Eli Lilly and Company (NYSE: LLY) today announced it has acquired two investigational positron emission tomography (PET) tracers from Siemens Medical Solutions USA, Inc. The tracers are intended to image tau (or neurofibrillary) tangles in the brain, one of two known hallmarks of Alzheimer's disease. Based on studies of samples obtained at autopsy, the amount and location of tau tangles in an Alzheimer's disease patient's brain is thought to correlate with the severity of the disease. There are currently no approved diagnostics to detect tau tangles in living patients, creating challenges for scientists working to understand the progression of the disease and how therapies may impact it.