Neurodegenerative disorders are progressive diseases characterized by loss of neurons, synapses, and brain atrophy. Imaging plays a crucial role in early detection, differential diagnosis, and monitoring progression.
📌 General Imaging Approach
- CT Brain: Shows global and regional atrophy, but insensitive in early disease.
- MRI Brain: Gold standard for structural assessment. Detects regional patterns of atrophy and excludes mimics.
- Advanced Imaging: PET, SPECT, and functional MRI provide metabolic and molecular insights.
🔍 Key Disorders and Imaging Features
1. Alzheimer’s Disease (AD)
- MRI findings:
- Medial temporal lobe atrophy (MTA): hippocampus, parahippocampal gyrus.
- Parietal and posterior cingulate atrophy in later stages.
- Enlarged temporal horns and Sylvian fissures.
- FDG-PET: Hypometabolism in temporoparietal cortex and posterior cingulate.
- Amyloid PET / Tau PET: Specific tracers for early diagnosis.
2. Frontotemporal Dementia (FTD)
- MRI findings:
- Focal atrophy of frontal and anterior temporal lobes.
- Asymmetrical atrophy common.
- PET: Frontal and anterior temporal hypometabolism.
3. Vascular Dementia
- MRI findings:
- Multiple lacunar infarcts.
- Extensive white matter hyperintensities (WMH) on T2/FLAIR.
- Cortical/subcortical infarcts, microbleeds (seen on SWI).
- CT/MRI can show chronic ischemic changes.
4. Lewy Body Dementia (DLB)
- MRI: Often mild generalized atrophy.
- DAT-SPECT (dopamine transporter imaging): Shows reduced uptake in basal ganglia.
- PET: Occipital hypometabolism (vs Alzheimer’s).
5. Parkinson’s Disease & Parkinson Plus Syndromes
- Parkinson’s Disease (PD):
- MRI usually normal in early stages.
- Advanced techniques (SWI, neuromelanin imaging) may show substantia nigra signal loss.
- Progressive Supranuclear Palsy (PSP): Midbrain atrophy (“hummingbird sign”).
- Multiple System Atrophy (MSA): Pontine atrophy, “hot cross bun sign” on T2.
🧾 MRI Patterns of Atrophy in Dementias
| Disorder | Primary Atrophy Pattern | Additional Imaging Clues |
|---|---|---|
| Alzheimer’s Disease (AD) | Medial temporal lobe (hippocampus, parahippocampal gyrus), posterior parietal | FDG-PET: temporoparietal & posterior cingulate hypometabolism; Amyloid/Tau PET positive |
| Frontotemporal Dementia (FTD) | Frontal and anterior temporal lobes (often asymmetric) | PET: frontal & temporal hypometabolism |
| Vascular Dementia | Diffuse white matter changes (T2/FLAIR hyperintensity), lacunar infarcts, cortical/subcortical infarcts | SWI: microbleeds; look for ischemic burden |
| Dementia with Lewy Bodies (DLB) | Mild generalized atrophy (less hippocampal atrophy vs AD) | Occipital hypometabolism on PET; abnormal DAT-SPECT |
| Parkinson’s Disease Dementia (PDD) | Usually mild cortical atrophy | Substantia nigra signal loss (SWI/neuromelanin MRI); DAT-SPECT abnormal |
| Progressive Supranuclear Palsy (PSP) | Midbrain atrophy (“hummingbird sign”) | MRI sagittal view useful |
| Multiple System Atrophy (MSA) | Pontocerebellar atrophy, putaminal atrophy | “Hot cross bun sign” in pons on T2 |
📌 Teaching Pearls
- Alzheimer’s → hippocampal atrophy.
- FTD → frontal + anterior temporal atrophy.
- Vascular dementia → infarcts + WMH.
- Lewy body dementia → occipital hypometabolism on PET.
- Parkinson-plus syndromes → characteristic brainstem signs.
