Nuclear Medicine Urinary System PDF

Nuclear Medicine Urinary System Introduction Radionuclide evaluation of the genitourinary system includes quantitative estimates of renal perfusion and function. Ultrasound and CT are used for the evaluation of renal anatomy, the role of nuclear renal imaging has become more confined t...

Nuclear Medicine Urinary System Introduction Radionuclide evaluation of the genitourinary system includes quantitative estimates of renal perfusion and function. Ultrasound and CT are used for the evaluation of renal anatomy, the role of nuclear renal imaging has become more confined to functional analysis. Nuclear medicine techniques maintain “gold standard” status in the diagnosis of upper urinary tract obstruction and pyelonephritic scarring secondary to urinary tract infection (UTI). Indications Assessment of renal blood flow Quantitative functional assessment of both native and transplanted kidneys. Evaluating ureteral or renal pelvic obstruction Vesicoureteral reflux Suspected renovascular hypertension PHYSIOLOGY The excretory function of the kidneys consists of: Passive filtration mechanism: through the glomerulus (20%) Active secretion: by the tubules (80%) http://unckidneycenter.org/kidneyhealthlibrary/glomerular-disease Radiopharmaceuticals Radiopharmaceuticals commonly used for evaluating renal function and anatomy fall into three main categories: Those excreted by tubular secretion Those excreted by glomerular filtration Those bound in the renal tubules for a sufficiently long time to permit cortical anatomic imaging Radiopharmaceuticals Glomerular Filtration Tc-99m-DTPA Tc-99m-MDP 51Cr- EDTA Glomerular Filtration + Tubular Function Tc-99m-MAG3 Tubular Function Tc-99m-DMSA Radiopharmaceuticals The minimal protein binding of 99mTc-DTPA and 51Cr- EDTA means that they rapidly diffuse into the extravascular space and are freely filtered at the glomerulus, which represents their only excretory pathway under normal circumstances. As neither agent is reabsorbed in the renal tubule, so either can be used to determine glomerular filtration rate. RADIONUCLIDE RENAL EVALUATION Evaluation of the kidneys may be performed by using a the following methods: Functional imaging (visual assessment of perfusion and function) Renography (time-activity curves representative of function) Quantification of renal function (GFR and ERPF determinations) Anatomic imaging (visual assessment of the renal cortex) Functional Renal Imaging Imaging may be adequately performed in most patients by using either 99mTc-MAG3 or 99mTc-DPTA. Functional imaging of the kidneys may be divided into assessment of blood flow, parenchyma, and excretion. Renal Perfusion Imaging Evaluation of renal blood flow and function of native kidneys is performed from the posterior projection. Normally, a small bolus of high-activity of (99mTc-DTPA or 99mTc-MAG3) is injected intravenously, preferably into a large antecubital vein. Imaging renal perfusion is usually begun as the bolus is visualized in the proximal abdominal aorta, with subsequent serial images made every 1 to 5 seconds, depending on the instrumentation available and the preferences of the interpreter. Renal Perfusion Imaging The activity reaches the kidneys about 1 second after the bolus in the abdominal aorta passes the renal arteries. Time-activity curves reflecting renal perfusion during the first minute may be generated by drawing regions of interest over the aorta and each kidney. Each of the renal curves may then be compared with the time- activity curve of the abdominal aorta to assess relative renal perfusion. In some cases, the spleen overlies the left kidney, giving a false impression of asymmetrically increased left renal perfusion or of a “phantom kidney” in patients with prior left nephrectomy. Normal renal blood flow A bolus of 99mTc-DTPA in the lungs is visualized at the top of the serial images at 1 second. By 3 seconds, the aorta is fully visualized. By 5 to 6 seconds, both kidneys are clearly seen. The flow is symmetric to both kidneys. Maximal activity in the kidneys usually is reached later, between 30 and 60 seconds. Renal Function Imaging At the end of the renal perfusion sequence, imaging for renal function begins. Dynamic or sequential static, 3- to 5-minute 99mTc- DTPA or 99mTc- MAG3 images are then obtained over 20 to 30 minutes. Evaluation of the images includes attention to renal anatomy and position, symmetry and adequacy of function, and collecting system patency. post-void or post-ambulation images to enhance collecting system drainage may be obtained as needed. Time-activity (renogram) curves for each kidney, reflective of relative renal function, are also usually created from regions of interest over the renal parenchyma. Normal renogram After administration of 99mTc-mertiatide (MAG3), maximal kidney activity is seen