<qa>  What is Parkinsons disease characterized by?Parkinsons disease PD is the second most common neurodegenerative disorder. PD is characterized primarily by motor impairment, but cognitive impairment is common and dementia may occur in up to 50 of patients. Cholinergic deficits have been widely demonstrated in PD patients with and without dementia. Cholinergic deficits are more profound in the setting of PD with dementia. The cholinergic system mediates aspects of learning and memory and deficits in cholinergic function are correlated with the degree of dementia in PD. Additionally, the cholinergic system may mediate certain aspects of motor function in PD. Data on cholinergic function, particularly relating to nicotinic cholinergic receptors, are limited to post mortem studies, and little is known about the role of the cholinergic system in vivo and how it relates to clinical phenomena. The aim of the current study is to assess the feasibility and usefulness of imaging of the high affinity alpha 4 beta 2 nicotinic cholinergic receptor in vivo, using SPECT single photon emission computed tomography imaging in patients with PD. The long-term goal of the present study is to further understanding of the role of the cholinergic system in the pathogenesis of PD and to elucidate potential diagnostic and therapeutic targets. 