Figure 1: Axial STIR image demonstrates perinephric edema adjacent to the right kidney consistent with perirenal inflammation.

Figure 2: Axial VIBE image demonstrates two ill-defined areas of decreased signal intensity in the lower pole of the right kidney. A simple renal cyst is seen in the left kidney

Acute focal pyelonephritis.

Acute pyelonephritis is a bacterial infection of the kidney that causes an acute suppurative tubulo-interstitial inflammation of the renal parenchyma. It primarily results from an ascending infection from the lower urinary tract, typically due to gram negative enteric pathogens (e.g. E. coli, Proteus mirabilis, Pseudomonas aeruginosa, and Klebsiella spp.). Conditions that predispose patients with lower urinary tract infections to renal involvement include: vesicoureteral reflux, urinary tract obstruction, calculi, altered bladder function, altered host resistance, pregnancy, and congenital urinary tract anomalies. In small number of patients, acute pyelonephrtitis can be caused by hematogenous seeding (septic emboli) of the kidney secondary to intravenous drug abuse, subacute bacterial endocarditis, or from a distant primary focus e.g. tooth abscess, respiratory tract infection. More than 90% of these types of renal infections are caused by Staphylococcus aureus and Streptococcus. Acute pyelonephritis can be of varying severity, which can range from uncomplicated to progressively worsening stages of interstitial inflammation to frank abscess formation. Women under the age of 50 years are more commonly affected by acute pyelonephritis than men. Over the age of 50, the incidence of acute pyelonephritis in men increases as a result of urinary stasis secondary to benign prostatic hypertrophy and other factors. Patients often present with fever, chills, flank pain, pyruia, nausea, vomiting, and malaise.

In the majority of cases, the diagnosis of acute pyleonephritis in adults can be based on clinical and laboratory findings and quick response to antibiotics is the rule. Routine renal imaging is generally not indicated in uncomplicated bacterial pyelonephritis in adults. Imaging in adult patients is indicated for the following cases:

• Adult patients who respond poorly to appropriate antibiotic therapy after 3 days.
• When a definite diagnosis of acute renal infection is not established.
• When patients present with recurrent episodes of infection. In these cases renal imaging is indicated because of an increased likelihood of stones, obstruction, abscess, or congenital anomaly.
• Patients with history of poorly controlled diabetes mellitus, AIDS, organ transplants or other immunocompromised diseased states. In these patients imaging may be required initially because there is an increased risk of developing a complicated acute renal infection including a renal or perinephric abscess.

The primary goal of renal imaging is to provide information regarding the nature and extent of disease process and to identify significant complications (e.g. gas-forming infection, abscess, and urinary obstruction) and predisposing abnormalities (e.g. obstruction and calculi). This information helps to guide immediate intervention and appropriate antimicrobial treatment. The reported results of MR imaging for the evaluation of acute pyelonephritis are promising from both animal studies and from clinical studies in pediatric patients. Clinical experience in adults is still limited. MRI can be used in evaluating adult patients with suspected acute pyelonephritis who have contraindications to iodinated contrast material. MR findings of acute pyelonephritis include a striated nephrogram that radiates from the renal medulla to the cortex, globular renal enlargement, and perinephric fluid. Acute pyelonephritis is best demonstrated after contrast material administration and with fast inversion recovery or T2-weighted pulse sequences. The affected areas have persistent increased signal intensity on these images compared with the normal renal parenchyma, which becomes lower in signal intensity after administration of contrast material. Perinephric fluid may be observed, which is best shown on post-gadolinium images. Proteinaceous material in the renal tubules may occasionally be visualized as high signal intensity substance in the renal medulla on non-contrast T1-weighted fat-suppressed images. It should be noted that gas, calculi, and blood flow appear as areas of signal void on MRI and may be indistinguishable.

References:

1. Semelka, Richard. Abdominal-Pelvic MRI. New York: Wiley-Liss Inc, 2002. pp. 830-832.
2. Dunnick NR, Sandler CM, Newhouse JH, and ES Amis. Textbook of Uroradiology, 3rd Edition. Philadelphia: Lippincott Williams & Wilkins, c2001. pp. 150-151.
3. Rickhardt PJ, Lonergan GJ, Davis FJ, Kashitani N, and BJ Wagner. Infiltrative Renal Lesions: Radiologic-Pathologic Correlation. RadioGraphics. 2000; 20: 215-243.
4. Kawashima A, Sandler CM, and SM Goldman. Imaging in Acute Renal Infection. British Journal of Urology International. 2000; 86 (Suppl.): 70-79.
5. Kawashima A. and A.J. LeRoy. Radiologic Evaluation of Patients with Renal Infections. Infectious Disease Clinics of North America. 2003; 17(2): 433-456.