Microvessels may Confound the "Swallow Tail Sign" in Normal Aged Midbrains: A Postmortem 7 T SW-MRI Study

Background and purpose: Susceptibility weighted imaging (SWI) plays a role in the differential diagnosis of Parkinson's disease, but lacks widespread acceptance in clinical routine. In a descriptive pilot study, we assessed hypointense microstructures of the normal substantia nigra pars compacta at ultrahigh-field strength for interpretation of the "swallow tail sign."

Methods: Magnetic resonance imaging at 7 Tesla was performed in five postmortem samples obtained from subjects not affected by Parkinson's disease. Susceptibility weighted images, including minimum intensity projections, were created followed by consensus assessment for microvascular confound. Histological workup in this case-control study included iron and myelin staining. Seven Tesla SWI images from the reference cohort of nine living subjects, all of which showed a positive "swallow tail sign" in their midbrains, were assessed visually.

Results: All specimens showed microvessels running through the dorsal pars compacta and along the caudolateral circumference of the red nucleus. Hypointense imaging patterns in the medial part of the "swallow tail" were due to susceptibility effects of iron deposits and microvessels. In eight out of nine control subjects, one or more microvessels were detected medial to the dorsolateral nigral hyperintensity or at least unilaterally in the medial part of the "swallow tail." One microvessel crossing nigrosome 1 was found in two in-vivo cases.

Conclusion: Both iron deposits and microvessels contribute to the hyposignal surrounding nigrosome 1 in susceptibility weighted imaging of normal aged midbrains at ultrahigh-field strength. When assessing the substantia nigra for the presence or absence of the "swallow tail sign," intrinsic vessels may be a sporadic confounder.