Radionuclide study of the kidneys

Radionuclide methods are firmly established in the practice of urological and nephrological clinics. They make it possible to detect violations of kidney function in the initial stages, which is difficult to accomplish with the help of other methods. Clinicians are attracted to the physiology of the method of radioindication, its relative simplicity and the possibility of conducting repeated studies during the treatment of the patient. It is also important that radionuclide compounds can be used in patients with increased sensitivity to radiopaque substances. Depending on the research objectives, one of the radionuclide indicators is selected from the group of nephrotropic RFPs.

^{99 m} Tc-DTPA is selectively filtered by the glomeruli, ^{99 m} Tc-MAG-3 and I-hippuran are also filtered by glomerulus, but are secreted mainly by tubular cells. Thus, all these three RFPs can be used to study kidney function - glomerular filtration and tubular secretion. This study was called "renography." Two other drugs, ^{99 m} Tc-DMSA and ^99m Tc-glucoheptonate, accumulate for a relatively long time in functioning tubular cells, so they can be used for static scintigraphy. After intravenous administration of these drugs, they are held for several hours in the tubular epithelium of the kidneys. The maximum accumulation is noted 2 hours after the injection. Therefore, at this time and it is necessary to conduct a scintigraphy. Usually take a few pictures: in a direct projection in front and behind, 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 location and dimensions correspond to areas of non-functioning or missing kidney tissue. Static scintigraphy can be used not only to detect volumetric processes in the kidney, but also to diagnose stenosis of the renal artery. For this purpose, a captopril test is performed. Static scintigraphy is performed twice - before and after intravenous administration of the drug. In response to the introduction of captopril, the scintigraphic image of the kidney, which is "covered" by stenosis, - the so-called drug nephrectomy - disappears.

A much broader indication for radionuclide studies of kidneys is renography. As is known, the total kidney function is made up of the following particular functions: renal blood flow, glomerular filtration, tubular secretion, tubular reabsorption. All these aspects of kidney activity can be studied using radionuclide techniques.

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

In the study of the functional state of the kidneys, an important place is assigned to determining the rate of glomerular filtration. For this purpose, substances that are not subjected to tubular reabsorption, tubular secretion, destruction, and not formed in the tubules and urinary tract are used. These substances include inulin, mannitol and, to some extent, creatinine. Determination of their concentration in the laboratory is difficult. In addition, it is necessary for him to collect urine that has been released over certain periods of time.

The radionuclide method made it possible to significantly simplify the evaluation of glomerular filtration. The patient is intravenously injected with ^{99 m} Tc-DTPA. Since this drug is isolated exclusively by glomerular filtration, by determining the rate of blood purification from the RFP, it is possible to calculate the intensity of the filtration function of the kidneys. Usually, the concentration of these RFPs in the blood is determined twice: 2 and 4 hours after intravenous administration. Then, according to a special formula, the glomerular filtration rate is calculated. Normally, it is 90-130 ml / min.

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

A common method of assessing the function of the renal parenchyma is dynamic scintigraphy, or renography. As the RFE , ¹³¹ I-hippuran or ^{99 m} Tc-MAG-3 is used. The study is carried out on a gamma camera. Usually the duration of the study is 20-25 minutes, and if the kidney function is up to 30-40 minutes. 4 "zones of interest" (both kidneys, aorta and bladder) are selected on the screen of the display and curves - renograms reflecting the kidney function are plotted on them.

First RFP, administered intravenously, is brought with blood to the kidneys. This causes a rapid appearance and a significant increase in the intensity of radiation 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 seconds. Of course, this segment of the curve reflects the presence of the radionuclide not only in the vascular bed of the kidneys, but also in the perineal tissues and soft tissues of the back, and also the beginning of the RFP transit into the lumen of the tubules. Then, the amount of RFP in the kidneys gradually increases. The curve on this segment is less steep - this is its second phase. The content of the tubules decreases, and within a few minutes an approximate equilibrium is observed between the intake and excretion of the RFP, which corresponds to the apex of the curve (T max is 4-5 min). Since the moment when the concentration of RFP in the kidney begins to decrease, i.e. The outflow of the RFP predominates over the intake, the third phase of the curve is noted. The duration of half-life of RFP from the kidneys varies from person to person, but on average it varies from 5 to 8 minutes.

To characterize the renographic curve, three indicators are usually used: the time to reach the maximum of radioactivity, the height of its maximum rise, and the duration of half-life of the RFP from the kidney. When the renal and urinary tract function is impaired, the renographic curves change. We indicate four characteristic variants of the curves.

• The first option is a slowdown in the receipt of RFPs into the "zone of interest" of the kidney. This is manifested by a decrease in the height of the curve and the elongation of its first two phases. This type is observed with a decrease in blood flow in the kidney (for example, with narrowing of the renal artery) or a decrease in the secretory function of the tubules (for example, in patients with pyelonephritis).
• The second option is to slow the excretion of RFP by the kidney. This increases the steepness and duration of the second phase of the curve. Sometimes within 20 minutes the curve does not reach a peak and the subsequent decline does not occur. In such cases, one speaks of a curve of the obstructive type. In order to distinguish the true obstruction of the urinary tract by a stone or other mechanical obstruction from dilatation uropathy, intravenously inject a diuretic, for example, a lasix. When urinary tract obstruction, the introduction of a diuretic does not affect the shape of the curve. In the cases of the functional delay of the RFP transit, an immediate drop in the curve occurs.
• The third option is the delayed intake and excretion of RFP from the kidneys. This is manifested by a decrease in the total height of the curve, deformation and elongation of the second and third segments of the rheogram, the absence of a clearly expressed maximum. This variant is observed mainly in chronic diffuse diseases of the kidneys: glomerulonephritis, pyelonephritis, amyloidosis, and the severity of changes depends on the severity of kidney damage.
• The fourth option is the repeated rise of the renographic curve. It is observed with vesicoureteral reflux. Sometimes this option is detected with conventional scintigraphy. If it is not, and on the basis of clinical data suspect reflux, then at the end of the renography the patient is offered to urinate in a bed-liner. If a new rise occurs on the curve, this means that the urine containing urine from the radionuclide returned to the ureter and further to the renal pelvis.

[1], [2], [3], [4], [5], [6], [7], [8], [9]