Background
Parkinson's disease (PD) and multiple system atrophy (MSA) are neurodegenerative diseases that can be difficult to diagnose and distinguish from each other.

Study aims and methods
Patients with PD and MSA and controls were studied with magnetic resonance imaging (MRI) using tissue segmentation and outlining of regions in order to identify regional volume changes that might be useful in the diagnosis of the two diseases.

Results
Patients with PD had significantly larger intracranial volumes (ICVs) and significantly smaller putaminal and sustantia nigra volumes than controls. MSA patients had significantly smaller substantia nigra and caudate volumes than controls but normal intracranial volume. In both patient groups there was a further trend towards smaller amygdala volumes.

Discussion
Increased ICV in PD patients is a new finding that may be explained by genetic factors or compensatory responses to early CNS damage. Atrophy of the amygdala in MSA patients has not been demonstrated with MR before. It might explain why these patients can have hyposmia. The putaminal atrophy found in the PD group may be a trait of the later stages of PD. Segmentation of the substantia nigra can be a useful aid in the diagnosis of PD and MSA.