at about 3 to 5 minutes, and, by 4 to 5 minutes, the bladder can be identified. By about 8 to 12 minutes, most of the activity has cleared the parenchyma and is seen in the collecting systems, making the kidneys appear slightly smaller than on the early images. Renography A renogram is a time-activity curve that provides a graphic representation of the uptake and excretion of a radiopharmaceutical by the kidneys In basic renography a series of 10 second duration digital images of the urinary tract are recorded over a period of about 30 minutes following injection of a suitable radiopharmaceutical. The field of view should include the heart, kidneys, and at least part of the bladder. Renography- Typical regions of interest for computer analysis. Renogram curves are generated by placing a region of interest around each kidney, usually the entire kidney. Background subtraction ROIs are selected just inferior to each kidney An aortic ROI may be used to assess the adequacy of the injected bolus as well as relative renal perfusion. Outlined regions of interest are drawn over the aorta, the kidneys, and the bladder. Areas of background activity (Bkd) are also drawn. Time-activity curves are then generated for each of these after appropriate background subtraction has been made. Renography Image processing is then performed using computer- generated ROI in order to obtain graphs showing the variation of radioactive count rate with time within organs and tissues of interest “Time–activity curve” (TAC) associated with an individual kidney is called a renogram. The renogram curves and associated images can be visually inspected to determine whether the drainage function from each kidney is normal or abnormal BUT In this basic form, renography can not determine the cause of any abnormal drainage. Typical renogram curves Schematic drawing demonstrates the Actual renogram shows symmetric activity conceptual portions of the time-activity between right and left kidneys, rapid curve within the kidney. drop-off after the peak, and a long tail extending to the right. The curve also shows increasing activity within the bladder after about 4 minutes. Typical renogram curves 1. Vascular transit phase: initial renal perfusion, lasts about 30 to 60 seconds and represents the initial arrival of the radiopharmaceutical in each kidney. 2. Cortical or tubular concentration phase of initial parenchymal transit. 1- 5 minutes and contains the peak of the curve. 3. Clearance or excretion phase, which represents the down slope of the curve and is produced by excretion of the radiopharmaceutical from the kidney and clearance from the collecting system. Patient preparation and renogram curves Patients should be well hydrated when renography is performed because in the presence of dehydration, an abnormal renogram curve demonstrating delayed peak activity, delayed radiopharmaceutical clearance, or an elevation of the excretion slope may result. Overall, the renogram curves for each kidney should be reasonably symmetric, although slight asymmetries are not unusual. The shapes of curves should also be inspected individually for alterations in the normal configuration. Renogram curves Data commonly derived from 99mTc-MAG3 renograms include the following: Time to peak activity: Normal is about 3 to 5 minutes. Relative renal uptake ratios at 2 to 3 minutes. This is an index of relative renal function between the two kidneys. Activity in each kidney should be equal, ideally 50%. A value of 40% or less in one kidney should be considered abnormal. Half-time excretion is the time for half of the peak activity to be cleared from the kidney. Normal is about 8 to 12 minutes. Differential cortical retention at 15 minutes. The percentage of retained activity about 15 minutes after injection in each kidney should be relatively equal. Differences of 20% or more should be considered abnormal. The 20 minute-to-peak count ratio. This is the activity measured in each kidney at 20 minutes and expressed as a percentage of peak curve activity. In the absence of pelvic calyceal retention, or if only a cortical region of interest is used, a normal 20-minute maximal cortical ratio for 99mTc-MAG3 is less than 0.3 (or 30%). Indirect Radionuclide Cystography Indirect radionuclide cystography (IRC) is effectively a combination of a basic renogram and a subsequent relatively short dynamic study monitoring free voiding into a receptacle (voiding study). The purpose of this test is to detect vesico-ureteric reflux (VUR), VUR is an obvious source of upper urinary tract infection (UTI) confirming retrograde flow of urine from bladder into the kidneys. (reflux) Voiding stage of an indirect radionuclide cystogram. The peak in the left kidney (LK) curve (bold line) indicates vesico-ureteric reflux occurring at the onset of micturition. The flat right kidney (RK) curve (- - line) is normal. The subsequent rise in the bladder curve (... line) indicates refilling from above (i.e. the left kidney), confirming the diagnosis of left-sided reflux. Anatomic (Cortical) Imaging Using 99mTc-DMSA A pinhole or a high-resolution collimator, or single-photon emission computed tomography (SPECT). Renal cortical imaging is usually performed for the evaluation of: Space-occupying lesions Functioning pseudotumors Edema or scarring associated with acute or chronic pyelonephritis, especially in children. Clinical applications Diffuse Renal Disease In the evaluation of diffuse renal diseases producing acute or chronic impairment of renal function, such as acute pyelonephritis or chronic glomerulonephritis, radionuclide techniques are often sensitive but not disease-specific. Most often, there is simply demonstration of unilaterally or bilaterally poor vascular perfusion and poor radiopharmaceutical excretion. The renogram provides quantitative estimates of the function of each kidney Generally, poor renal function results in flattening of the renogram curve as concentration and excretion of the radiopharmaceutical become increasingly impaired. Acute pyelonephritis Posterior perfusion images obtained after intravenous administration of 99mTc- mertiatide (MAG3) show decreased perfusion to the right kidney (arrows). Subsequent static images show decreased general activity in the right kidney throughout the study. Right renogram curve (arrow) demonstrates a near-normal shape but depressed function. Acute tubular necrosis There may be normal or only modestly reduced renal perfusion and preserved parenchymal accumulation but bilaterally poor excretion of 99mTc-MAG3. This frequently presents with rising renogram curves. Reasonably good visualization of the kidneys indicates a favorable prognostic outcome, whereas poor visualization correlates with a prolonged or absent recovery.  Acute tubular necrosis. A, Posterior flow images done after IV administration of 99mTc-(MAG3) show symmetric and normal perfusion to both kidneys. B, Computer curves of blood flow during the first minute demonstrate a normal pattern, with aortic activity decreasing quickly after about 10 seconds and renal activity increasing rapidly up to about 30 seconds after injection, ultimately exceeding the peak aortic activity. Clinical applications Vascular Abnormalities In acute renal vein thrombosis, there is generally decreased or absent perfusion and delayed and diminished accumulation and excretion of 99mTc-MAG3 by an enlarged edematous kidney. Obstructive Uropathy Standard imaging techniques, such as ultrasonography, evaluate structure but do not depict urodynamics. Acute Pyelonephritis The diagnosis of acute pyelonephritis in children based on clinical and laboratory observations is frequently difficult, even in the presence of fever, flank pain, or a positive urine culture. CT and renal sonography have a low sensitivity and underestimate the degree of parenchymal involvement. Radionuclide renal cortical imaging is a highly sensitive technique for diagnosis of renal parenchymal infection and should be considered for the diagnosis of acute pyelonephritis in children. Acute pyelonephritis Technetium-99m DMSA in a dose of 50 μCi/kg is the radiopharmaceutical of choice, with a minimum administered dose of 500 μCi. In normal patients, cortical uptake of DMSA appears homogeneous throughout the kidneys, except for relative defects in the regions of the collecting systems. High-resolution magnified images with the use of a pinhole collimator, may show heterogeneous uptake and may provide better differentiation of the cortex from the medulla. Acute pyelonephritis In patients with acute pyelonephritis, there are three common patterns of presentation: (1) focal cortical defects, (2) multifocal cortical defects, and (3) diffusely decreased activity. Renal Masses Renal space-occupying lesions identified by sonography, CT, or magnetic resonance imaging (MRI) may occasionally warrant further evaluation with renal cortical imaging using 99mTc-DMSA to distinguish nonfunctioning from functioning renal tissue. Masses not representing renal parenchyma, such as neoplasms, abscess, cysts, hematoma, or infarcts, present as photopenic lesions in the renal parenchyma. Demonstration of functioning renal tissue in the region of the suspected mass confirms its benign nature. Static and SPECT Renal Imaging Using 99mTc- DMSA Abnormal DMSA scan in a 13-year-old boy with a history of UTIs and bilateral reflux. The relatively small right kidney shows a wedge-shaped defect in the upper pole that is typical of renal scarring. There is also an area of significantly reduced uptake in the lower pole of the left kidney References Fred A. Mettler, J., MD, MPH and Milton J. Guiberteau, MD. Essentials of nuclear medicine. Sixth edition