Utility of renal artery ultrasound for detecting stenosis or obstruction: A narrative review

Article information

Precis Future Med. 2023;7(2):55-61

Publication date (electronic) : 2023 April 7

doi : https://doi.org/10.23838/pfm.2023.00009

Department of Radiology, Samsung Medical Center, Sungkyunkwan University School of Medicine, Seoul, Korea

Corresponding author: Byung Kwan Park Department of Radiology, Samsung Medical Center, Sungkyunkwan University School of Medicine, 81 Irwon-ro, Gangnam-gu, Seoul 06351, Korea Tel: +82-2-3410-6457 E-mail: rapark@skku.edu

Received 2023 January 15; Revised 2023 March 9; Accepted 2023 March 14.

Abstract

Acute renal failure can result from many conditions, in which renal artery stenosis (RAS) or renal artery obstruction (RAO) can be corrected with appropriate interventional or surgical procedures. The renal artery can be directly depicted with currently available ultrasound (US) scanners. Therefore, we can determine if there is RAS or RAO in patients with poor renal function using renal artery US. However, there are few reports about the utility of renal artery US in excluding stenosis or obstruction. The purpose of this review is to show clinical applications and diagnostic pitfalls of renal artery US in detecting RAS or RAO.

Keywords: Acute kidney injury; Kidney; Obstruction; Renal artery; Ultrasonography

INTRODUCTION

Computed tomography (CT) [1-4] or magnetic resonance imaging (MRI) [5-8] angiography has been accepted for assessing the renal artery and is most commonly performed prior to donor nephrectomy [9-12]. However, this imaging modality is difficult to conduct in patients with poor renal function because intravenous injection of contrast material is essential. Iodine or gadolinium can result in contrast-induced nephrotoxcity toxicity [13-16] or nephrogenic systemic fibrosis [17-20].

Ongoing development of ultrasound (US) scanner technology has improved image resolution so that deep-seated organs can be assessed. Prior to 2000, several investigations reported that US has difficulties in assessing the renal artery for stenosis or obstruction [21-23]. Accordingly, they relied on intra-renal Doppler US in diagnosing renal artery stenosis (RAS) or renal artery obstruction (RAO). Currently, the renal artery can be directly depicted with currently available US scanners [24-27]. RAS or RAO can be precisely identified with renal artery US. This imaging modality does not require CT or MRI contrast material, which should be excreted from the kidney in patients with poor renal function. Even though US contrast materials can be used to assess RAS or RAO, microbubbles can be excreted to the pulmonary circulation [28-31]. Therefore, renal artery US is a safe screening examination for those with suspected RAS or RAO. However, there are few reports about how renal artery US can be applied or its limitations. The purpose of this review is to show clinical applications and diagnostic pitfalls of renal artery US in detecting stenosis or obstruction.

CLINICAL APPLICATIONS

Follow-up of RAS

Renal artery US is useful for follow-up after angioplasty [32- 35] or stenting is performed [36-39]. It is important to identify re-stenosis when renal function or blood pressure is not well controlled during the follow-up period. The early stage of re-stenosis does not show pulsus tardus or parvus pattern in the kidney (Fig. 1). Therefore, the renal artery should be carefully evaluated to detect re-stenosis. Color or spectral Doppler US can provide key findings to determine re-stenosis following angioplasty or stenting (Fig. 1). Frequently, a renal artery stent obscures frequency shift from Doppler US due to posterior shadowing. If peak systolic velocity (PSV) is elevated in the renal artery distal to the stent, re-stenosis can be diagnosed. A large decrease in blood flow is another finding suggesting re-stenosis. Contrast-enhanced US can offer added value in depicting blood flow within a stent (Fig. 2).

Fig. 1.

Follow-up renal artery ultrasound (US) in a 66-year-old woman with angioplasty. (A) Color Doppler US shows no sign of re-stenosis in the left renal artery (white arrow) behind the left renal vein (LRV). She underwent angioplasty of the left renal artery stenosis 1 month prior. (B) Spectral Doppler US does not show pulsus tardus and parvus in the left kidney. This US scan was performed 2 years after the last angioplasty due to decreased renal function. There was no sign of re-stenosis on renal Doppler US. (C) Color Doppler US shows clear restenosis (black arrow) in the left renal artery. (D) Spectral Doppler US shows a high peak systolic velocity (270 cm/sec) in the post-stenotic area (white arrow). Systolic velocity is difficult to measure in the stenosis, in which the frequency shift is too small to detect. Subsequent angiography confirmed re-stenosis, and angioplasty was repeated.

