Radionuclide methods used in nephrourology

 

            Renal scintigraphy offers a rapid, noninvasive means of evaluating suspected urinary tract disorders. It provides sensitive indices of relative renal blood flow, glomerular filtration, tubular function, and urinary excretion. We can use different radiopharmaceuticals a also different method to collect data. From this point of view it is very important to clearly know the clinician diagnotic question. This knowledge makes possible to choose proper radiopharmaceutical and imaging method to answer this question. On the other hand we must take in mind that not all renal scans are diagnostic.

 

 

            Radiopharmaceuticals.

 

            99mTc DTPA:

            It has a clearance comparable to inulin and may be used to measure glomerular filtration rate.

 

                                                                                                                                                         123I or 131I hippurate                                                                                 

            Orthoiodohippurate is cleared similarly to paraaminohippuric acid and provides a measure of effective renal plasma flow and tubular function. Approximately 60 to 80% of OIH entering the kidneys is extracted with each pass. A small percentage is cleared by glomerular filtration while the remainder is removed by the tubules. In normal conditions, the rate of OIH clearance by the kidneys is dependent of renal blood flow. In certain disease states, the extraction fraction may fall and OIH clearance is no longer a measure of renal plasma flow.

 

            99mTc DMSA:

            It binds to the cortical tubules and is considered to represent functioning tubular mass. 99mTc DMSA reaches the tubules via the glomerular filtrate as well as tubular extraction from the peritubular capillaries. About 50% of the injected dose accumulates in the cortex within 1 hour of injection and remains in the kidneys for 24 hours.

 

            99mTc MAG3:

            It has similar properties as OIH does. The clearance of 99mTc MAG3 can be used as an independent measure of renal function. The clearance of 99mTc MAG3 can be calculated based on the dose injected and a single sample of plasma obtained 43 minutes postinjection. The image quality of 99mTc MAG3 is superior to that obtained with 99mTc DTPA.

 

 

            51Cr- EDTA:

            It is excreted by means of glomerular filtration and so it is used for in vitro measurement of glomerular filtration rate. This method is suposed to be the most accurate.

 

 

 

 

 

 

 

         Diagnostic methods:

 

         In vivo – non-imaging methods:

           

GFR and ERPF measurement:

            They are not widely used. They require several plasma samples and their measurements. They use several mathematical models to calculate particular physiologic parameters. They can assess only global parameters.

 

            Radionuclide nephrography:

            They are still used by some departments. They use scintillation probe to measure radioactivity over both kidneys and sometimes also blood clearance. They are very simple and are used namely in patient follow-up. They make possible evaluation of both kidneys separately.

 

         Imaging methods:

            They are most often used methods for kidney and urinary tract evaluation. They can be divided into two types - static and dynamic imaging.

 

            Static imaging:

            Renal images are usually obtained 2 to 3 hours following intravenous injection of 99mTc DMSA. Since rapid loss of tracer does not occur, several views of the kidneys ca be obtained, including SPECT imaging. The static renal images obtained provide good definition of the cortical outline and, in addition, shouw the relative distribution of functional tissue. The ratio of tracer uptake between kidneys provides a measure of divided renal function.

 

            Dynamic imaging:

            Dynamic imaging is performed most often with 99mTc DTPA or 99mTc MAG3.In contrast to DMSA the tracers are rapidly excreted, and thus rapid sequential renal imaging must be performed. The images that are obtained provide information relating to renal vascularity, renal function and excretion. Following an intravenous bolus of these tracers an image obtained for the first 30 seconds provides a so called vascular image, with the major blood vessels and perfusion to both kidneys, liver and spleen being visualized. The amont of activity at each site reflects the relative vascularity. Renal function is assessed at 2 minutes after injection, when there is good renal visualization and an image show the relative distribution of function between the kidneys. Thereafter, cortical activity rapidly diminishes as the tracer is excreted by glomerular filtration or tubular secretion. By 5 minutes activity is normally seen in the collecting systems and serial imges are obtained up to 20 to 30 minutes which show progressive excretion of tracer. If there is any suggestion of obstruction, it is important to mobilize the patient and obtain a subsequent image to ensure that there is no functional hold-up caused by patient positioning. If the question of obstruction has not been resolved, the study will need to be extended and further images obtained following diuretic administration.

