Radionuclide examination of the kidneys

Radionuclide methods have become firmly established in the practice of urological and nephrological clinics. They allow detecting renal dysfunction at early stages, which is difficult to do using other methods. Clinicians are attracted by the physiological nature of the radioindication method, its relative simplicity and the possibility of conducting repeated studies during the patient's treatment. It is also important that radionuclide compounds can be used in patients with increased sensitivity to radiocontrast agents. Depending on the objectives of the study, one of the radionuclide indicators is selected from the group of nephrotropic RFPs.

^99m Tc-DTPA is selectively filtered by the glomeruli, ^99m Tc-MAG-3 and I-hippuran are also filtered by the glomeruli, but are excreted mainly by tubular cells. Thus, all three of these radiopharmaceuticals can be used to study kidney functions - glomerular filtration and tubular secretion. This study is called "renography". Two other drugs - ^99m Tc-DMSA and ^99m Tc-glucoheptonate accumulate in functioning tubular cells for a relatively long time, so they can be used for static scintigraphy. After intravenous administration of these drugs, they are retained in the tubular epithelium of the kidneys for several hours. The maximum accumulation is observed 2 hours after injection. Therefore, scintigraphy should be performed at this time. Usually, several images are taken: in a direct projection from the front and back, in lateral and oblique projections.

Changes in the renal parenchyma associated with the loss of its function or replacement of its tissue with pathological formations (tumor, cyst, abscess) lead to the appearance of "cold" foci on the scintigram. Their localization and size correspond to areas of non-functioning or absent renal tissue. Static scintigraphy can be used not only to identify volumetric processes in the kidney, but also to diagnose renal artery stenosis. For this purpose, a test with captopril is performed. Static scintigraphy is performed twice - before and after intravenous administration of the specified drug. In response to the introduction of captopril, the scintigraphic image of the kidney "covered" by stenosis disappears - the so-called drug nephrectomy.

The indications for radionuclide examination of the kidneys - renography - are much broader. As is known, the total function of the kidney consists of the following partial functions: renal blood flow, glomerular filtration, tubular secretion, tubular reabsorption. All these aspects of kidney activity can be studied using radionuclide methods.

Determining renal plasma flow is of great importance in the clinic of internal diseases. This can be done by studying clearance, i.e. the rate of purification of the kidneys from substances that are completely or almost completely removed when blood flows through the kidney. Since purification from these substances does not occur in the entire renal parenchyma, but only in its functioning part, which is about 90%, the renal clearance determined by the purification method is called "effective renal plasma flow". Hippuran labeled with ¹³¹ I is used as a radiopharmaceutical. After intravenous administration of a small amount of this radiopharmaceutical, its concentration in the blood is measured 20 and 40 minutes after injection and compared with the level of radioactivity using a special formula. In healthy people, the effective renal plasma flow is 500-800 ml/min. Selective reduction in effective renal plasma flow is observed in arterial hypertension, cardiac and acute vascular insufficiency.

In studying the functional state of the kidneys, an important place is given to determining the glomerular filtration rate. For this purpose, substances that are not subject to tubular reabsorption, tubular secretion, destruction, and are not formed in the tubules and urinary tract are used. Such substances include inulin, mannitol, and to some extent creatinine. Determining their concentration in laboratory conditions is difficult. In addition, it is necessary to collect urine excreted over certain periods of time.

The radionuclide method has made it possible to significantly simplify the assessment of glomerular filtration. The patient is given ^99m Tc-DTPA intravenously. Since this drug is excreted exclusively by glomerular filtration, by determining the rate of blood purification from the radiopharmaceutical, it is possible to calculate the intensity of the renal filtration function. Usually, the concentration of the specified radiopharmaceuticals in the blood is determined twice: 2 and 4 hours after intravenous administration. Then, the glomerular filtration rate is calculated using a special formula. Normally, it is 90-130 ml/min.

