Standards of the Polish Ultrasound Society – Update. Ultrasound examination of renal arteries

Abstract

This paper has been prepared on the basis of the Standards of the Polish Ultrasound Society and updated based on the latest reports from the relevant literature. The author presents a renal artery examination technique, patient preparation for the testing, limitations of the method, currently recommended proper blood flow standards and criteria for the diagnosis of significant stenoses. Renal artery ultrasound is performed using a 2–5 MHz probe, usually a convex 3.5 MHz one. The ultrasound machine must be equipped with the Doppler options for the evaluation of color coded blood flow and recording of the blood flow spectrum, including the triplex Doppler mode. Patients have to fast for at least eight hours before testing. Ultrasound always begins with the assessment of renal structure using the grayscale. Next, color coded blood flow imaging is used, followed by placing a sampling gate in the lumen of the target vessel to record the spectral image. The aim of renal artery ultrasound is to assess the course and position of arteries, evaluate blood flow parameters, as well as visualize possible changes: stenoses, occlusions, aneurysms, or arteriovenous fistulas. Blood flow velocity is always measured in a longitudinal projection/ longitudinal section of the vessel, after placing the sampling gate in the central part of the flowing bloodstream, which normally corresponds to the central part of the vascular cross-section. When diagnosing renal artery stenosis, it is necessary to know the nature of the blood flow and norms for flow parameters in healthy vessels. The spectrum of the blood flow velocity in renal arteries and their branches is a low resistance one. The following parameters are used to evaluate normal renal arteries and to identify the narrowed ones: V_max, V_min, RAR, AT, AI, RI and PI.

Introduction

Due to the increasing possibilities of treating stenoses and vascular occlusions through surgical or endovascular procedures, there is a growing number of patients referred for diagnostic vascular tests, including the Doppler ultrasonography. In recent years, hypertension has been more and more frequently diagnosed in increasingly younger age groups. The most common form of secondary hypertension is renal artery stenosis caused by the narrowing of one or both renal arteries. An early diagnosis of this pathology and effective endovascular or surgical treatment offer a real chance for the normalization of blood pressure.

Equipment

Ultrasound of renal arteries is performed using a 2–5 MHz transducer, usually a convex 3.5 MHz one. The ultrasound equipment must be equipped with the Doppler options for the evaluation of color coded blood flow and recording of the coded blood flow spectrum, including the triplex Doppler mode^{(1, 2)}. In very slim patients the periaortic sections of renal arteries can be screened using a linear6–12 MHz probe.

Preparation

Patients have to fast for at least eight hours prior to the testing. In some ca ses (obese people, patient s whose intestinal loops are filled with gas) special preparation is recommended: for three days preceding the test two capsules of Espumisan should be taken three times a day. The last easily digestible meal should be consumed at about 4 p.m. on the day preceding the test. On the day of the examination the patient needs to be fasted – regardless of whether the test is performed in the morning or in the afternoon. It is recommended that the patient also refrain from chewing gum and smoking on that day⁽¹⁾.

Limitations

1. Anatomy and malformations – nephroptosis, multiple renal arteries, horseshoe kidney.

2. Patient with severe condition – the inability to take a deep breath and hold it.

3. Difficult testing conditions – a large amount of intesti-nal gas, obese patient.

Technique

Renal artery ultrasound always begins with the assessment of renal structure – the location, size and morphological image of the kidneys on the grayscale. Standard as well as additional kidney examination methods are applied. The next step is the evaluation of blood vessels.

The aim is to visualize both renal arteries throughout their course and to start from the proximal, i.e. periaortic, segments. The right renal artery usually exits the anterior-right contour of the abdominal aorta, the left renal artery – the posterior-left one, that is why it is best to scan the initial segments on the abdominal aortic cross-section in the midline of the body, bearing in mind that the left artery exits the aorta lower than the right one. If atherosclerotic changes are found in the abdominal aorta or its shape is tortuous, the application of additional projections (oblique) becomes necessary and the transducer needs to be shifted to the right or to the left depending on the course of the aorta, so that the initial segments of renal arteries can be scanned. In thin patients the trunk of the left and right renal artery can be visualized throughout its entire course by placing the transducer in the midclavicular line in the lateral projection. In order to obtain optimal images of renal arteries, after the transducer is pressed fairly strong on the midline the patient must be asked to breathe freely and take fairly shallow breaths.

Next, the color coded blood flow imaging is used, followed by placing a sampling gate in the lumen of the target vessel to record the spectral image. While the spectrum of the periaortic section of the right and left renal artery is being recorded, the patient is asked to hold his breath, which allows to eliminate the “noise”.

