Ultrasonic dopplerography of the kidneys

Ultrasonic dopplerography is an important addition to ultrasound examination of the kidneys. With the help of ultrasound dopplerography, stenosis of the renal artery can be detected, as a result of which doctors no longer need to resort to a streamlined diagnosis of "vascular kidney atrophy". Dopplerography can reveal pathological conditions even before they lead to structural tissue changes.

Renal allografts with ultrasound can be clearly visualized with their location in the iliac fossa. The rejection of the graft can be determined at an early stage. In addition, the arteries and veins of the transplant are accurately detected. Ultrasonic Doppler ultrasound can replace almost all radionuclide and angiographic studies in assessing the transplanted kidney.

Ultrasonic dopplerography also plays an important role in urological and andrological studies. Due to its speed, it is performed in the differential diagnosis of acute scrotal disease and facilitates making the right decision about operative or conservative treatment. Ultrasonic Doppler ultrasound also provides important etiological information when evaluating erectile dysfunction. This method increasingly replaces invasive diagnostic procedures.

Situations when ultrasound dopplerography is indicated:

• Hypertension in persons younger than 30 years of age
• The difference between the size of the right and left kidneys is more than 1.5 cm
• The diastolic pressure is higher than 105 mm Hg. Despite antihypertensive therapy with three drugs, especially in severe generalized atherosclerosis
• | Increase of creatinine in the treatment of ACE blockers or antagonists of AT-1 receptors

Indications for ultrasonic dopplerography of the renal arteries

Ultrasonic Doppler ultrasound is indicated only with clinical data that allow one to suspect renovascular hypertension. It makes no sense to examine every patient with 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 great depth, a low-frequency sensor with a frequency of 2.0 to 3.5 MHz

Anatomy and location of the sensor

The right renal artery departs from the aorta at the 10 o'clock position (in cross section), starting slightly below the site of the superior mesenteric artery. It goes to the back and passes behind the inferior vena cava to the gates of the right kidney. The left renal artery departs from the aorta approximately at the 4 o'clock position, usually at the same level as the right one. It can be traced about 3 cm from the aorta towards the gate. Visualization of the left renal artery is usually more difficult than the right one, since it is more often concealed by gas in the ligating loops of the small intestine.

Speed measurements with angle correction are made at 5 points along the main renal arteries. The normal peak speed is from 50 to 160 cm / s.

Additional renal arteries are found in 20% of the subjects. In order not to miss them, the aorta should be scanned in the cranial and caudal direction from the site of the main renal arteries.

The renal arteries can be visualized in an oblique coronal longitudinal section with the location of the sensor on the right median-inclusive line or in the transverse position when scanning the abdominal cavity.

The best images are obtained by placing the sensor in the middle point between the xiphoid process and the navel. If aortic visualization interferes with gas in the gut, move the sensor higher to the sub-obvious level and tilt it downwards, or scan at a more caudal level and tilt the sensor upward. The best acoustic window is selected depending on the location of the gas during the study.

Normal ultrasound picture of the kidneys

When examining the location of the right renal artery in the color regime, a zone of color inversion in crimped vessels is often determined. Relatively dark shades make it possible to distinguish this normal phenomenon from a bright color change caused by blurring due to proximal stenosis of the renal artery.

Obtaining oblique coronal longitudinal images is carried out in the position of the subject on the left side. The sensor is placed longitudinally along the median-inclusive line. It is inclined at an angle until a hollow vein appears on the longitudinal section. If gas in the intestine makes it difficult to visualize, the sensor 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 straight in the direction of the sensor. The blood flow towards the sensor causes a significant shift in the Doppler frequencies and a clear Doppler spectrum. The left renal artery, moving away from the aorta, is directed to the opposite side from the sensor. This plane is best suited for the detection of multiple renal arteries.

