Several publications confirm the observation that up to 90% of PD patients have increased echogenicity of the substantia nigra. In healthy subjects and in patients with essential tremor this hyperintensity of the substantia nigra is only found in 10%. However 60% of the healthy subjects with increased echogenicity also have decreased nigra-striatal function on (18)-F-dopa-PET. So TCD might possibly be an early (presymptomatic) marker for PD.