Ultrasound Doppler renal ultrasonography

Ultrasound Doppler is an important complement to renal ultrasound. With the help of ultrasound Doppler, renal artery stenosis can be detected, as a result of which physicians no longer need to resort to the vague diagnosis of "renal vascular atrophy". Doppler can detect pathological conditions even before they lead to structural tissue changes.

Kidney allografts can be clearly visualized by ultrasound when they are located in the iliac fossa. Transplant rejection can be detected at an early stage. In addition, the arteries and veins of the transplant are accurately identified. Ultrasound Dopplerography can replace almost all radionuclide and angiographic studies in assessing the transplanted kidney.

Ultrasound Dopplerography also plays an important role in urological and andrological studies. Due to its speed, it is performed in the differential diagnosis of acute diseases of the scrotum and facilitates the correct decision on surgical or conservative treatment. Ultrasound Dopplerography also provides important etiological information in the assessment of erectile dysfunction. This method is increasingly replacing invasive diagnostic procedures.

Situations when ultrasound Dopplerography is indicated:

• Hypertension in people under 30 years of age
• The difference between the sizes of the right and left kidneys is more than 1.5 cm
• Diastolic pressure above 105 mmHg, despite antihypertensive therapy with three drugs, especially in severe generalized atherosclerosis
• |Increased creatinine during treatment with ACE inhibitors or AT-1 receptor antagonists

Indications for ultrasound Dopplerography of the renal arteries

Ultrasound Dopplerography is indicated only when clinical data allow one to suspect renovascular hypertension. There is no point in examining every patient with arterial hypertension, this will lead to an unjustified number of false-positive results.

Kidney examination: technique and normal ultrasound anatomy

The patient is examined on an empty stomach. Due to the fact that the renal arteries usually pass at a great depth, a low-frequency probe with a frequency of 2.0 to 3.5 MHz is used.

Anatomy and location of the sensor

The right renal artery arises from the aorta at the 10 o'clock position (in cross-section), starting slightly below the origin of the superior mesenteric artery. It courses posteriorly and passes behind the inferior vena cava to the hilum of the right kidney. The left renal artery arises from the aorta at approximately the 4 o'clock position, usually at the same level as the right. It can be followed for about 3 cm from the aorta toward the hilum. Visualization of the left renal artery is usually more difficult than that of the right, since it is more often obscured by gas in the superimposed loops of the small intestine.

Angle-corrected velocity measurements are made at 5 points along the main renal arteries. Normal peak velocity ranges from 50 to 160 cm/s.

Additional renal arteries are present in 20% of patients. To avoid missing them, the aorta should be scanned in the cranial and caudal directions from the origin of the main renal arteries.

The renal arteries can be visualized in an oblique coronal longitudinal section with the transducer positioned along the right midclavicular line or in a transverse position when scanning the abdominal cavity.

The best images are obtained by placing the transducer at the midpoint between the xiphoid process and the umbilicus. If visualization of the aorta is obstructed by gas in the bowel, move the transducer higher to the subxiphoid level and tilt it downwards, or scan at a more caudal level and tilt the transducer upwards. The best acoustic window is selected based on the location of the gas during the examination.

Normal ultrasound image of the kidneys

When examining the origin of the right renal artery in color mode, a zone of color inversion is often seen in the tortuous vessels. The relatively dark shades help distinguish this normal phenomenon from the bright color change caused by blurring due to proximal renal artery stenosis.

Oblique coronal longitudinal images are obtained with the patient lying on the left side. The transducer is positioned longitudinally along the midclavicular line. It is tilted at an angle until the vena cava appears on the longitudinal section. If the presence of gas in the intestine makes visualization difficult, the transducer should be moved and tilted until a satisfactory acoustic window is selected. The aorta is visualized "behind" the vena cava. The right renal artery goes from the aorta directly towards the transducer. Blood flow towards the transducer causes a significant shift in Doppler frequencies and a clear Doppler spectrum. The left renal artery, departing from the aorta, is directed in the opposite direction from the transducer. This plane is best suited for identifying multiple renal arteries.

Doppler spectra from intrarenal interlobar arteries

The kidneys are best visualized in B-mode with the patient in the right and left lateral positions. In most patients, they can also be visualized in the standard supine position. Once an optimal B-mode image is obtained, activate color mode and duplex scanning and measure the resistance index values sequentially in the proximal, middle, and distal thirds of the three interlobar arteries. In healthy individuals, the resistance index values vary slightly between one kidney and both kidneys. The mean value is calculated from the resistance indices for each kidney.

