abnormality. Colour Doppler can show a perfusion defect, but power Doppler is often used to characterise any slow flow in this region. If no flow is identified, optimisation of the Doppler settings should be performed using the contralateral kidney prior to an additional insonation of the affected kidney.

A regional lack of power Doppler flow in the renal parenchyma with wedge-shaped appearance suggests segmental infarction (Fig. 9-17). Search for other similar abnormalities in the contralateral kidney or other organs should be performed since this may be due to showering of emboli from a remote source.

RENAL ARTERY ANEURYSM

The diagnosis of renal artery aneurysm or pseudoaneurysm is most commonly made by CT or MRI. However, there are findings on Doppler that may be seen in this abnormality. It may appear as a vascular structure that is fusiform, eccentric, and saccular. RAA commonly arise from a branch point within the artery. On colour Doppler, there may be circular flow with a ‘yin-yang’ appearance (Fig. 9-18). A portion of the aneurysm may be thrombosed. It is common for RAA to have peripheral calcifications, and these may limit sonographic evaluation of central flow.

ARTERIOVENOUS FISTULA AND MALFORMATIONS (AVF AND AVM)

Arteriovenous fistula in the native kidney is rare except in cases of prior renal biopsy and will not be suspected sonographically if colour or power Doppler is not evaluated. Colour or power Doppler characteristically will show a large tortuous cluster of vessels. Spectral Doppler waveforms of the renal arteries feeding the fistula will show high velocity and low resistance (Fig. 9-19). The main renal vein may be dilated and arterialised waveforms may be found in veins near the fistula (Fig. 9-20).

VENOUS DISEASE

Renal Vein Thrombosis

Renal vein thrombosis (RVT) of the native kidneys is seen more commonly in the paediatric population than in adults. In adults, the underlying aetiology may include dehydration and nephrotic syndrome,41 hypercoagulable state, or trauma to the renal vein. Visualisation of renal vein flow with Doppler is critical in patients with clinical suspicion for acute RVT. It is important to demonstrate the entire renal vein before excluding renal vein thrombosis because the native kidney may develop collaterals quickly.

The diagnosis of RVT is based upon demonstration of thrombus filling the renal vein (Fig. 9-21) or non-occlusive thrombus surrounded by venous flow. In some cases, the vein may be expanded by the thrombus. The absence of flow in the renal vein without demonstration of thrombus may suggest RVT; however, demonstration of low-level colour signal within the vein on Doppler does not exclude the possibility of non-occlusive or occlusive renal vein thrombus. Monophasic venous waveforms are abnormal but not specific for RVT.42 Because of the potential for thrombus to extend cranially within the IVC (Fig. 9-22) and the effect on clinical management, the IVC should be imaged as part of the sonographic examination when evaluating for RVT.

Renal vein thrombus may also be seen in patients with renal cell carcinoma if there is tumour extension into the renal vein. In these cases arterial flow may be seen within the thrombus on spectral Doppler.

Nutcracker Syndrome

The ‘nutcracker’ phenomenon results from compression of the left renal vein between the superior mesenteric artery and the aorta and may lead to left renal vein hypertension, haematuria, and varix formation. It is important to remember that a distended left renal vein may be seen in some of the normal population by CT, MR, or ultrasound. Therefore, other criteria should be applied, including a high Doppler velocity ratio (Fig. 9-23) and high venous diameter ratio as described in recent literature.43 Due to the complexity of the potential surgical repair, measurement of a pressure gradient between the IVC and the left renal vein may be needed as confirmation before clinically significant renal vein compression is diagnosed. Visualisation of blood from the ureteral orifice on retrograde ureteroscopy may also be supportive. Colour flow Doppler may provide noninvasive evidence of renal vein compression with peak velocity ratio greater than 5:1 when collateral veins are demonstrated. On CT, there may be a sharp change in venous calibre as the vein crosses the SMA, usually with a ‘beaked’ appearance.

Varicocele Formation

Varicocele is dilatation of the pampiniform venous plexus seen on testicular ultrasound examination, and is more common on the left due to anatomical factors of angle of insertion of the gonadal vein into the left renal vein and potential for compression of the left renal vein by the SMA. The primary type of varicocele is associated with incompetent valves, and the secondary type is associated with increased venous pressure due to obstructed venous outflow. Isolated right varicocele is uncommon and should warrant further evaluation of the right retroperitoneal area and kidney to exclude a right renal hilar mass or adenopathy compressing venous outflow. Diagnosis of varicocele is made on testicular ultrasound examination when the veins in the spermatic cord area are dilated to greater than 2 to 3 mm in diameter (Fig. 9-24). Rarely, the varix may be intratesticular in location. In some cases, colour Doppler of the dilated veins may show intraluminal thrombus. Reversal of venous flow at rest with increased reversed flow during Valsalva is suggestive of the diagnosis, but in some patients there may only be reversal during Valsalva.44 Techniques used to improve detection of varicocele include Valsalva or standing position. As noted earlier, a recent study by Karazincir et al. showed the incidence of retroaortic left renal vein was significantly higher in patients with varicocele compared with controls.12

Summary

Ultrasound plays an important role in the diagnosis and management of renal disease. Ultrasound also plays an extremely important role in the initial evaluation for RAS. The quality and accuracy of vascular ultrasound is dependent on the volume of cases, as well as the skill and experience of the sonographers and sonologists interpreting the examination. The high attention to detail required in performing and interpreting renal vascular ultrasound examinations will continue to limit its widespread application, as compared to CTA and MRA examinations. However, renal vascular ultrasound will continue to offer the strong advantage of not requiring potentially nephrotoxic intravenous contrast agents in patients with diminished renal function or acute renal failure. Future improvements in ultrasound technology and more widespread practice of optimal techniques should be able to keep up with the predicted increase in patients with renal insufficiency.

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