What are the typical brain imaging findings for Multiple System Atrophy (MSA) versus Parkinson's disease?

Brain Imaging Findings in Multiple System Atrophy vs. Parkinson's Disease

MRI is the optimal imaging modality for differentiating Multiple System Atrophy (MSA) from Parkinson's Disease (PD), with MSA showing characteristic putaminal atrophy, brainstem atrophy, and infratentorial abnormalities that are typically absent or mild in PD. 1, 2

Key MRI Findings

Multiple System Atrophy (MSA)

• Putaminal abnormalities:

  • Moderate to severe putaminal atrophy (hallmark finding) 1, 3, 4
  • Putaminal hypointensity on T2-weighted images (moderate to severe) 3, 5
  • Lateral slitlike hyperintensity along the putaminal border (more pronounced than in PD) 3
  • Significantly reduced putamen/caudate volume ratio (<1.6) 4

• Infratentorial abnormalities:

  • Brainstem atrophy (particularly pons) 1, 3, 5
  • Cerebellar abnormalities (more severe than in PD) 3
  • Signal increase in middle cerebellar peduncle (highly specific for MSA-C with 100% specificity) 6
  • "Hot cross bun" sign in the pons (in MSA-C variant)

Parkinson's Disease (PD)

• Generally normal or near-normal MRI appearance 1
• When abnormalities present, they are typically mild:
  • Mild putaminal hypointensity may be seen 3, 5
  • Mild cerebellar abnormalities in some cases 3
  • No significant putaminal or brainstem atrophy 1, 3
  • Putamen/caudate volume ratio typically >1.6 4

MSA Variants Differentiation

• MSA-P (parkinsonian variant):

  • Predominant putaminal atrophy 6
  • Hyperintense putaminal rim (72.2% sensitivity, 90% specificity) 6

• MSA-C (cerebellar variant):

  • More prominent cerebellar and pontine abnormalities 6
  • Signal increase in middle cerebellar peduncle (85.2% sensitivity, 100% specificity) 6

Functional Imaging

• DaT Scan (I-123 ioflupane SPECT/CT):
  • Shows abnormal patterns of dopaminergic depletion in both MSA and PD 1, 2
  • Cannot reliably distinguish between MSA and PD as both show decreased radiotracer uptake in the striatum 2
  • Useful for differentiating parkinsonian syndromes from essential tremor or drug-induced parkinsonism 1, 2

Diagnostic Algorithm

1. Initial MRI evaluation:

   • Look for putaminal atrophy and hypointensity (moderate-severe suggests MSA)
   • Assess brainstem and cerebellar structures (atrophy suggests MSA)
   • Calculate putamen/caudate volume ratio (ratio <1.6 suggests MSA) 4

2. For uncertain cases:

   • Multimodal MRI combining structural, iron-sensitive, and diffusion tensor imaging improves diagnostic accuracy 7
   • DaT scan to confirm parkinsonism but not to differentiate between MSA and PD 1, 2

Common Pitfalls

• Relying solely on DaT scan for differentiation (cannot distinguish MSA from PD) 2
• Overlooking mild structural abnormalities in early MSA
• Not considering MSA variants (MSA-P vs MSA-C) which have different imaging emphasis 6
• Failing to use quantitative measures like putamen/caudate volume ratio which can improve diagnostic accuracy 4

The combination of these MRI findings, particularly putaminal atrophy, brainstem atrophy, and cerebellar abnormalities, provides the most reliable imaging-based differentiation between MSA and PD, with approximately 80-90% diagnostic accuracy when using appropriate criteria 3, 4.