Emerg Radiol (2007) 14:13–22 DOI 10.1007/s10140-007-0591-3
REVIEW ARTICLE
State of the art: imaging of renal infections Jennifer Demertzis & Christine O. Menias
Received: 22 December 2006 / Accepted: 30 December 2006 / Published online: 21 February 2007 # Am Soc Emergency Radiol 2007
Abstract Infections of the urinary tract are common and account for a large number of patient encounters in the ambulatory and emergency department setting. Their severity and presentation varies, and although uncomplicated infections can often be diagnosed and treated based on clinical and laboratory findings alone, diagnostic imaging may be necessary in some cases to guide appropriate medical or surgical therapy. We will briefly describe the indications for imaging known or suspected upper urinary tract infections, and the relative benefits and limitations of the different imaging modalities for a given clinical presentation. This will be followed by a discussion of various types of renal infections with a pictorial review of their imaging appearances and differential diagnoses. Keywords Urinary tract infection . Kidney diseases . Pyelonephritis . Interstitial nephritis . Diagnostic imaging Abbreviations UTI Urinary tract infection CT computed tomography IVU intravenous urography
This lecture was originally presented at the American Society of Emergency Radiology annual meeting in Washington DC on September 30, 2006. J. Demertzis (*) : C. O. Menias Mallinckrodt Institute of Radiology, 510 South Kingshighway Boulevard, Campus Box 8131, St. Louis, MO 63110, USA e-mail:
[email protected] C. O. Menias e-mail:
[email protected]
MRI US XGPN
magnetic resonance imaging ultrasound xanthogranulomatous pyelonephritis
Introduction Urinary tract infections (UTIs) are the most commonly reported bacterial infection, accounting for seven million office visits annually and 100,000 hospitalizations [1]. In many instances, patients present with uncomplicated cystitis or pyelonephritis. Uncomplicated UTIs are distinguished from complicated infections in that they occur in otherwise young, healthy, nonpregnant women and are therefore expected to respond favorably to appropriate antibiotic therapy [2]. Diagnostic imaging is generally not indicated for this population. However, for patients with complicated UTIs, radiologic evaluation can be useful in directing appropriate medical or surgical therapy, thereby preventing unfavorable or potentially catastrophic outcomes. At most institutions, available imaging modalities include ultrasound (US), intravenous urography (IVU), computed tomography (CT) with or without CT urography, and magnetic resonance imaging (MRI), each with their own potential benefits and limitations. In this article, we will review the imaging findings of upper tract UTIs found primarily in the adult population including acute pyelonephritis, renal and perirenal abscess, pyonephrosis, emphysematous pyelonephritis, and xanthogranulomatous pyelonephritis (XGPN). While the clinical presentation will often narrow the differential diagnosis to an infectious etiology, it is important to recognize alternate entities that may appear similarly on radiologic imaging. Several of these conditions will be reviewed here. Understanding the
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role of imaging in suspected renal infections and recognizing their imaging characteristics will hopefully promote the judicious use of radiologic studies and improve outcomes for patients with complicated renal infections.
Indications for imaging The diagnosis of pyelonephritis can often be made with a fair degree of certainty based on history, physical examination, and standard laboratory evaluation. Infections are often heralded by fever, flank pain, costovertebral angle tenderness (with or without suprapubic tenderness), and/or nausea and vomiting. Pyuria and bacteriuria are almost invariably present on urinalysis, potentially accompanied by hematuria or white cell casts (a finding which verifies that the infection involves the kidney). Positive blood cultures may support these findings. In a young woman who is not pregnant and is otherwise healthy, this is an uncomplicated UTI, which is expected to resolve with standard antibiotic therapy and does not require further evaluation with diagnostic imaging. Complicated UTIs include those in which the patient is infected with an uncommon organism, has findings suggestive of obstruction such as renal colic, or remains febrile after 72 h of appropriate antibiotic therapy. Rapid recrudescence after completing a course of antimicrobial therapy is also worrisome for complication [3]. Certain groups of patients, regardless of their presentation, should be viewed with a higher index of suspicion. Patients with diabetes are more likely to have upper tract UTIs, bilateral involvement, and atypical causative organisms [4]. Additionally, severe manifestations of renal infection such as emphysematous pyelonephritis are almost exclusively seen in diabetics. Male patients, who are typically protected from UTIs by the length of the urethra, are more likely to harbor an
Fig. 1 Noncontrast CT images demonstrate marked asymmetry in renal size with associated thickening of the right perirenal fascia, findings consistent with pyelonephritis of the right kidney. Rewindowing in b to exaggerate attenuation differences in the right renal parenchyma reveals a striated nephrogram
Fig. 2 Contrast-enhanced CT image obtained during the early pyelographic phase demonstrates excellent detection of affected renal parenchyma, characterized by the striated pattern of low attenuation in this patient with pyelonephritis of the right kidney
underlying urologic abnormality such as prostatic hypertrophy when evaluated for the cause of their infection. Diagnostic imaging is a valuable tool in the evaluation of these patients. It may reveal a causative or contributory factor to the infection such as an obstruction or congenital anatomic anomaly, either of which may alter therapeutic management. Investigation of the full extent of disease and further characterization of the involvement can be performed with proper radiologic assessment. Finally, imaging may serve as an arbiter when the diagnosis is unclear despite clinical and laboratory evaluation.
