DIAGNOSING Parkinson’s disease (PD) in its early or uncertain stages remains a clinical challenge. In cases where the diagnosis of a parkinsonian syndrome is unclear, imaging techniques play a key role in helping clinicians distinguish PD from other conditions. Traditionally, ¹²³I-FP-CIT SPECT imaging has been used to detect dopaminergic deficits indicative of PD, but it requires radioactive tracers and can be costly. A recent multicentre study assessed whether the absence of a characteristic feature known as the “swallow tail sign” (STS) on susceptibility-weighted MRI scans could offer a reliable, non-radioactive alternative in these ambiguous cases. A key finding was that when radiologists were confident in their assessment of the STS, the MRI’s accuracy in diagnosing PD rose significantly to 90%.
The study was prospective and included 106 participants with clinically uncertain parkinsonian syndrome, recruited between May 2016 and May 2019. All patients underwent 3-Tesla susceptibility-weighted MRI and ¹²³I-FP-CIT SPECT scans, which were interpreted independently by imaging experts blinded to the clinical picture. Diagnostic performance for each modality was compared against clinical diagnosis at follow-up, and reliability between raters and modalities was assessed using Cohen’s kappa.
Overall, the absence of the STS on MRI showed a sensitivity of 81% (95% CI: 70–90) and a specificity of 75% (95% CI: 58–88) for diagnosing PD. In cases where diagnostic confidence in the MRI assessment was high (60 of 106 participants), sensitivity increased to 91% (95% CI: 79–98) and specificity to 87% (95% CI: 60–98), resulting in a diagnostic accuracy of 90% (95% CI: 79–96; p=0.002). In comparison, ¹²³I-FP-CIT SPECT demonstrated a higher sensitivity (98%; 95% CI: 91–100; p=p.001) but lower specificity (55%; 95% CI: 39–70; p=0.06).
These findings suggest that in cases of diagnostic uncertainty, the STS on MRI could provide a reliable, radiation-free alternative for supporting the diagnosis of Parkinson’s disease, especially when expert confidence in the imaging interpretation is high. However, limitations include the reliance on subjective assessment of the STS and the variability in confidence levels. Further work is needed to refine imaging protocols and training to increase consistency. In clinical practice, MRI could complement or, in select cases, reduce the need for nuclear imaging in suspected PD.
Reference
Xing Y et al. Diagnostic Value of Swallow Tail Sign at Brain MRI in Patients with Clinically Uncertain Parkinsonian Syndrome. Radiology. 2025;316(1):e240680.
