Parkinson’s disease is a progressive neurological disorder characterized by tremors, rigidity, slow movements, and difficulty with walking and coordination. It is caused by the loss of nerve cells in the brain that produce dopamine, a chemical messenger that helps control movement. Parkinson’s is diagnosed through a neurological exam, but various scans can provide supporting evidence and rule out other possible causes. Scans alone cannot definitively diagnose Parkinson’s, but they may reveal changes in the brain that are suggestive of the disease.
MRI
An MRI (magnetic resonance imaging) scan uses radio waves and strong magnets to create detailed images of the brain and other structures. An MRI does not use radiation like CT scans or x-rays. In Parkinson’s disease, an MRI may show:
- Atrophy (shrinking) in an area of the brain stem called the substantia nigra. This is where dopamine-producing neurons start to be lost in Parkinson’s.
- Iron deposits. Increased iron levels create dark areas on T2-weighted MRI images.
- Alterations in other brain regions as the disease progresses and more neurons are affected.
However, changes in the substantia nigra and other areas may not be visible on an MRI until later stages of Parkinson’s. A normal scan does not rule out a diagnosis. MRIs are more useful for ruling out other disorders with similar symptoms, like tumors, strokes, or hydrocephalus (fluid buildup).
DAT scan
A DAT scan (dopamine transporter scan) uses small amounts of a radioactive tracer to detect dopamine activity in the brain. The scan highlights the dopamine-producing neurons, allowing doctors to look for patterns of loss or reduced dopamine uptake consistent with Parkinson’s:
- Decreased signal in the substantia nigra, indicating loss of dopamine neurons
- Asymmetrical signal, one side of the brain affected more than the other
- Reduced dopamine uptake throughout the striatum, the area where dopamine is released
DAT scans are very useful in early or uncertain Parkinson’s cases. But false positives and negatives are possible. And the scan is expensive and not always covered by insurance.
SPECT
SPECT (single photon emission computed tomography) scans also use a radioactive tracer to measure brain activity and blood flow. SPECT scans may reveal reduced blood flow in areas affected by Parkinson’s, like the substantia nigra. SPECT images can detect changes early, but they are not specific to Parkinson’s. The scan may be useful for ruling out other conditions.
PET
PET (positron emission tomography) scans involve injection of a radioactive glucose solution to map brain activity through glucose metabolism. PET scans show problems with dopamine production and use in Parkinson’s:
- Decreased glucose metabolism in the substantia nigra
- Lower dopamine levels in the striatum
- Impaired glucose metabolism in the cortex and associated regions
Like SPECT, PET can detect Parkinson’s in early stages. But it is nonspecific and may show changes from other conditions.
Which scan is used?
There is no one definitive scan for Parkinson’s disease. The type of scan used depends on the individual factors and symptoms. Here is a table summarizing the main brain imaging options for Parkinson’s:
| Scan Type | What is detected | Usefulness |
|---|---|---|
| MRI | Atrophy in substantia nigra and other regions; iron deposits | Rule out other disorders; changes often visible in later Parkinson’s |
| DAT | Reduced dopamine and dopamine cell loss in substantia nigra and striatum | Early diagnosis; detect dopamine deficiency |
| SPECT | Reduced blood flow in affected areas like substantia nigra | Early detection; not specific to Parkinson’s |
| PET | Decreased dopamine production and metabolism | Early detection; not specific to Parkinson’s |
The choice of scan depends on factors like:
- Age of onset of symptoms
- Types of symptoms present
- Disease stage
- Need to rule out other disorders
- Insurance coverage for expensive scans like DAT and PET
Often, a combination of scans is used over the course of diagnosis, monitoring, and treatment.
Accuracy of Scans
Brain scans can provide supporting evidence for a Parkinson’s diagnosis, but have limitations:
- Scans like MRI may be normal early in Parkinson’s.
- The scans can appear abnormal for other reasons not related to Parkinson’s.
- The findings are usually not specific enough to definitively diagnose Parkinson’s on their own without clinical correlation.
- There is a margin of error in interpreting the images.
Experienced clinicians factor in the scan results with the full clinical picture including symptoms, response to medication, disease progression, age, and other factors. Scans provide useful data, but cannot provide a Parkinson’s diagnosis by themselves.
False positives
Sometimes a scan suggests Parkinson’s but the person does not actually have the disease. This false positive result may occur because:
- The changes seen can occur in normal aging brains.
- Other disorders like Alzheimer’s disease, strokes, and brain tumors can mimic Parkinson’s on scans.
- There are inaccuracies in image acquisition and interpretation.
Further clinical correlation is needed to rule out other conditions and confirm Parkinson’s disease.
False negatives
In other cases, the scan may not show typical Parkinson’s patterns, but the person does have the disease. Sources of these false negative scan findings include:
- Early disease stage before changes are visible.
- Atypical symptom patterns.
- Inaccuracies in image acquisition and interpretation.
- Parkinson’s variants like Lewy body dementia that affect different brain regions.
Normal scans do not definitively rule out a Parkinson’s diagnosis, especially early on. Clinical features and progression over time remain most important.
Role of Scans
Though imperfect, brain scans are useful tools in Parkinson’s diagnosis and management when combined with a full clinical picture. Key roles of imaging include:
- Suggesting Parkinson’s if typical changes are seen.
- Detecting early loss of dopamine-producing neurons.
- Ruling out other neurological disorders.
- Mapping the disease stage and progression.
- Evaluating responses to therapy over time.
- Identifying atypical Parkinson’s variants.
- Improving accuracy of diagnosis when combined with clinical data.
While no scan can definitively diagnose Parkinson’s on its own, scans provide valuable additional data when interpreted by an experienced clinician along with the full clinical context. Ongoing advances in imaging technology and interpretation aim to improve the accuracy for detecting Parkinson’s.
Conclusion
Parkinson’s disease progressively harms dopamine-producing neurons in characteristic brain regions like the substantia nigra. Scans like MRI, DAT, SPECT, and PET can reveal changes in these regions and help evaluate dopamine levels. Scan findings can provide supporting evidence for a Parkinson’s diagnosis when considered together with clinical features. However, scans have limitations including margins of error and inability to definitively diagnose Parkinson’s independently. They cannot replace a skilled clinician’s overall diagnostic process. Ongoing advances in neuroimaging continue to enhance the detection and management of Parkinson’s disease. But the optimal role of scans remains adjunctive to the clinical skills of an experienced neurologist or movement disorder specialist.