Options for imaging Once the decision has been made to proceed with diagnostic imaging of a renal infection, choosing the
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Fig. 3 This tomogram from an IVU demonstrates a mass within the upper pole of the left kidney. This defect in the nephrogram correlated to infected renal parenchyma in a patient with acute pyelonephritis
appropriate modality becomes the task at hand. At most institutions, IVU, US, CT with or without CT urography, and MRI are available, each with their own potential benefits. Intravenous urography IVU remains the most cost-effective initial study in patients with complicated UTIs and can demonstrate an underlying cause for unresolving infection such as a renal calculus or papillary necrosis. However, many clinicians opt for CT over IVU for their initial assessment because of the lower sensitivity of IVU in detecting potential complications such as emphysema or perinephric abscess. IVU requires the use of intravenous iodinated contrast, which may be contraindicated in some patients and exposes the patient to ionizing radiation, although it typically has a lower effective dose than that incurred during CT of the abdomen or pelvis [6]. Ultrasound Sonography is an accessible and cost-effective modality for evaluating renal infections, and the lack of ionizing Fig. 4 Nephrographic-phase coronal image of the kidneys obtained during a CT urogram in a patients with pyelonephritis. Note the low attenuation in the lower pole of the right kidney corresponding to the area of infection. Three-dimensional reconstruction of that image again demonstrates the lower pole defect, similar to the abnormal nephrogram on IVU seen in Fig. 3
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Fig. 5 A single image from a contrast-enhanced CT scan during the late nephrographic/early pyelographic phase demonstrates an enlarged, edematous left kidney with striated areas of low attenuation extending to the renal surface. Thickening of the perirenal fascia is noted anterolateral to the left kidney
radiation and iodinated contrast are additional benefits. US is especially effective in evaluating for hydronephrosis or pyonephrosis. However, it is an operator-dependent modality that can also be limited by patient’s large body habitus. Even under ideal conditions, it is less sensitive than CT for evaluating potential complications of upper tract UTIs. Additionally, functional assessment of the kidneys is often unattainable with US. Regardless of these shortcomings, it remains a mainstay of evaluation of renal pathology for its accessibility and problem-solving capacity in differentiating cystic versus solid structures. Computed tomography Despite increased cost, radiation exposure, and use of iodinated contrast, CT is often the preferred modality for evaluating complicated UTIs. It is a readily available and sensitive method for evaluating many of the potential complications of upper tract disease, and can provide a global assessment of the extent of involvement within the abdomen or pelvis. Furthermore, the development of the
16 Fig. 6 In this CT image of a patient with acute pyelonephritis, there is vague low attenuation of the posterior renal parenchyma, perinephric stranding and thickening of the urothelium
CT urography protocol with noncontrast, nephrographicand pyelographic-phase imaging processed in multiple imaging planes with three-dimensional reconstruction capabilities has further improved sensitivity for detecting underlying renal abnormalities. This modality has largely supplanted IVU at some institutions. Noncontrast CT imaging can demonstrate enlargement, inflammatory changes, and variable attenuation of the kidney (attenuation is usually decreased in affected areas secondary to edema). Specific complications such as stone disease, hydronephrosis, or gas within the renal parenchyma or collecting system can also be evaluated. When imaging without intravenous contrast agents, rewindowing the images for improved contrast can be a useful tool for revealing subtle differences in attenuation (Fig. 1a and b). The use of intravenous contrast during CT evaluation not only improves anatomic detail but may also provide functional information about the kidney. Imaging can be performed during several different phases of contrast enhancement. The corticomedullary phase occurs 20–
Fig. 7 Sonographic images of the kidney in a patient with pyelonephritis in a demonstrate a focal area of increased echogenicity in the renal parenchyma with corresponding decreased vascularity on color Doppler imaging in b, confirming the presence of infection
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45 s after contrast injection and is characterized by the distinction between the hyperattenuating cortex and relatively low-attenuation renal medulla. This phase of imaging provides excellent anatomic detail, although subtle changes in the nephrogram may be missed. Imaging 45 s to 2 min after contrast administration captures the nephrographic phase with relatively homogenous enhancement of both the renal cortex and medulla. Contrast is normally excreted into the collecting system 2–3 min after injection, and imaging performed during this late nephrographic and early pyelographic phase improves visualization of regions within the kidney involved in the infectious process (Fig. 2). Additionally, areas of low attenuation during early phases of imaging may demonstrate delayed enhancement during this phase, differentiating functional from damaged parenchyma [5]. Magnetic resonance imaging MRI offers high sensitivity for detecting underlying renal abnormalities that contribute to complicated upper tract UTIs. Extent of disease within the abdomen or pelvis can also be evaluated. Unlike CT, the lack of ionizing radiation and iodinated intravenous contrast make MRI an attractive imaging option for patients in which either of these may be unfavorable or contraindicated. However, MRI is often the least accessible imaging modality of those mentioned, especially in the emergency department setting. Longer imaging times and effects of motion artifact on interpretation require significant patient cooperation. Additionally, sensitivity for detecting renal calculi is generally less than with CT [6].
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Fig. 8 Grayscale ultrasound images of the renal pelvis demonstrate urothelial thickening
Acute pyelonephritis Acute pyelonephritis is a bacterial infection resulting in tubulointerstitial inflammation of the renal parenchyma. The infection is usually the result of an ascending infection from the bladder or, less commonly, hematogenous spread. Nomenclature to describe the salient features of acute
Fig. 9 a, b The patient in this study initially presented with symptoms of acute pyelonephritis. Contrast-enhanced CT demonstrates an enlarged, edematous left kidney with thickening of the perirenal
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pyelonephritis has evolved over the years to the current simplified schema: unilateral or bilateral, focal or diffuse, focal swelling or no focal swelling, and renal enlargement or no renal enlargement [7]. Abnormalities of contrast enhancement and excretion secondary to interstitial inflammation explain many of the findings seen on IVU (Fig. 3) and CT (Fig. 4a and b). In the corticomedullary or early nephrographic phases, diffuse involvement may present as an enlarged, edematous kidney with decreased enhancement. However, the nephrogram is variable on CT, as there may be a persistent corticomedullary phase or a delayed or persistent nephrogram. Wedge-shaped areas of decreased attenuation or rounded low attenuation masses with delayed enhancement can also be seen. These findings are typical in the corticomedullary or early nephrographic phases. Obstruction of the renal tubules by inflammatory debris and impaired function from tubular ischemia results in decreased concentration of excreted contrast and the classic “striated nephrogram” in the excretory phase (Fig. 5) [5]. In addition to abnormalities within the renal cortex and medulla, inflammatory changes in Gerota’s fascia, the renal sinuses, and thickening of the urothelium may be seen (Fig. 6). Similar patterns may also be noted on MRI examination.
fascia (a). Repeat imaging one week later in (b) demonstrates liquefactive necrosis of the affected areas, resulting in a left renal abscess
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Fig. 10 There is a complexappearing mass in the lower pole of the kidney with mixed solid and cystic features (a). Power Doppler imaging (b) demonstrates distinct lack of vascularity within the lesion, correlating with suspected abscess
US of acute pyelonephritis may present with areas of increased or decreased echogenicity relative to surrounding renal parenchyma, simulating the features seen on CT, resulting in a striated echogenic pattern. Doppler evaluation will often demonstrate decreased vascularity of the affected region, corresponding to physiologic tubular ischemia (Fig. 7a and b). Because its appearance can mimic a tumor, US evaluation of pyelonephritis may warrant follow-up imaging after treatment to document resolution of the findings. Interrogation of the renal pelvis may demonstrate urothelial thickening (Fig. 8).
