A 58-year-old woman presents with a 3-year history of progressive asymmetric resting tremor, rigidity, and bradykinesia. She has shown excellent response to levodopa therapy. Her neurologist now wants to confirm the diagnosis of idiopathic Parkinson disease and exclude secondary causes such as structural lesions, vascular parkinsonism, or normal-pressure hydrocephalus. Which neuroimaging investigation is most appropriate as the first-line structural assessment?

Explanation

## First-Line Structural Neuroimaging in Parkinson Disease **Key Point:** MRI brain with standard sequences (T1, T2, FLAIR) is the most appropriate first-line structural neuroimaging investigation in suspected Parkinson disease. It excludes structural mimics (tumors, stroke, hydrocephalus, atrophy patterns) and identifies red flags suggestive of atypical parkinsonian syndromes. **High-Yield:** MRI findings that raise suspicion for **atypical parkinsonism** rather than idiopathic PD include: - **Midbrain atrophy** (PSP) - **Putaminal rim sign** or **hot cross bun sign** (MSA) - **Cerebellar atrophy** (MSA-C) - **Cortical atrophy** (CBD) - **Periventricular white matter changes** (vascular parkinsonism) - **Ventriculomegaly** (normal-pressure hydrocephalus) **Clinical Pearl:** In idiopathic PD, MRI is typically **normal or shows only mild nonspecific changes**. The absence of red flags on MRI, combined with excellent levodopa response and asymmetric onset, supports the diagnosis of idiopathic PD. **Mnemonic:** MRI **rules out mimics** — structural lesions, atypical patterns, and secondary causes. It is the gatekeeper investigation before functional imaging. ## Role of Each Investigation in PD Diagnosis | Investigation | Purpose | When to Use | Limitation | | --- | --- | --- | --- | | MRI brain (T1/T2/FLAIR) | Exclude structural mimics; identify atypical patterns | **First-line** in all suspected PD | Does not assess dopaminergic function | | 123I-FP-CIT SPECT | Confirm dopaminergic denervation | After MRI if diagnosis remains uncertain | Functional imaging; requires nuclear medicine facility | | 18F-FDG PET | Identify metabolic patterns in atypical syndromes | Suspected atypical parkinsonism (PSP, MSA, CBD) | Not first-line for idiopathic PD confirmation | | Transcranial ultrasound | Assess substantia nigra echogenicity | Research tool; not routine clinical practice | Limited availability; operator-dependent | **Warning:** Do not skip MRI in the initial workup. Even though idiopathic PD is a clinical diagnosis, MRI is essential to exclude structural and secondary causes. Functional imaging (DaTscan) is reserved for cases where clinical diagnosis is uncertain after MRI has ruled out structural mimics. ## Diagnostic Algorithm ```mermaid flowchart TD A[Suspected Parkinson Disease]:::outcome --> B[Clinical assessment:<br/>asymmetric onset, rest tremor,<br/>rigidity, bradykinesia]:::action B --> C{Red flags for<br/>atypical syndrome?}:::decision C -->|Yes or uncertain| D[MRI brain with<br/>standard sequences]:::action C -->|No, typical PD| D D --> E{Structural lesion<br/>or atypical pattern?}:::decision E -->|Yes| F[Diagnosis: Secondary<br/>or atypical parkinsonism]:::outcome E -->|No| G{Clinical diagnosis<br/>still uncertain?}:::decision G -->|Yes| H[123I-FP-CIT SPECT<br/>DaTscan]:::action G -->|No| I[Diagnosis: Idiopathic PD]:::outcome H --> J{DAT loss<br/>present?}:::decision J -->|Yes| I J -->|No| K[Diagnosis: Essential tremor<br/>or other mimic]:::outcome ``` [cite:Harrison 21e Ch 429]