The values of the resistance index in healthy individuals depend on age and the area being measured. In the main artery, they are higher in the hilum area (0.65+0.17) than in the more distal small arteries, and they are lowest in the interlobar arteries (0.54±0.20). Comparable data can only be obtained by examining arteries of equal order. It is best to choose segmental and interlobar arteries, since these vessels are easy to visualize in the area of the junction of the renal pelvis and parenchyma. They are usually located under the sensor and cause a significant shift in Doppler frequencies, which leads to obtaining color and spectral images of good quality.

Age-related changes in the resistance index in renal arteries

The values of the resistance index depend on age: the older the person, the higher they are. In older patients, the blood flow is more "pulsating". Due to interstitial fibrosis, the resistance of the renal blood flow increases, and the concentration function decreases.

Factors Affecting Renal Perfusion

Age is not the only factor that affects the renal vascular resistance index. The table lists intrarenal and extrarenal factors that must be taken into account when interpreting the resistance index values. These factors are much more common in transplanted kidneys than in native kidneys. When present on both sides, they do not affect the comparison of the resistance index of the right and left kidneys in the diagnosis of renal artery stenosis (RAS).

Reason for the increase	Pathophysiology of resistance to blood flow
Acute renal failure	Swelling of the kidneys due to interstitial edema, tubulo-juxtaglomerular reversal with contraction of the mesangium and constriction of the afferent vessels
Renal pelvis obstruction	Interstitial edema due to backfiltration of fluid within the tubules into the interstitium
Extrarenal compression	Increased interstitial pressure due to subcapsular hematoma or other mass
Low diastolic blood pressure	Deficit of propulsive force in diastole (eg, due to severe aortic valve insufficiency)
Bradycaria	Insufficient blood flow at the end of prolonged diastole
Interstitial scarring	Interstitial fibrosis or sclerosis of small arteries, leading to rarefaction of terminal arterial branches with increased resistance to blood flow
Acute rejection	Interstitial rejection: graft enlargement due to lymphocytic interstitial infiltrate Vascular rejection: increased resistance due to narrowing of small intrarenal arteries
Toxic effects of cyclosporine A	Cyclosporine A has a vasoconstrictive effect on the afferent vessels

Narrowing of the arterial lumen usually results in an acceleration of blood flow. Stenosis of less than 50% causes only a slight acceleration, the velocity increases sharply only as its degree increases, and then drops sharply when the stenosis approaches 100%. Because of this acceleration of blood flow, stenoses are coded in bright colors on Doppler ultrasound. High-resolution scanning allows one to detect turbulence in the form of a yellow-green mosaic extending distally from the stenosis. However, stenosis cannot be diagnosed using color mode alone. In suspicious areas, a spectral picture should be obtained from which blood flow velocities can be determined.

An experienced specialist (who has performed more than 500 ultrasound Doppler sonographies of the renal arteries) using modern equipment can visualize 70-90% of the renal arteries. Visualization of additional renal arteries is a more difficult task and is successful only in 20-50% of cases. An experienced doctor can perform a complete examination in 30-45 minutes.

Typical ultrasound signs of high-grade renal artery stenosis are blood flow acceleration greater than 20 cm/s (438 cm/s in this figure) and poststenotic turbulence in the lumen of the affected renal artery.

Diagnostic criteria for renal artery stenosis:

• Peak blood flow velocity > 200 cm/s (direct sign).
• The difference between the resistance index of the right and left points is > 0.05 (indirect sign) - stenosis of the renal artery in the kidney with a low resistance index.
• The resistance index on each side is lower than the age-appropriate value - bilateral renal artery stenosis (indirect sign).
• Increase time > 70 ms (measured in 10 segmental arteries).

Diagnostic criteria for renal artery stenosis

A direct sign of renal artery stenosis is an increase in the blood flow velocity in the main renal artery of more than 200 cm/s. Indirect signs are based on the fact that each stenosis above 70% causes blood flow disturbances in the poststenotic segment of the vessel. Poststenotic peaks are rounded), the peak blood flow velocity in this case is only 8 cm/s. This leads to a decrease in the resistance index values in the poststenotic segment. Comparison with the opposite kidney demonstrates a normal wave in one of the right interlobar arteries.

