Renal Infections

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Acute Pyelonephritis and Renal Scarring

Most upper urinary tract infections in children originate from bacterial contamination of the perineum and lower urinary tract by uropathogenic fecal flora. Although vesicoureteral reflux is the most widely described mechanism for upward transport of bacteria from the bladder to the renal collecting systems and kidneys, it is not invariably present in patients who develop ascending pyelonephritis. Other, as yet uncharacterized, mechanisms for upward conveyance of bacteria clearly exist. Pyelonephritis secondary to hematogenous seeding of the kidney is, by comparison, quite rare in childhood and generally occurs only in the context of serious extra-urinary disease of the cardiovascular, gastrointestinal, or musculoskeletal system that predisposes to recurrent bacteremia.

Ascending pyelonephritis in children is typically segmental in distribution and occurs most frequently at the renal poles. Although pyelonephritis can be unifocal, multifocal involvement is much more common. Bilateral disease is often present. The polar predilection in ascending pyelonephritis is thought to be due to the greater tendency of the papillary orifices at the renal poles to be incompetent, thereby allowing for "intrarenal" reflux of infected urine from the collecting system back into the collecting tubules. This phenomenon of intrarenal reflux is also commonly referred to as pyelotubular backflow.

Frequently, it can be difficult to reliably differentiate between upper and lower urinary tract infections in children based upon clinical and laboratory parameters alone.

Imaging modalities that have been used for diagnosing acute pyelonephritis in children who have fever and bacteruria include DMSA renal cortical scintigraphy, power Doppler ultrasonography, CT, and MRI (29,93) (104-112). With each of these modalities, the diagnosis of acute pyelonephritis depends on the demonstration of both anatomic and physiologic changes in the appearance of the kidneys. Anatomic changes include parenchymal edema resulting in local or generalized renal enlargement, often with focal alterations in the renal contours, as well as alterations in renal parenchymal imaging characteristics, such as changes in tissue echogenicity, attenuation, or signal intensity. Alterations in the appearance of the renal collecting systems, such as urothelial thickening secondary to edema, pelvocaliectasis from bacterial endo-toxins, vesicoureteral reflux, and the presence of debris in the collecting systems, ureters, or bladder are also common. Physiologic changes include segmental or lobar reduction in renal parenchymal perfusion and excretory function in the affected areas.

The vast majority of upper urinary tract infections in children are bacterial, with more than 90% being due to Escherichia coli. Nevertheless, the imaging appearance of acute pyelonephritis is not microbiologically specific and is frequently the same in fungal disease, as in bacterial disease. The abnormal pattern of enhancement of the infected areas can persist for several months after completion of antibiotic therapy, before resolving or progressing to visible scar formation. In the event of scar formation, evolution in the appearance and size of the scar can continue for years following the inciting infection, with the scar becoming increasingly conspicuous as the surrounding unaffected tissue continues to grow. The sensitivity of CT in acute pyelonephritis is comparable with that of DMSA nuclear scintigraphy and superior to that of combined power Doppler and conventional gray-scale US (93,104). The ability of CT or MRI to differentiate between scars and areas of acute infection in the absence of a previous examination is a significant advantage over conventional DMSA nuclear scintigraphy (12,113).

US is the most widely used imaging modality for evaluating children who have acute urinary tract infection. Inflammatory renal edema secondary to acute pyelonephritis can produce local or generalized renal enlargement, as well as focal alterations in the renal contours and cortical echogenicity (13,31,87,105,106). Focal polar enlargement can be dramatic and can occasionally be misinterpreted as being due to the presence of a renal mass. Areas of pyelonephritis typically appear abnormally hyperechoic with reduced or absent corticomedullary differentiation. However, areas that are severely hypovascular and those that are undergoing necrosis can appear hypoechoic and can be difficult to distinguish from early abscess development (13,31,114).

Renal Abscess Mri

Figure 15 An infant girl with acute multifocal pyelonephritis. A coronal postcontrast inversion recovery image shows multiple wedge-shaped, high-intensity and, therefore, nonenhan-cing lesions throughout the right kidney, consistent with acute multifocal pyelonephritis. A very small amount of perinephric fluid is present around the lower pole with adjacent edema of the perinephric fat.