Fig. 2.

Poor breath-holding in an 81-year-old woman with bilateral renal artery stenosis. (A) Color Doppler ultrasound (US) shows markedly decreased perfusion in the right kidney. Spectral Doppler US could not be obtained due to poor breath-holding in addition to poor renal perfusion. The size of the right kidney was 8.5 cm. (B) Gray-scale US shows smaller left kidney (7.5 cm), in which the left renal cortex is thinner and hyperechoic than those in the right. (C) Color Doppler US shows focal stenosis (white arrow) and post-stenotic dilatation (asterisk) in the right renal artery. (D) Color Doppler US shows another stenosis (white arrow) and post-stenotic dilatation (asterisk) in the left renal artery. The left renal artery stenosis is more severe, but post-stenotic dilatation is smaller compared to those in the right. The right renal artery is larger than the left. (E) Aortography shows clear bilateral renal artery stenoses (white arrows) and post-stenotic dilatations (asterisks). The stenotic degree is larger, but the post-stenotic dilatation is smaller in the right renal artery than in the left. The right renal artery is larger than the left. After considering imaging features of renal artery US and aortography, we decided to conduct angio-intervention in the right renal artery because it appears more effective in controlling renal function or blood pressure. (F) Angioplasty and stenting (white arrow) are successfully performed to dilate stenosis in the right renal artery alone. (G) Contrast-enhanced US shows the morphologic changes of right renal artery stenosis before and after angio-intervention (white arrow). This US technique clearly shows good blood flow within the stent.

Poor breath-holding

Breath-holding is necessary to obtain precise Doppler spectrum in the kidney. Accordingly, patient cooperation is essential during renal Doppler US. Many patients with RAS or RAO are old and/or have co-existing morbidities, and it is frequently difficult for them to hold their breath (Fig. 2). However, renal artery US does not require breath-holding for obtaining a renal artery Doppler spectrum (Fig. 2). First of all, the left renal vein should be searched with gray-scale US because renal arteries can be present behind it [24-27]. Then, gray-scale and Doppler US can depict RAS or RAO directly. As RAS becomes severe, the frequency shift is often not sufficient for detection. Thus, post-stenotic dilatation should be determined on a Doppler spectrum (Fig. 1). A PSV greater than 180 to 200 cm/sec in this area [24,40-44] or a PSV reno-aortic ratio (RAR) [44-47] greater than 3.5 strongly suggest RAS. The sensitivity and specificity of PSV > 180 to 200 cm/sec for RAS range from 85%–97% and 72%–98%, respectively [27,42,43,48]. The sensitivity and specificity of RAR > 3.5 for RAS range from 91%– 92% and 71%–95%, respectively [45-47].

Acute kidney injury

Contrast-enhanced CT or MRI is difficult to perform in patients with acute kidney injury (AKI) because of poor renal function [13-20]. Renal artery US helps to differentiate various conditions resulting AKI (Fig. 3). This US technique can exclude the possibility of RAS as an etiology of AKI. Therefore, if the renal artery is normal, radiologists should evaluate kidney size, echogenicity, perfusion, or obstruction with renal US to determine if AKI is pre-renal, renal, or post-renal [49-51]. For this reason, renal artery or renal US scan is recommended as a primary test for assessing patients with AKI rather than CT or MRI examination. A pre-renal etiology for AKI is most common [51]. The other etiologies include acute tubular necrosis, sepsis, hypovolemia, and drug toxicity [50,52]. Renal artery US helps nephrologists to manage AKI by excluding the likelihood of RAS [49-51].

Fig. 3.

Acute obstruction of the left renal artery in a 62-year-old man. (A) Non-contrast computed tomography (CT) shows a hyperdense sign (white arrows) in the distal left renal artery, suggesting acute thrombus. His renal function relies on a single left kidney because the right kidney is severely atrophic. (B) Color Doppler ultrasound (US) shows very weak blood flow in the proximal left renal artery (white arrows). (C) Arteriography shows acute thrombus (black arrow) as a filling defect in the left renal artery. Thrombolysis was performed to treat it. (D) Color Doppler US shows re-canalized left renal artery (white arrows), in which good blood flow is restored after thrombolysis.