 

 

            Patient positioning:

            Some clinicians image the patient in a seated or prone position, but most image the patient supine with the camera positioned beneath the kidneys. In the supine position the kidneys are most likely to be eqiudistant from the camera, and differences in uptake will represent differences in relative function rather than defferences in attenuation. Imaging the patient in an upright or seated position may interfere with measurements of relative function. Occasionally, patients will be imaged in the upright or seated position to facilitate drainage of the urine from the renal pelvis when the clinical question is obstruction.

 

 

            The renogram curve:

            The renogram curve is a time-activity curve descriging the transit of any tracer through the kidney. The curve is obtained by placing a computer-assested region of interest over the whole kidney or the cortex, obtaining the counts in the ROI for each period of data acquisition, and plotting these counts as a function of time. The renogram curve is often divided into the period of tracer appearance, tracer extraction and tracer elimination denoted as phase 1, 2 and 3 respectively. Tracer appearance describes the period of blood flow beneath the detector, tracer extraction is proportional to renal plasma flow or glomerular filtration rate according to used tracer. The curve peaks when tracer exits from the kidney at the same rate it is entering the kidney.

The kidney is limited in the number of ways it can handle a tracer in response to disease; it may accumulate less of the tracer, accumulate it at a slower rate, or eliminate it at a slower rate. Tracer kinetics can be altered by disease as well as by the level of hydration or dehydration. Proper interpretation of the renogram curve requires the clinician to understand what the curve implies in terms of renal function and then to relate that functional information to potential desease processes. Nowadays mathematical function called deconvolution is also used for renogram curve evaluation. It can better detect renal artery stenosis and distinguish between obstruction and nonobstructed pelvic dilation and detect transplant rejection.

 

 

         Clinical applications:

            Renal function:

            Radionuclide renal imaging with quantitation provides the only non-invasive accurate method of measuring the contribution of an individual kideny to overall renal function. In addition, the technicque can be used to assess the regional distribution of function within an individual kidney.

                        Main clinical situation for renal function evaluation:

                        Renal function in urinary tract infection

                        Renal tubular dysfunction

                        Renal function with calculi

                        Renal function after renal stone removal.

            It is important to specify the parameter being measure (GFR, ERPF, DMSA uptake) rather than using the more general term „function“.

 

            Obstruction.

            Hydronephrosis, the dilatation of the renal pelvis and collection system, is often due to obstruction. Intravenous urography, CT and ultrasound can imply the presence of obstruction based on pelvic or ureteral dilatation. If obstruction is present, intervention is usually indicated. It is well recognized, however, that the ureters and renal pelvices may be dilated in patients who have no obstruction to urine flow; dilatation can occur in those with reflux, congenital anomalies, previous obstruction, and prior urinary tract surgery.

The diuretic-augmented renogram is used to distinguish between them. It can also be used to determine if surgery has been successful in relieving a known obstruction.

                        Main clinical situation for renal obstruction evaluation:

                        Measurement of renal function in known obstruction

                        Assessment of eqiuvocal obstruction after IVU or ultrasound

                        Baseline during a period of observation

                        Preoperative evaluation

                        Postoperative comparison.

 

 

            Reflux nephropathy.

            Radionuclide studies are routinely used in the management of vesicoureteral reflux and can be used to help predict which patients will have spontaneous resolution of their reflux. There are two radionuclide techniques for reflux detection. Indirect method means intravenous administration of tracer. When the tracer reaches the bladder, patient is asked to void. The kidneys, ureters, and bladder are monitored by use of a gamma camera and the diagnosis of reflux is based on a singificant increase in activity in the upper urinary tract during or after voiding. This indirect method is simple, but has several limitations. The second technique is direct cystography. It serves as an important role in the management of patients with reflux or suspected reflux. Bladder catheterization is needed. The bladder volume at which reflux occurs should be noted and may be a useful prognostic factor.