Figure 15 An infant girl with acute multifocal pyelonephritis. A coronal postcontrast inversion recovery image shows multiple wedge-shaped, high-intensity and, therefore, nonenhan-cing lesions throughout the right kidney, consistent with acute multifocal pyelonephritis. A very small amount of perinephric fluid is present around the lower pole with adjacent edema of the perinephric fat.

Urinary Collecting System

On nonenhanced CT, the diagnosis of acute pyelonephritis is often more difficult because infected segments are generally isodense or only slightly hypodense in relation to adjacent normal parenchyma. Increased attenuation in a focus of acute pyelonephritis on nonenhanced CT is consistent with localized hemorrhage secondary to segmental venous thrombosis and parenchymal infarction, an appearance frequently associated with scarring and severe parenchymal atrophy (13,31,114). On contrast-enhanced CT, acute pyelonephritis usually produces one or more wedge-shaped or triangular areas of decreased parenchymal enhancement (13,31,87,93,104,106,112). The infected areas in the kidney appear edematous, typically having convex margins and causing the renal contour to bulge, occasionally even mimicking an intrarenal mass. With more extensive involvement, the affected portion of the kidney appears enlarged, although it usually still maintains its reniform shape, with patchy enhancement of the less involved areas. The need for intravenous contrast enhancement as well as for exposure to ionizing radiation are both drawbacks to using CT. In addition, some authors have suggested that multiple contrast-enhanced imaging series are necessary to assure maximum sensitivity in detecting acute pyelonephritis. They have observed that in some patients, the diagnosis is not apparent on early contrast-enhanced "cortical" phase CT images (12,108). In comparison, in these patients the findings are frequently more apparent on delayed enhanced images.

The use of MRI for the diagnosis of acute pyelonephritis and renal cortical scarring has been studied in both piglets and humans (12,107-110,113). On gadolinium-enhanced fast spin-echo T2 and inversion recovery sequences, normally enhancing renal parenchyma have a much reduced signal intensity due to the shortening of the T1 and T2 relaxation times by gadolinium. In contrast, areas of pyelonephritis remain bright because of the lower gadolinium concentration in these areas (Fig. 15). The infected areas in the kidney are also typically edematous and can produce a localized bulge in the renal contour. As with CT, edema of the perinephric fat is common. In addition, small amounts of perinephric fluid are occasionally visible on MRI. Their presence does not necessarily indicate the presence of a perinephric abscess that requires drainage (13,31).

In a comprehensive study of acute pyelonephritis in a piglet model, Majd et al. (108) compared the accuracy of DMSA-single photon emission CT (SPECT), MRI, CT, and power Doppler ultrasonography, using histology as the reference standard. In this study, there were no statistically significant differences between DMSA-SPECT, MRI, and CT either in the diagnosis of pyelonephritis or in the localization of lesions within the kidneys. The sensitivity for the diagnosis of pyelonephritis exceeded 90% for all three modalities. Power Doppler ultrasonography, on the other hand, was significantly less sensitive than any of these other modalities.

Post-pyelonephritis renal cortical scarring and atrophy are also usually readily visualized on MRI as areas of parenchymal loss without change in signal intensity between pre- and postgadolinium inversion recovery sequences (12,13,31,108,113).

Figure 16 (Figure on facing page) A 10-year-old girl with a history of recurrent urinary tract infections and bilateral vesicoureteral reflux resulting in bilateral reflux nephropathy with marked right renal atrophy and malignant hypertension. (A) A coronal postcontrast T1-weighted MIP image shows a markedly atrophic right kidney with clubbed calyces bilaterally. (B) A coronal 2 mm thick slice through the kidneys postcontrast demonstrates the marked par-enchymal loss on the right. Both kidneys are excreting the contrast material, but the right kidney contributes only 14% of total renal function. Abbreviation: MIP, maximum intensity projection.

Retraction of the renal contour is often evident in affected areas and can progress as the scar matures, with loss of renal parenchymal volume (Fig. 16) (113). The sensitivity of MRI using fat-saturated, Tl-weighted, and an enhanced inversion recovery sequence is comparable with that of DMSA renal cortical scintigraphy in detecting renal scarring (108,113). Whether perfusion imaging of the renal cortex in the early contrast-enhanced phase is more sensitive in detecting renal scarring than is either DMSA or conventional MRI has not yet been studied objectively.

