Neuroimaging of the monoaminergic system in Parkinson's disease: A narrative review

Parkinson's disease (PD) is a complex neurodegenerative disorder characterized by both motor and non-motor symptoms, primarily attributed to dopaminergic dysfunction in the substantia nigra pars compacta. However, growing evidence indicates that serotonergic and noradrenergic alterations also contribute significantly to PD pathophysiology and progression. This growing understanding has driven the development of advanced neuroimaging techniques aimed at visualizing not only dopaminergic deficits but also serotonergic and noradrenergic alterations, providing deeper insights into PD pathophysiology and progression. Positron emission tomography (PET) and single-photon emission computed tomography (SPECT) have been instrumental in visualizing dopaminergic deficits, particularly dopamine transporter loss, aiding in diagnosis and disease progression tracking. While PET and SPECT remain crucial in assessing dopaminergic dysfunction, novel MRI techniques, including neuromelanin-sensitive MRI, iron-sensitive MRI, diffusion-weighted imaging, and pharmacological MRI, have emerged as promising non-invasive alternatives for evaluating monoaminergic dysfunction in PD. In this narrative review, we have discussed the recent neuroimaging advancements in assessing monoaminergic dysfunction in PD, highlighting how these advances not only improve our understanding of the distinct contributions of dopaminergic, noradrenergic, and serotonergic systems to motor and non-motor symptoms, but also offer promising biomarkers for disease progression and treatment response.