Acute pyelonephritis (APN) is a bacterial infection of the renal parenchyma and urinary tract. The diagnosis of urinary tract infection is usually made based on clinical presentations, such as fever and pain in the loin region, and confirmation is achieved by conducting a urine analysis with culture.

Accurate and early diagnosis, along with appropriate clinical management, are crucial to prevent severe complications, including sepsis, shock, and even fatal outcomes. In the majority of patients, uncomplicated APN is diagnosed clinically and responds to treatment with appropriate antibiotics, while in high-risk patients, complications like abscesses and sepsis are common and can result in severe health problems. By "high-risk population," we mean primarily those with a prior history of pyelonephritis, anatomic or congenital abnormalities of the urinary system, pregnancy, renal transplant recipients, and immunosuppressed or immunocompromised patients.

The value of DWI in the detection and assessment of infectious renal disease has been reported in both non-transplanted and kidney transplant patients. Typical patterns in diffusion-weighted imaging (DWI) include wedge-shaped, striated, multifocal areas of high signal intensity  with corresponding low signal on the apparent diffusion coefficient (ADC). The diffusion restriction observed on DWI is believed to result from increased cellular density due to the accumulation of leukocytes in the infected areas of the kidneys.

To avoid the risks associated with gadolinium-based contrast agents in the aforementioned patients, we use a biparametric MRI (bpMRI) approach, incorporating T1-weighted (T1W), T2-weighted (T2W), and DWI sequences, along with ADC reconstruction maps.

Our analysis focuses on ADC maps to provide both qualitative and quantitative assessments of renal parenchymal areas affected by focal nephritis.