                        Main clinical situation for reflux evaluation:

                        Determination of presence or absence of renal scars

                        Measurement of individual kidney function

                        Identification of presence or absence of reflux.

 

 

            Bacterial infection

            Clinical singns of  the bacterial of infection like fever, leukocytosis, bacteriuria are not able to distinguish between upper and lower urinary tract infection, but this disicion is very important. For this purpose, especially in neonates and infants, static renal scintigraphy using 99mTc DMSA is routinely used. It can detect focal abnormalities of tubular function before distortion of the normal anatomy occurs.

 

 

            Trauma.

            Scintigraphy is both sensitive and specific for detecting traumatic renal injuries. It can rapidly assess perfusion to both kidneys, it has also possibility of detecting active intraperitoneal or retroperitoneal hemorhage. Also excretory function can be evaluated in a few minutes.

 

 

 

            Renal failure.

            We can classify renal failure from the point of view of scintigraphic findings to several categories - obstruction, pre-renal (haemorrhage, hypotension, diarrhoea), acute tubular necrosis, parenchymal disease (glomerulonephritis), vascular cause and chronic renal failure.

            Main scintigraphic features are as follow:

            In obstruction, dilated calyces can be seen.

            In pre-renal failure, we can see normal blood flow and good uptake, but delayed intrarenal transit and minimal excretion.

            In acute tubular necrosis, we can see almost normal blood flow, but absent or minimal uptake and no excretion.

            In parenchymal disease, minimal blood flow, poor uptake and poor or absent excretion can be seen.

            In vascular disorders the findings are almost the same.

            In chronic renal failure, small kidneys, poor blood flow and little or no uptake and excretion can be seen.

 

 

            Space-occupying lesions

            Radionuclide scanning has a limited use in investigating renal masses because other investigations provide the information necessary for the diagnosis and management of these patients.

            Tumour

            Renal cysts

            Renal calculus.

 

 

            Congenital and ectopic abnormalities.

            Cystic disease

            Horseshoe kideny

            Crossed renal ectopia

            Pelvic kidney

 

           

 

            Vascular disorders and hypertension

            A functional agent is often used to evaluate patients with suspected renovascular hypertension. For this reason mostly the Captopril-Augmented renography is used. It often consists of a baseline scan followed at a later time by oral administration of 25 to 50 mg of captopril and a repeat renogram 1 to 2 hours after the captopril dose. The study is positive if there is a major change in relative uptake or in the washout phase of the renogram curve.

                        Main clinical situation for this evaluation:

                        Hypertension due to renal artery stenosis

                        Renal infarction

                        Before surgery - angioplasty

 

 

 

 

            Renal transplant

            Technical problems that require prompt intervention include obstruction of the ureter, leakage at the ureteral anastomosis and renal artery of renal vein stenosis or occlusion. Later problems include renal artery stenosis, cyclosporin toxicity and ureteral stricture. When the graft fails to function normally, the clinician must distinguish between these various possibilities.

            Acute renal vessel occlusion, poor parenchymal function, renal infarction, ureteral obstruction and extravasation can be usually detected by renal scintigraphy. Serial scan obtained during the first 1 to 2 weeks post-transplantation are useful in monitoring recovery from transplantation.

            Acute rejection is characterized by decreased blood flow, decreased uptake, prolonged intrarenal transit time, decreased excretion and increased renal size.

 

 

            Scrotal imaging:

            It make possible to distinguish between torsion and epididymitis. It enables surgeon to avoid unnecessary surgery. The patient is placed uder the gamma camery supine, bolus of 99mTc pertechnetate is injected intravenously and a radionuclide angiogram is obtained. The statid images are acquired several minutes thereafter. An area of decreased perfusion corresponding to the involved testis indicated a high probability of torsion. Normally perfused or hypervascular testis can exclude surgery.

 

 

 

 

         Non-imaging procedures

 

 

            Measurement of plasma volume

            Measurement of red cells volume