Distal to the stenosis, an increased acceleration time can be measured. This is the time from the onset of systolic acceleration until the curve becomes flat. Looking for these indirect signs of stenosis leads to improved detection of renal artery stenosis even in cases where the renal arteries cannot be visualized due to the presence of large amounts of gas in the intestine.

In patients with atrial fibrillation, peak blood flow velocity can vary significantly from one cardiac cycle to another due to beat-to-beat changes in stroke volume. Although the quality of the color images of the flow on each side was poor due to the obesity of the patient in this case, it is clear that peak blood flow velocity is elevated to approximately 395 cm/s in the right and approximately 410 cm/s in the left renal artery.

Transplanted kidney - research method

The technique for examining a transplanted kidney should take into account the fact that the graft artery and vein may have a more bizarre shape than the artery and vein of the native kidney, which is due to the position of the graft and the configuration of the surgical anastomoses. Examination is usually easier than with the native kidney, since the graft is closer to the skin. Modern equipment allows for complete visualization of more than 95% of all graft arteries.

Graft artery stenosis

The graft is a functioning solitary kidney that may undergo compensatory hypertrophy. Since renal blood flow is highly dependent on renal function, a threshold level of blood flow velocity sufficient to diagnose renal artery stenosis cannot be defined as for native kidneys. In the presence of a hypertrophied functioning graft, the blood flow velocity in the non-stenotic artery may be higher than 250 cm/s. In the case of chronic dysfunction of the transplanted kidney with a decrease in its size, a regional increase in blood flow velocity up to 250 cm/s may indicate significant renal artery stenosis if the blood flow velocities in the remaining sections of the basilar artery are only 50 cm/s.

Thus, local acceleration of blood flow by 2.5 times from prestenotic or remote poststenotic (for example, 260 cm/s versus 100 cm/s) is the first sign of stenosis in the artery of the transplanted kidney. The sensitivity and specificity of ultrasound Dopplerography in detecting stenoses exceed 90%. Unlike native kidneys, there are no indirect signs of stenosis for transplants, because the right and left kidneys cannot be compared with each other, and blood flow resistance depends on many other factors.

Vein graft thrombosis

Complete thrombosis of the graft vein is recognized by the inability to detect veins in the hilum area and by the pathognomonic bidirectional blood flow in the intrarenal arteries.

This pattern is the result of a maximal increase in resistance to blood flow caused by complete renal vein thrombosis. Blood flowing through the renal arteries in systole reverses in diastole. Blood flow through the renal arteries decreases to zero, and the average blood flow velocity over one cardiac cycle is also zero. This means that on the Doppler spectrum, the areas above the base during periods of systolic blood flow are equal to the areas of diastolic reverse blood flow below the base. This pattern is so specific for graft vein thrombosis that its visualization requires immediate surgical intervention without any additional studies.

Arteriovenous fistulas in transplanted kidneys

Most often, they are caused by biopsies. Fistula on color Doppler sonography looks like a non-specific mosaic pattern of red and blue. The diagnosis is confirmed if a decrease in resistance with increased diastolic blood flow is determined in the feeding arteries, and a pulsating pattern of increased blood flow is detected in the draining veins. Patients with a large fistula have a high risk of hemorrhagic complications when performing a repeat biopsy.

Transplant rejection

Doppler ultrasound is of particular importance in detecting early signs of kidney transplant rejection. Increased blood flow resistance is an early sign of rejection, preceding renal function impairment (creatinine level) by almost two days. Increased resistance is not a specific sign, since various intrarenal and extrarenal factors can increase the resistance index and pulsatility index in the transplanted kidney.

A single detection of an elevated resistance index does not indicate whether it is due to acute postischemic renal failure or transplant rejection. Determination of an elevated resistance index over a series of studies (every 3-4 days) is a more reliable indicator of rejection than a single change in its value. Since almost all studies have shown approximately the same diagnostic value for the resistance index and the pulsatility index, a daily increase in the pulsatility index is a better criterion for rejection than the resistance index, since the pulsatility index in patients with constant zero diastolic blood flow better reflects small changes in systolic inflow than the resistance index.

If the pulsation index increases, it is advisable to perform a transplant biopsy. A biopsy allows for earlier confirmation of transplant rejection and treatment.

If the elevated pulsatility index does not decrease in response to treatment, therapy may be inadequate. In such cases, repeat biopsy is recommended to assess the need for further immunosuppression.