Imaging

Imaging techniques can be used as tools to visualize both the function and structure of the brain. Functional imaging has been used to assess neurotransmitter function, metabolic processes, and immune responses in order to identify and track PD pathology. Functional imaging is also used to assess the efficacy of new drugs by tracking changes in neuronal or metabolic processes pre- and post-treatment (Seibyl, 2008) and is often used to assess the effect of placebo treatment during clinical trials (Diederich and Goetz, 2008). Structural imaging can be used to measure changes in brain anatomy associated with progression of the disease, specifically volume of brain structure and molecular composition of brain tissues. Magnetic resonance imaging (MRI), ultrasound, and optical coherence tomography (OCT) are all structural imaging techniques.

A Brain PET / MRI Fusion imageImage copywrite public domain.

Functional imaging techniques include:

• Positron Emission Tomography (PET) (detection of gamma rays emitted by a tracer introduced into the body)
• Single-Photon Emission Computed Tomography (SPECT) (Lower resolution than PET but less costly and more widely available)
• Magnetic resonance spectroscopy (MRS)
• Functional magnetic resonance imaging (fMRI) (Scherfler et al., 2007).

Some functional imaging techniques depend upon the development of imageable ligands that bind to molecules involved in processes related to neuronal health and function, including neurotransmission (Cowan et al., 2008; Pate et al., 1993), glucose metabolism (FDG) (Feng et al., 2008; Kwon et al., 2008; Yong et al., 2007), and immune activation (Ouchi et al., 2005). Imaging norepinepherine processing in cardiac tissue (metaiodobenzylguanidine scintigraphy MIBG-S) has also been reported to have some value as a diagnostic tool for PD (Braune, 2001; Sawada, 2009).

Structural imaging techniques include: 

• Magnetic Resonance Imaging (MRI)
• Magnetic Resonance Spectroscopy (MRS)

Unlike Alzheimer's disease, for which hippocampal atrophy is an established surrogate marker (Jack et al, 2000, Jack et al, 2004), similar structural changes do not appear to occur in PD.  More studies, including longitudinal studies, are needed to determine if MRI and MRS measures can be potential biomarkers for PD.

There is a large debate over dopamine imaging in PD and its utility in clinical trials due to the marked variability in the loss of dopaminergic markers (Marek et al., 2008). The Alzheimer's disease (AD) field has employed tracers to image the beta-amyloid plaques that accumulate in the brains of AD patients to determine whether experimental therapeutics lower the amyloid burden (Lockhart, 2006). These target-based biomarkers may be used to determine whether or not the therapeutic is affecting a biologic process within the brain. A similar approach could be employed in Parkinson's patients by imaging the alpha-synuclein protein found in Lewy bodies, once suitable imaging ligands are identified.