Diagnostic Value of Swallow Tail Sign at Brain MRI in Patients with Clinically Uncertain Parkinsonian Syndrome

Background Striatal dopaminergic deficits, established with iodine 123-2β-carbomethoxy-3β-(4-iodophenyl)-<i>N</i>-(3-fluoropropyl)-nortropan (¹²³I-FP-CIT) SPECT, support the diagnosis of Parkinson disease (PD) or atypical parkinsonian syndrome in clinical uncertainty. The swallow tail sign (STS) at susceptibility-weighted (SW) MRI helps differentiate patients with PD from controls, but its utility in clinically uncertain parkinsonian syndromes remains unclear. Purpose To compare the diagnostic performance of STS absence at SW MRI in diagnosing PD with that of ¹²³I-FP-CIT SPECT in participants with clinically uncertain parkinsonian syndrome. Materials and Methods This prospective, multicenter study included participants with clinically uncertain parkinsonian syndrome between May 2016 and May 2019. Imaging modality specialists independently assessed images from SW MRI and ¹²³I-FP-CIT SPECT while blinded to clinical findings. Diagnostic performance was assessed against diagnosis at clinical follow-up as the reference standard. Performance metrics were compared between modalities and post hoc subgroups split by diagnostic confidence and motor severity using Z tests. Cohen κ was computed for intrarater, interrater, and intermodality reliability. Results A total of 106 participants with clinically uncertain parkinsonian syndrome and diagnostic 3-T SW MRI examinations were included in the study sample (median age, 69 years [IQR, 14 years]; 59 male). STS assessment showed substantial agreement within (κ = 0.69; 95% CI: 0.55, 0.83) and between (κ = 0.70; 95% CI: 0.56, 0.84) raters. STS absence demonstrated a sensitivity of 81% (95% CI: 70, 90) and specificity of 75% (95% CI: 58, 88) in predicting PD. Post hoc analysis demonstrated better diagnostic performance in the subgroup with high (60 of 106 participants) versus low diagnostic confidence, with 91% sensitivity (95% CI: 79, 98; <i>Z</i> = 2.8; <i>P</i> = .005), 87% specificity (95% CI: 60, 98; <i>Z</i> = 1.4; <i>P</i> = .17), and 90% accuracy (95% CI: 79, 96; <i>Z</i> = 3.1; <i>P</i> = .002). ¹²³I-FP-CIT SPECT yielded higher sensitivity than STS (98% [95% CI: 91, 100]; <i>Z</i> = 3.3; <i>P</i> = .001) but only 55% specificity (95% CI: 39, 70; <i>Z</i> = -1.9; <i>P</i> = .06). Conclusion In participants with clinically uncertain parkinsonian syndrome, STS absence on 3-T brain SW MRI scans supported the diagnosis of PD with high accuracy overall and greater accuracy when the diagnostic confidence of STS assessment was high. Clinical trial registration no. NCT03022357 © RSNA, 2025 <i>Supplemental material is available for this article</i>.