Visualization of the entire course of the left renal artery is difficult or impossible to obtain in each patient. While in most subjects the right renal artery appears very well visible throughout its entire course – with patient lying on their left side with their right hand behind their head. The transducer is placed on the anterior-right axillary line in the right oblique projection, showing the cross-section of the kidney and the right renal artery which runs parallelly to the right renal vein and deeper away from it. Such access allows for obtaining the correct size of the Doppler angle of isonation in each patient, which translates into obtaining reliable measurements within the renal artery trunk. The positioning of the patient on their right side and the application of the transducer obliquely in the left posterior or central axillary line results in the imaging of the centeral and perihilar section of the left renal artery. Perihilar segments and intrarenal branchings of the renal arteries are assessed bilaterally with the patient positioned on their side: the right side to assess the left kidney and the left side in order to evaluate the right kidney.

The imaging of renal arteries and the measurement of blood flow parameters in a patient positioned laterally require the cooperation of the patient, i.e. taking a very deep breath and holding it for several or over a dozen seconds. This allows to perform the color coded blood flow Doppler examination and record the spectral flow. In addition, renal arteries can be visualized from the posterior access, after positioning the patient on their stomach. This applies in particular to children, but is also applicable in obese adults.

Indications for testing

The aim of renal artery ultrasound is to assess the course and location of kidneys, evaluate blood flow parameters and visualize changes: stenoses, occlusions, aneurysms, and arteriovenous fistulas.

Atherosclerotic stenoses are usually located in the initial segments of arteries, and fibromuscular dysplasia lesions – in the middle and downstream segments. Atherosclerotic stenoses are usually found in older people and fibromuscular dysplasia – in younger patients, especially in women. Obstruction is caused by atheroscleroses, embolisms or thrombi. Aneurysm and arteriovenous fistulas occur less frequently, e.g. as a result of an injury or a complication following a surgery (e.g. a biopsy).

The resulting image of renal arteries is analyzed particularly for abnormal – that is turbulent – blood flow. This type of flow may appear in a tortuous segment of the vessel and it is evidence of the physiological acceleration of velocity. However, it usually appears in significantly narrowed arter y segments (the so-called hemodynamically significant stenoses ≥60%) and is also visualized in widened sections. The hemodynamic image should always be analyzed in relation to the mor phological image of the vessel.

Technical aspects of the examination

Blood flow velocity measurements are carried out in the longitudinal projection/longitudinal section of the vessel, after placing the sampling gate in the central part of the flowing bloodstream, which normally corresponds to the central part of the vascular cross-section. The size of gate should amount to 1/2–1/3 of the patent blood flow channel, but in practice, it is 1.5–2.0 mm.

Blood flow parameters must be evaluated bilaterally in the initial, middle and perihilar segments of renal artery trunks, and i n i ntrarenal branches – in at least three locations: around the upper and lower pole, and in the middle part of the kidney. The Doppler spectrum should be recorded when the laterally positioned patient inspires, or when they hold their breath as the transducer head is placed in the midline of their body – that is why respiratory cooperation with the patient is important during the test.

Measurement method and sites

In the diagnosis of renal artery stenosis it is necessary to know the nature of the blood flow and parameter norms of the flow in healthy vessels.

The spectrum of the blood flow velocity in renal arteries and their branches is a low resistance one. As one moves toward the distal direction, the flow velocities in the branches of renal arteries decrease. However, the low-resistance character of the spectrum does not change.

Normal blood flow characteristics in renal artery trunks^{(3, 4)}:

• steep slope of the velocity curve in the contraction phase;

• spectral window present;

• absence of reversal blood flow direction;

• free return to the end-diastolic velocity;

• flow velocity in the trunk usually the same or slightly slower than in the aorta.

Normal values of blood flow parameters in renal arteries^(1–4):

• systolic flow velocity in the trunk of about 100 cm/s (V_max);

• RAR (renal aortic ratio – the ratio of maximum blood flow velocity in the renal artery to the maximum velocity in the aorta) of about 0.8–1.0;

• AI (acceleration index, the rate of acceleration specifying the slope of the curve expressed in m/s²) <3 m/s²;

• AT (acceleration time – the time counted in seconds since the beginning of the systolic phase to reaching the maximum velocity in the middle of the systolic phase) ≤0.07 s;

• PI (pulsatility index) within the limits of 0.78–1.33;

• RI (resistance index) of about 0.5–0.8.

The first two of these blood flow parameters, i.e. the maximum systolic velocity and RAR, are evaluated in the renal artery trunk, and the other ones, i.e. acceleration rate, acceleration time and the values of vascular resistance (PI and RI) – in intrarenal branches of renal arteries. The maximum blood flow velocity in the renal arteries must always be considered in relation to the velocity of blood flow in the abdominal aorta, where the range of the maximum blood flow velocities varies from 30 cm/s in people with circulatory failure or major atherosclerotic changes to 150 cm/s in young people with hyperkinetic circulation.

RI values are proportional to age: the older the patient, the lower the value. Velocity measurements are done when the Doppler angle of insonation is 60° or less. Measuring velocities in tortuous segments of the vessels must be avoided because of the physiological acceleration of velocity that is always recorded there. It is usually very difficult to confirm or rule out stenosis in this location. In these situations the morphological image of the vessel should be carefully analyzed and measurements should be made when the sampling gate is wide.

