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Understanding Your Imaging: Decoding the MRI and PET Scans

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Magnetic Resonance Imaging (MRI) and PET scans help diagnose Progressive Supranuclear Palsy (PSP) by revealing specific patterns of brain shrinkage, such as midbrain atrophy and the 'hummingbird sign'. These imaging markers help neurologists distinguish PSP from Parkinson's disease.

Key Takeaways

  • • The hummingbird sign on an MRI is a highly specific visual marker for Progressive Supranuclear Palsy (PSP).
  • • Radiologists use specific measurements like the Midbrain-to-Pons (M/P) ratio and MRPI score to calculate brain shrinkage.
  • • While a DaTscan cannot easily distinguish between Parkinson's and PSP, an FDG-PET scan can reveal PSP's unique energy-use pattern.
  • • In the early stages of PSP, an MRI may appear normal because brain atrophy happens slowly.

Diagnosing Progressive Supranuclear Palsy (PSP) is like putting together a complex puzzle. While your symptoms provide the most important clues, Magnetic Resonance Imaging (MRI) provides the “visual proof” by looking for specific patterns of brain shrinkage (atrophy) that aren’t usually found in Parkinson’s disease [1][2].

The “Hummingbird Sign”

One of the most famous visual markers for PSP is the hummingbird sign [2]. When a radiologist looks at a side-profile (sagittal) view of the brain, the midbrain—which controls balance and eye movement—may shrink in a way that makes it look like the head and beak of a hummingbird [2][3].

  • What it means: This sign is extremely specific; if your doctor sees it, there is a very high chance (often over 95%) that the diagnosis is PSP [4].
  • The limitation: Because the brain shrinks slowly, the hummingbird may not be visible in the early stages of the disease, so a “normal” report doesn’t always rule out PSP [4][2].

Measuring Atrophy: M/P and MRPI

Because looking at pictures can be subjective, radiologists use math to be more precise. They measure the size of the midbrain and compare it to the pons (another part of the brainstem) [5][6].

The Midbrain-to-Pons (M/P) Ratio

In healthy brains and those with Parkinson’s, the midbrain remains a certain size relative to the pons [6][7].

  • PSP Profile: In PSP, the midbrain shrinks while the pons stays largely the same [6].
  • The Cutoff: A ratio of 0.52 or lower is a strong indicator of PSP [8].

The Magnetic Resonance Parkinsonism Index (MRPI)

The MRPI is a more advanced score that combines four different measurements of the brainstem [9][10].

  • The Score: A “normal” score is low. If the MRPI is higher than 13.0, it points toward PSP [11][12].
  • MRPI 2.0: A newer version adds a measurement of the third ventricle (a fluid-filled space in the brain). This version is even better at catching the disease in its earliest stages [9][13].

Beyond MRI: DaTscan and FDG-PET

If the MRI is unclear, your doctor may order “functional” scans that look at how the brain is working rather than just its shape.

  • DaTscan: This scan looks at dopamine levels [14]. While it can distinguish Parkinson’s from a simple tremor, it cannot usually tell the difference between Parkinson’s and PSP, as both show low dopamine levels [14][15].
  • FDG-PET: This scan measures how the brain uses sugar (energy) [16]. PSP has a “signature” pattern of low energy (hypometabolism) in the frontal lobes and midbrain, which is very different from the pattern seen in Parkinson’s [17][18].

MRI Report “Completeness Checklist”

When you receive your radiology report, look for these specific terms. If they aren’t there, you can ask your neurologist to have the images re-evaluated for these “PSP markers”:

  1. Midbrain Atrophy: Is the midbrain described as “thinned” or “atrophied”? [2]
  2. Superior Cerebellar Peduncle (SCP): This area often shrinks in PSP but stays normal in Parkinson’s [19].
  3. Third Ventricle: Is it described as “enlarged” or “dilated”? [3]
  4. Hummingbird or Morning Glory Sign: Are any specific “visual signs” mentioned? [2]
  5. MRPI Score: If possible, ask if the MRPI was calculated (this often requires a specialist) [9].

Return to the Main Guide

Frequently Asked Questions

What is the hummingbird sign on an MRI?
The hummingbird sign is a visual pattern on an MRI where the shrinking midbrain looks like the head and beak of a hummingbird. If your doctor sees this specific sign, there is a very high chance the diagnosis is PSP rather than Parkinson's disease.
What does an MRPI score mean for my PSP diagnosis?
The Magnetic Resonance Parkinsonism Index (MRPI) is an advanced calculation that combines measurements of different parts of your brainstem. An MRPI score higher than 13.0 strongly points toward a diagnosis of Progressive Supranuclear Palsy.
Can a normal MRI rule out Progressive Supranuclear Palsy?
No, a normal MRI does not rule out PSP. Because brain shrinkage happens gradually, classic markers like the hummingbird sign or significant midbrain atrophy may not be visible in the early stages of the disease.
Will a DaTscan show if I have PSP instead of Parkinson's?
A DaTscan measures dopamine levels in the brain. While it can distinguish Parkinsonian conditions from standard tremors, it usually cannot tell the difference between Parkinson's disease and PSP because both conditions cause low dopamine levels.
Why might my doctor order an FDG-PET scan for PSP?
An FDG-PET scan measures how your brain uses energy (sugar). In PSP, there is a unique signature pattern of low energy in the frontal lobes and midbrain, which helps doctors differentiate it from Parkinson's disease.

Questions for Your Doctor

  • • Does my report mention the 'Magnetic Resonance Parkinsonism Index' (MRPI), and if so, was it calculated as version 1.0 or 2.0?
  • • I see the term 'midbrain atrophy' in the report; can you explain how this measurement helps distinguish my case from Parkinson's disease?
  • • Is the 'hummingbird sign' present in my sagittal MRI images, and how specific is that for my diagnosis?
  • • Would an FDG-PET scan be helpful to confirm the pattern of activity in my frontal lobes and midbrain?
  • • The report mentions 'third ventricle enlargement'—is this a standard finding for the subtype of PSP you suspect?

Questions for You

  • • When you look at your MRI report, do you see terms like 'midbrain,' 'pons,' or 'superior cerebellar peduncle'?
  • • Have you or your doctor noticed that your symptoms (like falling) match the 'midbrain' changes seen on the scan?
  • • Was your MRI a '3T' scan? (Higher-strength 3T scans often provide the detail needed to see subtle PSP signs.)

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References

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This page explains Progressive Supranuclear Palsy (PSP) imaging terms for educational purposes. Your neurologist and radiologist are the best sources for interpreting your specific MRI or PET scans.

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