Inflammatory Renal Diseases
Pyelonephritis is defined as an upper urinary tract infection with involvement of the renal pelvis. Risk factors include reflux disease in children and urinary obstruction or stones in adult patients. According to the course of the disease, acute and chronic pyelonephritis have to be differentiated.
The prevalence of pyelonephritis is about 3–9%. Acute pyelonephritis most often affects women between the ages of 15 and 40 years.
T2-weighted images may demonstrate enlargement of the kidneys, a decreased SI of the renal parenchyma, and a loss in corticomedullary contrast. Gerota’s fascia might be thickened.
T1-weighted images may show swelling of the kidneys, with a decrease in corticomedullary differentiation. A thickening of the Gerota’s fascia can be seen in some cases. Contrast-enhanced T1-weighted images or T2-weighted images may show-wedge shaped foci of persistently increased SI.
Chronic pyelonephritis can result from the acute form or develop insidiously. It often manifests with secondary symptoms, such as hypertension and uremia. The morphologic features of chronic pyelonephritis such as shrinkage, irregular borders, and thin renal parenchyma are easily detected with MRI.
Xanthogranulomatous pyelonephritis may develop in patients with abnormal immune response to bacterial infection. Particularly Proteus or Escherichia coli species are causative organisms. Often, a history of (partial) urinary obstruction (stone, ureteropelvic junction stenosis, tumor) can be found. Xanthogranulomatous pyelonephritis can occur as a diffuse disease or less often as a segmental or focal disease.
Histology is characterized by a diffuse infiltration of the affected tissue with plasma cells and lipid-laden macrophages.
This disease mainly affects women aged 45–65, but 10% of all patients are younger than 17 years.
MRI findings include fatty masses replacing the renal parenchyma, which may show strong enhancement on postcontrast T1-weighted images. One percent of all cases of xanthogranulomatous pyelonephritis are bilateral, and up to 90% show a complete loss of renal function. The bearclaw sign—a characteristic appearance of kidneys with xanthogranulomatous pyelonephritis, with a thinned-out cortex and dilated renal pelvis—has been described as a diagnostic finding suggestive of the presence of xanthogranulomatous pyelonephritis.
The different forms of glomerulonephritis have a loss of corticomedullary differentiation in common, which is a non-specific sign. The various forms of glomerulonephritis cannot be differentiated using MRI.
Initial results suggest that USPIO contrast agents, which are being taken up by macrophages, can demonstrate inflammatory changes in kidneys and hence help to establish the diagnosis of glomerulonephritis.
Malacoplakia is a rare inflammatory condition presenting as a plaque or a nodule that usually affects the genitourinary tract and the skin in immunocompromised patients and those with diabetes mellitus, status post–renal transplantation, and lymphoma. MRI depicts enlargement of the kidneys and inhomogeneous contrast enhancement. In T2-weighted images, inhomogeneous SI of the renal parenchyma is present. It is important to include this disease in the list of potential differential diagnoses as it may be cured with the use of antibiotics.