The standard measurement sites of the blood flow in renal arteries are:

• subaortic section;

• centeral section;

• perihilar section;

• intrarenal branchings – at least three locations: the area of the upper and lower poles and the midpoint of the kidney.

In addition, measurements must be made in the morphological segments of the narrowings in the lumen and during turbulences revealed in the Doppler color imaging.

Below are presented individual parameters of the blood flow used in the diagnosis of significant stenoses.

The diagnosis of hemodynamically significant renal artery stenosis (≥60%)^{(2, 4–7)}:

• maximum systolic velocity (V_max) ≥2,0 m/s;

• end-diastolic velocity (V_min) ≥1,5 m/s;

• RAR index ≥3,5;

• acceleration time (AT) ≥0.08 s;

• acceleration indicator (AI) ≥3 m/s²;

• RI difference >0.05 (stenosis occurs when there is lower RI value).

RAR index compares a potential stenosis site with a normal place of reference, i.e. with blood flow values in the abdominal aorta at the exit level of renal arteries. It must be remembered that if the V_max values in the abdominal aorta are incorrect (beyond the range of 40–100 cm/s), the calculation of the RAR index becomes useless⁽⁵⁾.

In addition, V_max values in the renal artery trunk >1.8 m/s and <2.0 m/s indicate <60% stenosis. RAR ratio >3.0 and <3.5 indicates a similar degree of stenosis⁽⁵⁾.

The first three of these blood flow parameters, i.e. maximum systolic velocity, end-diastolic velocity and RAR are evaluated in the renal artery trunk, and the others, i.e. acceleration rate, acceleration time and vascular resistance index – in intrarenal branches of renal arteries.

The following are the most important in the diagnosis of stenosis: V_max at the stenosis site, RAR and the acceleration time in the intrarenal segment.

Renal artery occlusion can be detected only when there is no evidence of the blood flow in the visualized renal artery trunk. By no means does the absence of a renal artery image allow for the diagnosis of its obstruction.

Description of the test

Each description of a renal artery examination should contain the following information:

• patient's name, age and PESEL number (Polish personal identification number);

• the date of the test;

• name of the apparatus, information about the type/types of transducer heads and their frequencies.

The description must be accompanied by photographic documentation (a printout from a videoprinter or a computer printer, or a CD) marked with the patient's name.

The description must provide information on the location, size and morphological image of the kidneys and it must include details concerning the segments of the visualized renal arteries, e.g.: “The right renal artery was visualized throughout its entire course and its intrarenal branching. No visualization of the left renal artery trunk in the initial and middle section, only the perihilar section and intrarenal branching visualized”.

The description of the test depends on whether the imaging revealed normal renal arteries or pathologies were diagnosed.

If the image of the renal arteries is normal, individual parameters of the blood flow in the trunk or intrarenal branches should not be included in the description (the photographic documentation contains images of blood vessels along with the recorded velocities).

In the event of diagnosing a pathology, details on the abnormal course and possible widenings of the renal artery lumen must be included in the description, e.g.: “Right renal artery tortuous”, “Aneurysm of the left renal artery in the hilum area- dimensions…”. If atherosclerotic plaques are present, their location must be indicated. If the plaques are the reason for the stenosis and increased blood flow velocity, the description must include the following values: V_max at the site of stenosis, RAR index, systolic acceleration time (AT) and the RI index – evaluated intrarenally. The size of the stenosis must be evaluated as significant, i.e. ≥60% and eligible for treatment, or non-significant, requiring observation or verification by other diagnostic methods, such as CT angiography.

If no renal artery/arteries are visualized in the examination, that fact must be included in the description and other imaging techniques (angio-MR, angio-CT) must be recommended.

Photographic documentation

The documentation consists of the images of the right and left kidney along with the dimensions.

If the image of renal arteries is normal, the documentation should include:

• a record of the spectrum and the measurement of the velocity in the periaortic segment of the renal artery – bilaterally;

• a record of the spectrum and the measurement of the velocity in the perihilar section of the renal artery – bilaterally;

• a record of the spectrum and the measurement of the velocity, AT and RI in intrarenal branches – bilaterally (only one photo of each side).

In the case of a stenosis:

• a record of the spectrum and the measurement of the acceleration at the stenosis site;

• a record of the spectrum and the measurement of the velocity in the abdominal aorta at the exit level of renal arteries;

• a record of the spectrum, the measurement of AT and RI and the velocity values in intrarenal branches;

• the image of stenosis with use of the color coded blood flow.

In case of other morphological changes:

• an image e.g. of the tortuous segment of the renal artery or an image of an aneurysm.

Summary

Renal artery ultrasound in many cases confirms or excludes the presence of a significant stenosis. The diagnostic value of this test depends on good patient preparation and respiratory cooperation with them, but primarily on the skills, knowledge and experience of the person conducting the test and also on the class of the available apparatus. The presented examination standards include parameters and data, the knowledge of which is necessary to optimize ultrasound examination results.

Conflict of interest

The author do not report any financial or personal links with other persons or organizations which might affect negatively the content of this publication and/or claim authorship rights to this publication.