Molecular Imaging of Neurodegeneration

Molecular imaging (MI) originated from the field of radiopharmacology due to the need to better understand the fundamental molecular pathways inside organisms in a noninvasive manner. It emerged in the early twenty-first century as a discipline at the intersection of molecular biology and in vivo imaging and enables the visualization of the cellular function and the follow-up of the molecular process in living organisms without perturbing them. Basic requirements for MI are probes whose concentration and/or spectral properties are altered by the specific biological process under investigation technology to monitor these probes in living organisms and to reconstruct images from their distribution patterns. Positron Emission Tomography (PET) ist the most sensitive molecular imaging tool and a well-established method for neuroimaging of neurodegeneration such as Alzheimer's disease (AD). An ideal PET biomarker for AD should allow a reliable estimation of disease risk and rate of disease progression long before first symptoms are clinically diagnosed. The main pathologic processes of AD, deposition of beta-amyloid, hyperphosphorylated tau protein, degeneration of cholinergic and other neurons, precede clinical symptoms by years providing potential targets for the identification of individuals at risk for AD. In the last few years, several PET tracers targeting beta-amyloid in AD have been developed. The suitability of these PET radiopharmaceuticals to differentiate AD patients and patients with mild cognitive impairment (MCI) from control subjects has been demonstrated and will be discussed. However beta-amyloid deposition has been found in about 20% of normal elderly subjects.