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3. CORTICAL SCINTIGRAPHY IN URINARY TRACT INFECTION
From the mid-1980 to the present, a series of studies have demonstrated the superiority of renal cortical imaging in detecting both acute pyelonephritis and renal scarring compared with both IVU and ultrasonography. Cortical scintigi-aphy overall detected approximately twice as many defects as USN and approximately four times as many defects as IVU.
Cortical scintigraphy should be performed to document the presence or absence of acute pyelonephritis early in the course of management of any patient in whom diagnosis is suspected clinically. If appropriate therapy is instituted promptly, renal scarring can be prevented. The study may also help determine which patients with clinical symptoms of pyelonephritis do not have upper tract involvement and can be managed with outpatient antibiotics. Cortical scintigraphy is cost-effective compared with a minimum hospitalization of 3 or 4 days to treat pyelonephritis.
Cortical scintigraphy also should be performed as a follow-up study in order to determine whether the kidney has healed or scarred. If scarring develops, the patient will need long-term management to prevent recurring renal injury and subsequent renal failure as well as to detect and treat hypertension, which may develop as a result of scaning .
Either 99mTcDMSA(dimercaptosuccinicacid) or 99mTc glucocheptonate can be used for cortical scintigraphy. 99mTc-DMSA is now recognized as the reference method to detect focal areas of renal parenchimal damage. This applies to acute pyelonephritis, renal scars regardless of the cause, renovascular disease, the poorly functioning kidney and the complex duplex kidney (see Table 1).
Approximately 40-50% of an injected dose is present in the cortex 2 hours after injection. The dose of radiopharmaceutical is calculated based on body weight, with a minimum dose necessary for adequate imaging( see later). Because DMSA is a fixed tubular agent, no dynamic excretory images can be obtained. Glucoheptonate is partially concentrated and excreted in the urine and partially bound to the renal tubule. Between 10% and 20% of glucoheptonate dose is present in the cortex 2 hours after injection. Extraction and drainage of the radiopharmaceutical can be obtained. Stasis in a hydronephrotic or dilated renal pelvis will interfere with cortical imaging. DMSA has an advantage over glucoheptonate in that it provides lower bladder and gonad exposure. 99mTcDMSA scintigraphy is emerging as method of the choice , because it combines high specificity sensitivity with convenience, repeatability, and acceptable radiation dose[l]. With the advent of newer technology there has been interest in evaluating the role of single photon emission tomography (SPECT) in renal cortical imaging. It has not yet been shown clinically whether the increased resolution of SPECT renal cortical imaging in children justifies sedation and doubling radiation exposure .
Although DMSA has been available and used for over 10 years, there is no general agreement on when to apply the DMSA scan in UTI, i.e. in the acute phase, in the follow-up or both. Factors such as the sort of health care organization, logistics and parental compliance to treatment may influence this decision. Several experimental studies in animals have shown a remarkable agreement between focal DMSA uptake defects and histologically proven pyelonephritis. In many studies the sensitivity and specificity has been over 90%. In recent study of piglets, Codley reported a sensitivity of 80%, but also that minor inflammatory changes could not be seen on scinitgraphy . After treatment 70% of the areas with mild to moderate reduction of uptake in the acute phase had normalized at follow up, while areas with initially absent uptake remained in over 90%.
The normal DMSA study demonstrates fairly uniform distribution of radiopharmaceutical throughout the cortex. The papillary pyramids and renal collecting systems don not accumulate DMSA and are seen as centrally located photopenic defects covered by a rim of cortex. These defects are particularly well seen on posterior oblique views. There are three recognizable patterns of pyelonephritis:
Acute pyelonephritis has been shown to be necessary etiologic factor for development of subsequent renal scarring , and the mechanism of renal injury in pyelonephritis has been extensively studied in the experimental models. The rate of resolution of defects due to pyelonephritis is age dependent, occurring more slowly in infants and smaller children and more rapidly in teenagers. With 99mDMSA scintigraphy, the true incidence of scarring with pyelonephritis can now be studies.
Six months is appropriate routine follow-up time. Studies to detect renal scarring should probably not be performed earlier than 3 months after acute infection. However, a study to evaluate a new acute febrile illness can be performed at any time.
Scarring also demonstrates a spectrum of appearances. The hallmark of chronic pyelonephritis or renal scarring is volume loss, either focal or global, in the affected kidney. Volume loss accompanies focal cortical defects or obvious cortical thinning. The scars may be large or small, single or multiple.
The role of cortical scintigraphy in covert bacteriuria, in patients with only lower tract infection clinically, and in siblings of patients with VUR remains to be more fully evaluated.
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