Doppler spectra from the intercostal interlobar arteries

The kidneys are best seen in B-mode in the patient's position on the right and left sides. In most patients, they can be visualized in the standard position on the back. After obtaining the optimum image in the B-mode, activate the color mode and duplex scanning and sequentially measure the values of the resistance index in the proximal, middle and distal thirds of the three interlobar arteries. In healthy individuals, the values of the resistance index have small differences among themselves in one kidney and in both kidneys. The average value is calculated from the resistance indices for each kidney.

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

Age-related changes in the resistance index in the arteries of the kidneys

The values of the resistance index depend on the age: the older the person, the higher they are. In older patients, the blood flow more "pulsates." 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 affecting the index of resistance in the vessels of the kidneys. The table lists the intrarenal and extrarenal factors that must be taken into account when interpreting the values of the resistance index. These factors are much more common in transplanted kidneys than in their own. If present on both sides, they do not affect the comparison of the right and left kidney resistance index in the diagnosis of renal artery stenosis (SPA).

The reason for increasing	Pathophysiology of resistance to blood flow
Acute kidney failure	Kidney swelling due to interstitial edema, tubulo-juxtaglomerular back current with contraction of mesangium and constriction of vasoconstrictors
Obstruction of the renal pelvis	Interstitial edema due to reverse filtration of fluid inside the tubes in interstitiums
Extrarenal compression	Increased interstitial pressure due to subcapsular hematoma or other formation
Low diastolic blood pressure	Deficiency of propulsive force in the diastole (for example, due to severe aortic valve insufficiency)
Bradicaria	Insufficient blood flow at the end of the elongated diastole
Interstitial scarring	Interstitial fibrosis or sclerosis of small arteries, leading to a thinning of terminal arterial branches with increased resistance to blood flow
Acute rejection	Interstitial rejection: an increase in the graft due to lymphocytic interstitial infiltrate Vascular rejection: increased resistance due to narrowing of small intracellular arteries
The toxic effect of cyclosporin A	Cyclosporin A has a vasoconstrictive effect on the delivery vessels

Constriction of the artery lumen usually leads to an acceleration of blood flow Stenosis less than 50% causes only a slight acceleration, the rate sharply increases only with an increase in its degree, and then falls sharply when the stenosis approaches 100%. Because of this acceleration of blood flow, stenosis with ultrasound dopplerography is encoded in bright colors. High-resolution scanning allows us to determine turbulence in the form of a yellow-green mosaic that extends from the stenosis in the distal direction. However, with the help of only the color regime, it is impossible to diagnose stenosis. In suspicious areas, a spectral picture should be obtained from which blood flow velocities can be determined.

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

Typical ultrasonic signs of renal artery stenosis of a high degree are acceleration of blood flow more than 20 cm / s (in this figure 438 cm / s) and poststhenotic turbulence in the lumen of the affected renal artery.

Diagnostic criteria of stenosis of the renal artery:

• Peak blood flow velocity > 200 cm / s (direct indication).
• The difference between the index of resistance of the right and left points> 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 corresponding age - bilateral stenosis of the renal artery (indirect sign).
• The growth time is> 70 ms (measured in segmental arteries).

Diagnostic criteria for renal artery stenosis

A direct sign of stenosis of the renal artery is an increase in the blood flow velocity in the main renal artery 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. Posthenstenotic peaks are rounded), the peak blood flow velocity in this case is only 8 cm / s. This leads to a decrease in the values of the resistance index in the post-stenotic segment. A comparison with the opposite kidney demonstrates a normal wave in one of the right-sided interlobar arteries.

Distal stenosis can be measured by an increased acceleration time. This is the time from the onset of systolic acceleration to the moment when the curve becomes flat. The search for these indirect signs of stenosis leads to an improvement in the detection of stenosis of the renal artery even in cases where the renal arteries can not be visualized due to the presence of a large amount of gas in the intestine.

