Swallow Tail Sign, Parkinson’s MRI Marker, in Distinct Brain Region
Study challenges long-held view of MRI brain marker that disappears with disease
A feature on MRI images called the “swallow tail sign” has long been thought to correspond to a dopaminergic brain region clearly affected by Parkinson’s disease — but a new study suggests this is not the case.
High-resolution MRI imaging shows only a partial overlap of these two regions, and they “probably correspond to distinct [brain] structures,” its scientists reported.
“We believe that with this new knowledge, it will be easier to understand how anatomy and MRI contrasts are related and how new MRI markers can be developed for the early diagnosis of Parkinson’s,” Malte Brammerloh, a  study author and neurophysicist with the Max Planck Institute for Human Cognitive and Brain Sciences, said in a press release.
The study, “Swallow Tail Sign: Revisited,” was published in Radiology.
Parkinson’s is caused by the death of dopamine-producing cells in the brain region called the substantia nigra. A specific part of the substantia nigra called nigrosome 1 is usually affected earliest and most strongly in Parkinson’s.
Swallow tail sign ‘disappears’ in Parkinson’s patients
MRI relies on powerful magnets to visualize structures inside the body. On MRI scans of a healthy brain, a particular feature called the “swallow tail sign” or STh — named for a Y-shaped pattern that looks somewhat like a swallow’s tail — is evident.
Based on research in the 1990s, it has long been thought that the STh corresponds to nigrosome 1. The loss of swallow tail sign in brain scans of Parkinson’s patients, accordingly, is taken to indicate a loss of nerve cells in this region. But recent research has challenged this assumed correspondence, though not the loss of STh in Parkinson’s patients.
“The neuroanatomic cellular underpinnings of the radiologic STh and its disappearance in Parkinson disease must be further investigated,” the Planck Institute researchers wrote.
These scientist conducted MRI brain scans of three healthy female volunteers with a mean age of 30, and they analyzed in detail brains collected postmortem from three people — two males and one female with no known history of neurological disease — who died at a mean age of 78. The small number of samples analyzed, and age and sex differences, were acknowledged as study limitations.
Results showed that, while there was some overlap between the swallow tail sign and nigrosome 1, there were also clear differences: the STh region generally had a round ovular shape, while the nigrosome 1 brain region was significantly thinner and longer, with a more flat, disk-like appearance.
“We showed that the widespread equation of the STh and nigrosome 1 is inaccurate because they are only partially overlapping. Therefore, STh and nigrosome 1 probably correspond to distinct structures and should not be used synonymously,” the researchers wrote.
This finding “does not affect the value of STh as a late-stage Parkinson disease biomarker,” the researchers added. Still, “a more accurate link of MRI features and the substantia nigra anatomy is expected to improve Parkinson disease diagnostics and disease monitoring.”
“This is relevant to the clinical field because the notion that the ‘swallow-tail sign corresponds to nigrosome 1’ is deeply engrained but should be revised,” Brammerloh said.
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