Guidelines for angio-intervention

Renal artery US can offer good imaging features for detecting early stage RAS, for which angioplasty or stenting is a useful treatment. However, many investigations have reported that this angio-intervention is not as effective in treating chronic stage RAS, and that renal function or renovascular hypertension frequently is aggravated despite angio-intervention [53-56]. Subsequently, radiologists should be familiar with imaging features suggesting early stage RAS. As RAS becomes severe, arterial supply decreases and renal ischemia increases. As a result, cortical echogenicity increases and kidney size decreases. Intra-renal Doppler US shows decreased renal perfusion and pulsus tardus and parvus [57-59]. Early stage RAS can be defined when stenosis is present but kidney size and echogenicity are not significantly changed (Fig. 2). As a result, RAS should be detected before such anatomical changes are clearly depicted on US. When RAS is present in both renal arteries, renal artery US can indicate the treatment priority (Fig. 2). Radiologists should choose a renal artery based on kidney size or echogenicity.

Acute obstruction of renal artery

Acute obstruction of the renal artery results in acute renal infarction and rapid reduction of renal function [60-62]. CT or magnetic resonance angiography is limited for use in patients with poor renal function because of the need for intravenous contrast material. Renal artery US helps to determine RAO resulting from arterial dissection, acute thrombus, or surgical ligation (Fig. 3) [63,64]. These clinical conditions can lead to acute renal failure in patients with single kidney. RAO can occur if the renal artery is ligated during lymph node dissection around the aorta or intravenous vena cava. Early detection with renal artery US can minimize rapid loss of viable renal tissue resulting from acute RAO. Accordingly, this US technique can also help surgeons to determine treatment plans such as re-implantation of renal artery, auto-transplantation, or thrombolysis.

DIAGNOSTIC PITFALLS

Renal artery US has some limitations in assessing stenosis or obstruction. First, in patients with multiple renal arteries, special techniques are needed to detect or assess each renal artery [21-23]. If RAS occurs in the segmental renal artery, beginners might miss it on renal artery US. Even US experts should pay attention to carefully determine the number of renal arteries. If an aberrant renal artery is far from the origin of the main renal artery, it can be difficult to detect.

Second, a PSV greater than 180 to 200 cm/sec or an RAR greater than 3.5 on renal artery US strongly suggest RAS. However, patients with single kidney resulting from nephrectomy have a high PSV or RAR even though RAS is not present [65]. This is a physiologic phenomenon in which a single kidney needs to increase the glomerular filtration rate to match that of two kidneys. Therefore, PSV in a renal artery of a single kidney is almost twice that in paired kidneys immediately after unilateral nephrectomy. This physiologic finding should be differentiated with RAS.

Third, a similar finding can be achieved in the early period of renal allograft. PSV or RAR is higher in the early post-transplantation period than in native renal artery (Fig. 4) [66,67]. Blood flow is markedly elevated in the transplanted renal artery, and pseudo-RAS can be detected in renal artery anastomosis. Because the diameter of the recipient artery is larger than that of the donor artery, anastomosis frequently appears stenotic. Therefore, this anatomical deformity should be differentiated from RAS.

Fig. 4.

High peak systolic velocity in a 67-year-old woman with renal allograft. (A) Spectral Doppler ultrasound (US) shows a high peak velocity (223 cm/sec) within the renal artery (arrow) of a renal allograft. This US scan was performed 3 days after the renal allograft was transplanted. (B) Right iliac arteriography shows no sign of stenosis in the allograft renal artery (arrow).

Fourth, renal artery US can detect RAS in the main or segmental renal artery. However, this US technique cannot easily detect small-vessel diseases in the subsegmental renal arteries due to limited image resolution [26]. This limitation is also found in CT or magnetic resonance angiography. Therefore, if small-vessel renal disease is suspected, conventional angiography is a management of choice.

SUMMARY

Renal artery US is useful in assessing poor renal function in patients who have poor breath-holding, follow-up after angio-intervention, screening for AKI, guidelines to angio-intervention, and acute obstruction of the renal artery. However, this US technique has limitations in patients who have multiple renal arteries, single kidney, allograft renal artery, or small-vessel disease.

CONCLUSION

Renal artery US can help to detect RAS or RAO in those in whom their renal function becomes poor. Therefore, radiologists or sonographers need to be familiar with this imaging techniques and imaging features.

Notes

No potential conflict of interest relevant to this article was reported.

AUTHOR CONTRIBUTIONS

Conception or design: BKP.

Acquisition, analysis, or interpretation of data: BKP.

Drafting the work or revising: BKP.

Final approval of the manuscript: BKP.

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