Renal and perirenal abscess Liquefactive necrosis that occurs in the setting of acute pyelonephritis may result in the development of a renal
Fig. 11 Transaxial CT demonstrates a large fluid collection containing small flecks of gas centered in the right renal fossa with anterior displacement of the kidney. The inflammatory process extends beyond the perirenal fascia to involve the right crus of the diaphragm, right quadratus lumorum muscle, and the thoracolumbar fascia. Findings are consistent with a perirenal abscess
abscess (Fig. 9a and b). Extension of the abscess beyond Gerota’s fascia into the perirenal space is one potential way in which a perirenal abscess may originate. Additional mechanisms include hematogenous spread, extension from extrarenal inflammatory processes such as diverticulitis, and pyelosinus extravasation of infected urine. The clinical presentation and treatment of renal and perirenal abscesses varies from that of acute pyelonephritis depending on adjacent affected structures (such as the psoas muscle), the size and extent of the abscess, which may result in referred pain, and the chronicity of the infection, which may determine how well confined the abscess is. Urinalysis may be negative if the abscess is isolated from the collecting system. The US appearance of renal abscess is variable. It can appear as either a hyper- or hypoechoic focal mass or complex cystic structure. There may be posterior acoustic enhancement, although early in development poor transmission of sound waves can be seen. Demonstrating a lack of vascularity on Doppler imaging is important in distin-
Fig. 12 Grayscale sonographic images demonstrate a dilated collecting system with urine-debris levels within enlarged calyces, findings consistent with pyonephrosis
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Fig. 13 Noncontrast CT images demonstrate inflammatory stranding around the left kidney and a focus of gas within the dilated collecting system, findings suggestive of pyonephrosis (a). Reformatted sagittal image (b) demonstrates obstructing stones within the mid to distal left ureter as the etiology of the abnormality
guishing a complex-appearing abscess from a malignant process such as renal cell carcinoma (Fig. 10a,b) [7]. CT is the imaging modality of choice for evaluating patients with suspected renal or perirenal abscess. It will appear as a low attenuation mass that may enhance after contrast administration, although not to the extent of a solid renal tumor. Perirenal fluid and inflammatory stranding with thickening of Gerota’s fascia may be present. With the anatomic detail that CT provides, assessment of the extent of infection (Fig. 11) and the potential source of a perirenal abscess is possible [6].
Fig. 14 CT image without intravenous contrast demonstrates gas dissecting through the left renal parenchyma, findings diagnostic of emphysematous pyelonephritis
Pyonephrosis Pyonephrosis, which refers to infected hydronephrosis or “pus under pressure,” is a true medical emergency that if not decompressed can lead to rapid destruction of renal parenchyma and sepsis. Common reasons for obstruction include renal and ureteral calculi or tumors, with iatrogenic strictures and retroperitoneal fibrosis also reported. Definitive diagnosis is made during decompression of the renal collecting system, often with concomitant placement of a percutaneous nephrostomy tube to relieve the obstruction. US findings include hydronephrosis with a shifting urine-debris level (Fig. 12). Poor sound wave penetration with dirty shadowing suggests the presence of gas in the collecting system, which is also diagnostic. Associated structures in the abdomen and pelvis can be imaged, which may reveal the cause or level of the obstruction.
Fig. 15 Contrast-enhanced CT image demonstrates an enlarged, nonfunctional left kidney with decreased contrast enhancement compared to the right. The calyces are enlarged, and there is extensive perirenal inflammation. There is an obstructive calculus at the ureteropelvic junction. Findings are consistent with XGPN
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Fig. 16 Sonographic image from a patient with XGPN demonstrates a large staghorn calculus in the left renal pelvis. Note the posterior acoustic shadowing produced by the stone
Differentiation of simple hydronephrosis from pyonephrosis is limited on CT; however, there may be indirect signs of an infected collecting system in the setting of hydronephrosis. These include fluid–fluid levels, gas in the collecting system, or urothelial thickening (Fig. 13a,b). An abnormal nephrogram with bulging of the renal contour in the setting of hydronephrosis is also suggestive of the diagnosis [7].
Fig. 17 T1-weighted MRI image with intravenous contrast demonstrates an enlarged left kidney with dilated calyces and inflammation of the retroperitoneum, findings consistent with XGPN
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Fig. 18 Contrast-associated nephropathy can mimic the striated nephrogram of pyelonephritis. Note, however, that there is no contrast within the aortic lumen
Emphysematous pyelonephritis An entity seen almost exclusively in patients with diabetes, emphysematous pyelonephritis is an infection of the renal parenchyma, perinephric tissues, and collecting system that is due to a gas-forming organism. The clinical presentation is often severe with rapid onset of fever, chills, and flank pain. Control of hyperglycemia, aggressive fluid resuscitation, and antibiotic therapy are important for early management. Surgical nephrectomy is often required, although cases of focal involvement may be amenable to a combination of percutaneous nephrostomy tube place-
Fig. 19 Peripheral low-attenuation wedge-shaped defects in the left renal parenchyma may be confused for pyelonephritis in this patient with arterial infarctions of the kidney. Note the thrombus within the lumen of the aorta
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Fig. 20 CT examples of renal enlargement with areas of low attenuation in leukemia (a), lymphoma (b), and Castleman’s disease (c). Note the extensive retroperitoneal lymphadenopathy in the patient with Castleman’s disease
ment and antibiotic therapy. CT is the preferred modality for evaluation. Gas will be seen in the renal interstitium, with or without gas in the collecting system (Fig. 14) [4].
