Pathognomonic MRI Findings in Progressive Supranuclear Palsy and Multiple System Atrophy

Pathognomonic MRI Findings in Progressive Supranuclear Palsy and Multiple System Atrophy Fifteen–Twenty High-Yield Points General principle

Structural MRI in parkinsonism serves two purposes: to exclude secondary causes (vascular disease, hydrocephalus, tumour) and to identify positive imaging signatures of atypical parkinsonism. MRI is normal in idiopathic PD.

The textbook states directly that a typical pattern of MRI imaging has been described in progressive supranuclear palsy and relates to midbrain atrophy, and illustrates the convex-to-concave change in the superior midbrain profile.

The normal superior margin of the midbrain is convex. In PSP it becomes flat and then concave — this single observation on a midsagittal T1 image is the basis of most PSP signs.

Hummingbird (penguin) sign — midsagittal T1: selective atrophy of the rostral midbrain tegmentum with a relatively preserved pons. The atrophic midbrain forms the bird's head and beak; the normal pons forms the body. Specificity about 99.5%, sensitivity about 51.6%.

Morning glory flower sign — axial T2: concavity of the lateral margin of the midbrain tegmentum, with relative sparing of the tectum and cerebral peduncles, resembling the flower.

Mickey Mouse sign — axial image at midbrain level where the atrophic tegmentum and preserved cerebral peduncles produce the outline of a mouse's head and ears.

Supporting indirect measures: reduced anteroposterior midbrain diameter, third ventricular dilatation, and atrophy of the superior cerebellar peduncle (SCP) — SCP atrophy is relatively specific for PSP.

Midbrain-to-pons area ratio (M/P) on midsagittal T1 is reduced in PSP; a ratio below approximately 0.52 is widely used as a threshold.

Magnetic Resonance Parkinsonism Index (MRPI) = (pons area / midbrain area) × (middle cerebellar peduncle width / superior cerebellar peduncle width). It is increased in PSP and has better accuracy than any single measure. MRPI 2.0 adds the third ventricle width-to-frontal horn width ratio and improves detection of PSP-P.

Important caveat: a normal MRI does not exclude PSP. Atrophy develops gradually, so the hummingbird sign and significant midbrain atrophy may be absent early in the disease.

Hot cross bun sign — axial T2 or proton-density image of the pons showing a cruciform hyperintensity. It results from selective degeneration of transverse pontocerebellar fibres and pontine neurons with preservation of the corticospinal tracts (which form the vertical limb) and the tegmentum.

Atrophy of the pons, cerebellum and middle cerebellar peduncles, best seen on midsagittal T1 — the imaging correlate of olivopontocerebellar atrophy.

Middle cerebellar peduncle (MCP) T2 hyperintensity, and a reduced MCP width and MCP/SCP ratio, are useful quantitative markers distinguishing MSA from PSP.

Putaminal rim sign — a hyperintense rim along the dorsolateral border of the putamen on T2/FLAIR, thought to represent gliosis and microglial activation.

Putaminal atrophy and putaminal T2 hypointensity (from iron deposition), best appreciated on axial T2 and susceptibility-weighted imaging.

Diagnostic performance: these putaminal signs have high specificity (87–100%) but widely variable sensitivity — 38–100% for putaminal T2 hypointensity and 56–90% for the hyperintense dorsolateral rim.

Increased putaminal diffusivity on diffusion-weighted imaging (raised apparent diffusion coefficient / mean diffusivity) is directly visible and is one of the most useful practical signs for separating MSA from PD and PSP.

The essential contrast for the examiner: in PSP the midbrain atrophies and the pons is spared; in MSA the pons and cerebellum atrophy and the midbrain is relatively spared. On the same midsagittal image, PD shows neither.

The hot cross bun sign is not entirely specific to MSA — it is also described in spinocerebellar ataxias (especially SCA-2, 3 and 7), variant CJD and some parainfectious pontine lesions, so it must be interpreted with the clinical picture.

Both PSP and MSA signs have high specificity but modest sensitivity: they confirm the diagnosis when present but never exclude it when absent. The diagnosis remains clinical, with MRI as supportive evidence.