In patients with atrial fibrillation, peak blood flow velocity may vary significantly in different cardiac cycles due to changes in stroke volume from contraction to contraction. Although the quality of the color images of the flow from each side due to the patient's obesity in this case was not high, it can be seen that the peak blood flow velocity is increased to about 395 cm / s in the right and about 410 cm / s in the left renal artery.

Transplanted kidney - a technique of research

The procedure for the examination of the transplanted kidney should take into account the fact that the artery and vein of the graft may have a more bizarre shape than the artery and vein of the kidney itself, which is due to the position of the graft and the configuration of surgical anastomoses. Examination is usually easier as compared to your own kidney, as the graft is closer to the skin. Modern equipment allows you to fully visualize more than 95% of all arteries of the transplant.

Transplant artery stenosis

The transplant is a functioning single kidney that can undergo compensatory hypertrophy. Since renal blood flow largely depends on kidney function, it is impossible to determine the threshold level of blood flow velocity sufficient for the diagnosis of renal artery stenosis, both for native kidneys. If there is a hypertrophic 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 the blood flow velocity to 250 cm / s may indicate a significant stenosis of the renal artery if the blood flow velocity in the remaining sections of the main artery is only 50 cm / s.

Thus, local acceleration of blood flow 2.5 times from a prestenotic or distant poststenotic (for example, 260 cm / s vs. 100 cm / s) is the fifth sign of stenosis in the artery of the transplanted kidney. The sensitivity and specificity of ultrasound dopplerography in the detection of stenoses exceed 90%. Unlike own kidneys for transplants, there are no indirect signs of stenosis, because it is impossible to compare the right and left kidneys, and the resistance of blood flow depends on many other factors.

Transplant vein thrombosis

Complete thrombosis of the transplant vein is recognized by the impossibility of detecting veins in the gate region and by pathognomonic bidirectional blood flow in the intrarenal arteries.

This picture is the result of the maximum increase in resistance to blood flow caused by complete thrombosis of the renal vein. The blood flowing along the renal arteries to the systole returns back to the diastole. Blood flow to the renal arteries decreases to zero, and the average blood flow velocity in one cardiac cycle is also zero. This means that on the Doppler spectrum of the region above the base during periods of systolic blood flow are equal to the areas of diastolic backflow below the base. This pattern is so specific for vein thrombosis that when imaging it requires immediate surgical intervention without any additional studies.

Arteriovenous fistulas in transplanted kidneys

The most common cause of their occurrence are biopsies. Fistula with color dopplerography looks like a nonspecific mosaic pattern of red and blue. The diagnosis is confirmed if in the feeding arteries a decrease in resistance is determined with an increase in the diastolic blood flow, and a pulsating picture of the increased blood flow appears in the drainage veins. Patients with a large fistula have a high risk of hemorrhagic complications during a repeated biopsy.

Graft rejection

Ultrasonic dopplerography is of particular importance in revealing early signs of rejection of the transplanted kidney. Increased resistance to blood flow is an early sign of rejection, preceding a violation of renal function (creatinine level) for almost two days. An increase in resistance is not a specific sign, as various intracellular and extrarenal factors can increase the resistance index and the pulsation index in the transplanted kidney.

With a single detection of an increased resistance index, it is not possible to determine whether this is due to acute post-ischemic renal failure or rejection of the graft. The definition of an increased resistance index during a series of studies (every 3-4 days) is a more reliable sign of rejection than a single change in its value. Since almost all the diagnostic values of the resistance index and the ripple index are shown in nearly all studies, the daily increase in the ripple index is the best rejection criterion than the resistance index, since the ripple index in patients with constant zero diastolic blood flow better reflects small changes in the systolic inflow than the resistance index .

If there is an increase in the ripple index, it is advisable to perform a biopsy of the transplant. A biopsy can be used to confirm rejection of the transplant and prescribe a treatment.

If the increased ripple index does not decrease in response to treatment, therapy may be inadequate. In such cases it is recommended to conduct a repeated biopsy to assess the need for further immunosuppression.