Xanthogranulomatous pyelonephritis XGPN is an uncommon reaction of the kidney to chronic infection in the setting of chronic obstruction. Escherichia coli and Proteus mirabilis are frequent inciting organisms, and a staghorn calculus is often responsible for the obstruction. The disease entity is named for the lipid-laden macrophages (xanthoma cells) that are seen histologically, although other inflammatory cells are present. Both a diffuse form and a localized, or “tumefactive,” form of XGPN have been described with the former more common than the latter. Because of its localized swelling, the tumefactive form is sometimes confused with a renal mass on imaging. The perirenal and pararenal spaces are commonly involved with either form, and nephrectomy is often required for treatment [7].
Fig. 21 Renal tumors such as renal medullary carcinoma (a), transitional cell carcinoma (b), and renal cell carcinoma (c) can mimic a renal infection on imaging studies as demonstrated on these CT
CT findings demonstrate an enlarged nonfunctioning kidney with decreased contrast enhancement, dilated calyces, and evidence of obstruction often by a renal stone. Evaluation of the perirenal and pararenal tissues may reveal extension of the inflammatory process into adjacent structures (Fig. 15). US demonstrates an enlarged kidney with cystic spaces representing enlarged calyces. In some cases, an obstructive renal calculus may be seen by US (Fig. 16). The characteristic features of XGPN can also be seen on MRI (Fig. 17).
Differential diagnosis Despite the characteristic imaging features and clinical presentation of patients with the renal infections presented above, it is important to consider the differential diagnoses for these entities. Contrast associated nephrotoxicity can mimic diffuse pyelonephritis with a persistent and striated nephrogram. However, contrary to the striated nephrogram in pyelonephritis, there is no contrast enhancement within
images. Note the large left retroperitoneal lymph node in c representing nodal metastasis
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the aorta on CT images (Fig. 18). Renal infarcts, either from arterial or venous thrombus, may present with focal or diffuse low attenuation lesions in the kidneys (Fig. 19). Patients may present with flank pain as a result of the vascular insult. Cancer may at times also be confused for renal infection on imaging. Leukemia, lymphoma, or Castleman’s disease can present as infiltrative processes of the kidneys with or without associated lymphadenopathy (Fig. 20a–c). Centrally based renal cell carcinoma, renal medullary carcinoma, and transitional cell carcinoma are also common mimics of pyelonephritis (Fig. 21a–c).
Conclusion Although diagnostic imaging is not indicated for uncomplicated UTI, it can be an important adjunct for the evaluation of patients with complicated infections or equivocal presentations. IVU, US, CT, and MRI are available at most institutions for this purpose, with the appropriateness of each modality determined by the clinical scenario. Following this pictorial review of common upper
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tract infections and differential diagnoses in the adult patient, their imaging characteristics should be recognizable, helping to direct appropriate therapy. References 1. Foxman B (2002) Epidemiology of urinary tract infections: incidence, morbidity, and economic costs. Am J Med 113(Suppl 1):5–13 2. Hooton T, Stamm W (2006) Acute pyelonephritis: symptoms; diagnosis; and treatment. In: Rose BD (ed) UpToDate. UpToDate, Waltham, MA 3. Hooton T, Stamm W (2006) Indications for radiologic evaluation in acute pyelonephritis. In: Rose BD (ed) UpToDate. UpToDate, Waltham, MA 4. Joshi N, Caputo G, Weitekamp M, Karchmer A (1999) Infections in patients with diabetes mellitus. N Engl J Med 341:1906–1912 5. Kawashima A, Sandler C, Goldman S, Raval B, Fishman E (1997) CT of renal inflammatory disease. Radiographics 17:851–866 6. Dunnick N, Sandler C, Newhouse J, Amis E (2001) Renal inflammatory disease. In: Dunnick N, Sandler C, Newhouse J, Amis E (eds) Textbook of uroradiology. Lippincott Williams & Wilkins, Philadelphia, pp 150–177 7. Kawashima A, LeRoy A (2003) Radiologic evaluation of patients with renal infections. Infect Dis Clin North Am 17:433–456
