Imaging in Parkinson's Disease

Idiopathic Parkinson’s disease (PD) is one of the commonest neurodegenerative diseases. Although, it is considered to be a Lewy body pathology and neuronal loss are not always associated with PD, suggesting other factors responsible for neuronal degeneration. Degenerative processes do not involve only dopaminergic system but are more widespread and involve other central neurotransmitter system including serotoninergic, cholinergic, noradrenergic and other neurotransmitters of central nervous system. Neuronal network involving basal ganglia including substantia nigra (SN), subthalamic nucleus (STN) and thalamus have important role in pathogenesis of PD and related disorders. Healthy substantia nigra (SN) possesses cluster of few important dopaminergic cells (nigrosomes) having special immune-histochemical properties. There are five types of nigrosomes, but nigrosome-1, the most important and largest one, are present in posterior third of SN. It can be visualized by special sequences of magnetic resonance imaging (MRI) in patients affected with PD. Dysfunctional nigrostriatal projections result in striatal dopamine deficiency (SDD) causing symptoms of PD. Other atypical parkinsonian syndromes associated with SDD include multiple system atrophy (MSA), progressive supranuclear palsy (PSP), corticobasal degeneration (CBD), vascular Parkinsonism and Wilson’s disease. Non-striatal dopamine deficiency (SDD) disorders mimicking PD include drug induced Parkinsonism; Parkinsonism due to toxins (manganese or CO poisoning); essential tremors; or psychogenic Parkinsonism. Detecting functional status of dopaminergic network is one of the potential targets to differentiate various related disorders. Because of multiple pathophysiological changes in PD including gliosis, neuronal loss, neuronal dysfunction, Lewy body accumulation and other related changes; it is practically difficult to have a single imaging modality to diagnose it. Hence, a good biomarker (imaging as well as biochemical) is needed to differentiate idiopathic PD from other parkinsonian syndrome especially in early stage of disease. In this chapter, imaging biomarker would be discussed in detail, which may help in differentiating one from other Parkinsonism syndromes. They are as follows: