A radionuclide cisternogram is a medical imaging study which involves injecting a radionuclide by lumbar puncture (spinal tap) into a patient's cerebral spinal fluid (CSF) to determine if there is abnormal CSF flow within the brain and spinal canal which can be altered by hydrocephalus, Arnold–Chiari malformation, syringomyelia, or an arachnoid cyst. It may also evaluate a suspected leak (also known as a CSF fistula) from the CSF cavity into the nasal cavity. A leak can also be confirmed by the presence of beta-2 transferrin in fluid collected from the nose before this more invasive procedure is performed.