What is the best initial imaging study for diagnosing PML?

The best initial imaging study for diagnosing progressive multifocal leukoencephalopathy (PML) is a head CT or MRI. PML is a demyelinating disease caused by the reactivation of the JC virus, primarily affecting the white matter of the brain. Typical imaging findings on MRI include areas of non-enhancing white matter lesions that often have a characteristic appearance. While a CT scan can be useful, MRI provides more detailed information about the brain's structure, including subtle changes that are more indicative of PML, such as the presence of lesions without significant mass effect or contrast enhancement.

Other imaging modalities, such as chest X-ray, ultrasound of the heart, or CT scans of the abdomen, are not relevant for the diagnosis of PML, as they do not provide the necessary visualization of the central nervous system where the disease manifests. Thus, head CT or MRI is the most appropriate choice for identifying the brain lesions associated with PML.