Overview of MRI findings in progressive multifocal leukoencephalopathy

Progressive multifocal leukoencephalopathy (PML) is a severe demyelinating disease of the central nervous system caused by JC virus (JCV) infection. PML affects patients with various underlying conditions, such as HIV/AIDS, hematological malignancies, organ transplants, autoimmune diseases, or multiple sclerosis particularly those receiving disease-modifying therapies. MRI plays a crucial role in diagnosis, demonstrating characteristic findings across multiple sequences, including T2-weighted imaging (T2WI)/fluid-attenuated inversion recovery (FLAIR), T1-weighted imaging (T1WI), diffusion-weighted imaging (DWI), and susceptibility-weighted imaging (SWI). Early stage markers first appear as a cluster of punctate high-signal areas in T2WI (the "punctate pattern") and later develop into a distribution of oval-shaped lesions of varying sizes, commonly referred to as the "milky way appearance." Lesions typically show T2WI/FLAIR hyperintensity, T1WI hypointensity, and DWI hyperintensity. Recent findings highlight the significance of SWI hypointensity as a potential early marker. The prognosis varies significantly depending on the underlying condition and timing of diagnosis, with mortality rates ranging from 20 to 90%. Early detection, particularly in asymptomatic stages, significantly improves survival rates, emphasizing the importance of regular MRI screening in high-risk patients. Diagnostic challenges include low JCV DNA levels in cerebrospinal fluid (CSF), particularly in early stages and drug-associated cases, necessitating ultrasensitive PCR testing. This review provides an overview of PML's imaging characteristics, with particular emphasis on early diagnostic features using MRI, with a detailed understanding of PML's imaging characteristics across various stages and clinical subtypes, aiming to improve patient outcomes through early detection and intervention.