In the nephrology clinic, another indicator of kidney function is of great importance - the filtration fraction. This is the ratio of the glomerular filtration rate to the effective renal plasma flow rate. According to the results of a radionuclide study, the normal value of the filtration fraction is equal to 20% on average. An increase in this indicator is observed in arterial hypertension, and a decrease - in glomerulonephritis and exacerbation of chronic pyelonephritis.

A common method for assessing renal parenchyma function is dynamic scintigraphy, or renography. In this case, ¹³¹ I-hippuran or ^99m Tc-MAG-3 are used as radiopharmaceuticals. The study is conducted on a gamma camera. The study usually lasts 20-25 minutes, and in case of renal dysfunction - up to 30-40 minutes. On the display screen, 4 "zones of interest" are selected (both kidneys, aorta and urinary bladder) and curves are plotted based on them - renograms, reflecting renal function.

At first, the intravenously administered radiopharmaceutical is carried with the blood to the kidneys. This causes a rapid appearance and a significant increase in the radiation intensity above the kidneys. This is the first phase of the renographic curve; it characterizes the perfusion of the kidney. The duration of this phase is approximately 30-60 sec. Of course, this section of the curve reflects the presence of the radionuclide not only in the vascular bed of the kidneys, but also in the perirenal tissues and soft tissues of the back, as well as the beginning of the transit of the radiopharmaceutical into the lumen of the tubules. Then the amount of the radiopharmaceutical in the kidneys gradually increases. The curve in this section is less steep - this is its second phase. The contents of the tubules decrease, and within a few minutes an approximate equilibrium between the intake and excretion of the radiopharmaceutical is observed, which corresponds to the peak of the curve (T max - 4-5 min). From the moment when the concentration of the radiopharmaceutical in the kidney begins to decrease, i.e. the outflow of the radiopharmaceutical prevails over the intake, the third phase of the curve is observed. The half-life of radiopharmaceuticals in the kidneys varies from person to person, but on average it ranges from 5 to 8 minutes.

Three parameters are usually used to characterize the renographic curve: the time of reaching the maximum radioactivity, the height of its maximum rise, and the duration of the half-life of the radiopharmaceutical from the kidney. Renographic curves change when the function of the kidneys and urinary tract is impaired. We will indicate 4 characteristic curve variants.

• The first option is a slowdown in the flow of the radiopharmaceutical into the "zone of interest" of the kidney. This is manifested by a decrease in the height of the curve and an extension of its first two phases. This type is observed when the blood flow in the kidney decreases (for example, when the renal artery is narrowed) or the secretory function of the tubules decreases (for example, in patients with pyelonephritis).
• The second option is a slowdown in the elimination of the radiopharmaceutical by the kidney. In this case, the steepness and duration of the second phase of the curve increase. Sometimes, within 20 minutes, the curve does not reach a peak and does not decline. In such cases, we speak of an obstructive curve. In order to distinguish true obstruction of the urinary tract by a stone or other mechanical obstruction from dilated uropathy, a diuretic, such as Lasix, is administered intravenously. In case of urinary tract obstruction, the administration of a diuretic does not affect the shape of the curve. In cases of functional delay in the transit of the radiopharmaceutical, the curve immediately declines.
• The third variant is a slow entry and elimination of the radiopharmaceutical from the kidneys. This is manifested by a decrease in the overall height of the curve, deformation and lengthening of the second and third segments of the renogram, and the absence of a clearly defined maximum. This variant is observed mainly in chronic diffuse kidney diseases: glomerulonephritis, pyelonephritis, amyloidosis, and the severity of the changes depends on the severity of the kidney damage.
• The fourth variant is a repeated rise in the renographic curve. It is observed in vesicoureteral reflux. Sometimes this variant is detected during conventional scintigraphy. If it is not present, and reflux is suspected based on clinical data, then at the end of renography the patient is asked to urinate into a bedpan. If a new rise occurs on the curve, this means that the urine containing the radionuclide has returned from the bladder to the ureter and then to the renal pelvis.

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