The loss of the swallow tail sign (STS), a hyperintensity within the substantia nigra on high-resolution T2*/SWI brain MRI, is present in early stages of PD, and has been suggested as a diagnostic biomarker because of its high accuracy when evaluated by experienced neuroradiologists and with sequences with slice thickness under 1.5mm. 

In ET, the STS is preserved, so its evaluation could help differentiate it from PD.  

There are no studies that evaluate the accuracy of STS detection in clinical practice by a movement disorders neurologist using non-optimal MRI slice thickness. 

Seventy-three 3T brain MRIs from patients who were clinically diagnosed with PD or ET were retrospectively assessed in a MacBook Pro using the software Horos by a movement disorders neurologist. In the SWAN sequence (0.68x0.68x2 mm3) five slices caudal to the upper limit of the red nucleus were selected and analyzed. 

The STS was assessed blindly on each side to define the PD diagnosis. Uncertainty about the presence/absence of the STS was also computed for each side. 

The overall accuracy was 79.5%, with 86.2% sensitivity and 75.0% specificity. Interestingly, when considering only the cases where there were no uncertainties about the diagnosis based on STS presence/absence (29/73, 39.7%), the accuracy remained similar (79.3%) with a decrease in sensitivity and slight increase in specificity (76.9%, and 81.3% respectively).

 The visual assessment of the STS performed by a movement disorders neurologist in clinical practice yields good diagnostic accuracy to discriminate between ET and PD even with suboptimal image quality (slice thickness >1.5mm). This supports the utility of this radiological sign in real life scenarios.