The purpose of this study is to determine if participants have changes in dopamine cells in their brain using DaTSCAN™ brain imaging. Dopamine cell loss occurs in Parkinson's disease (PD) and other degenerative Parkinsonian disorders, but does not occur in most other movement disorders such as essential tremor or dystonia. DaTSCAN, which is also known as 123I-Ioflupane, is a new compound that has been developed by General Electric, Inc. and has been approved by the US Food and Drug Administration (FDA) to help doctors detect changes in dopamine. This test is performed by injecting DaTSCAN into a vein in the arm, and after a few hours, a large amount of DaTSCAN temporarily accumulates in an area of the brain where there are a lot of dopamine brain cells. Because DaTSCAN contains a small amount of radioactive iodine, it allows doctors to use a special machine called single photon emission computed tomography (SPECT) scanning to detect the location and amount of radioactivity in the brain and help determine if there are changes in brain dopamine. It is hoped that this study will help doctors detect the presence of dopamine changes even before symptoms are present. This study will evaluate DaTSCAN in people with PD, those who are at risk for developing PD (e.g., those with idiopathic rapid eye movement sleep disorder (iRBD) and those who are heterozygous or homozygous for Gaucher's disease (GBA) mutations) and those who are healthy volunteers.

Inclusion Criteria:

• Written consent prior to study by the subject or their surrogate
• Subjects >/= 18 years and\

Investigators' clinical practice (e.g., neurology clinic, sleep disorder clinic, etc).