Complicated Renal Infections

Renal abscess and other serious acute complications of pyelonephritis are, fortunately, very rare in healthy children with ascending pyelonephritis. Renal abscesses are more commonly seen in children with hematogenous pyelonephritis, in those who are immunocompromised (115), and where infection occurs in an obstructed collecting system. Distinguishing between uncomplicated acute pyelonephritis and abscess is important because uncomplicated infection is typically treated with antibiotics alone, whereas an abscess usually requires surgical drainage (116).

Renal abscesses can be perirenal or intraparenchymal. Intraparenchymal abscesses appear as round or irregular cystic intrarenal structures, which may be uni-locular or multilocular. On sonography, abscesses typically contain echogenic purulent debris. Although CT remains the most widely used modality for the evaluation of complicated renal infections in children, as in adults (116,117), MRI is equally efficacious and would be the preferred modality in patients with a significantly impaired renal function or with a history of contrast allergy, in whom the use of iodi-nated contrast media may be contraindicated. CT and MRI can detect even small amounts of perinephric fluid. Careful monitoring of the clinical course of patients with such fluid is warranted to ensure that this fluid does not develop into a discrete abscess. If patients remain septic or febrile, close imaging follow-up is also critical. Sometimes, CT and MRI can suggest that perinephric fluid is infected. Debris within infected fluid may alter its attenuation or signal characteristics. Also, when contrast material is administered, an enhancing wall can often be seen around the fluid.

Both bacterial endotoxin and vesicoureteral reflux can produce transient dilatation of the renal collecting systems and ureters, mimicking obstructive renal collecting system and ureteral dilatation, or even pyonephrosis (when collecting system debris is also present). US, CT, and conventional MRI are not definitive in differentiating between an uninfected dilated collecting system and true pyonephrosis. Echogenic (as seen on US), high-attenuation (as seen on CT), or high-signal (as seen on MRI) debris in the renal collecting systems and ureters of children with pyelonephritis does not necessarily indicate either obstruction or pyonephrosis, even when the collecting system is dilated. Similarly, desquamated cells and crystalline material in chronically obstructed, but uninfected systems cannot necessarily be differentiated from purulent urine on standard imaging studies.

Diffusion-weighted MRI has been applied extensively in neuroimaging for evaluating patients with acute cerebral strokes and intracranial neoplasms and in demyelinating disorders, as well as in differentiating between cerebral abscess and cystic or necrotic neoplasms (118). Until recently, applications of diffusion-weighted imaging (DWI) have been quite limited outside of the central nervous system because of significant image degradation related to respiratory and cardiac motion. Ultrafast, single-shot echo planar imaging techniques now permit the application of DWI elsewhere in the body, including the urinary tract. By revealing the micromolecular motion of water within tissues, DWI provides information on the velocity and direction of movement of the water molecules in tissue under the influence of a diffusion gradient (119). Marked hyperintensity on diffusion-weighted sequences consistent with restricted diffusion has been reported in pyelonephritis and renal abscesses in animals and adult humans. Restricted diffusion in pyelonephritis is thought to be primarily a result of cytotoxic edema, as is the case in the central nervous system (119,120). However, intratubular inspissation of inflammatory cells might also be important. DWI also might be useful in differentiating between noninfected dilatation and pyonephrosis (119). Differences in the relative mobility or viscosity of water molecules in tissues create the contrast in DWI. Because pus is a thick, high-viscosity fluid consisting of water, inflammatory cells, necrotic tissue, and proteinaceous exu-dates causing a marked restriction of water proton mobility, the fluid in a renal abscess or pyonephrotic collecting system has a very low apparent diffusion coefficient (ADC). As a result, the fluid in the pyonephrotic collecting system has a very high signal intensity on diffusion-weighted images and relative signal hypointensity of ADC maps. Either acute or subacute hemorrhage into the collecting system or within a renal cyst can also produce a very high signal intensity because the very long T2 relaxation time of blood can mimic the appearance of purulent material. However, the ADC maps, which are free of this T2 effect, will differentiate between hemorrhage and pus. Nevertheless, at the current time, percutaneous puncture of the collecting system remains the only definitive procedure for the diagnosis or exclusion of pyonephrosis. In pyonephrosis and renal abscess, drainage is necessary and is usually accomplished percutaneously with US guidance. Although DWI is being utilized increasingly in pediatric neuroimaging, applications for this imaging approach in pediatric uroradiology are still limited.

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