When someone is evaluated for memory, thinking, or neurological changes, brain imaging is often part of the diagnostic process. MRI and PET scans allow clinicians to examine the brain in different ways - both its structure and its biology. These tests do not diagnose symptoms by themselves, but they provide critical context that helps explain why symptoms may be occurring and how best to plan care.
What Is a Brain MRI?
MRI (Magnetic Resonance Imaging) shows the structure of the brain. It uses magnetic fields and radio waves, not radiation, to create detailed images.
An MRI helps clinicians assess:
- Overall brain structure and volume
- Areas of shrinkage (atrophy)
- Evidence of strokes, small vessel disease, or prior injury
- Microbleeds or changes in blood vessels
- Brain swelling or inflammation
MRI answers the question: "What does the brain look like?"
This information is especially important for identifying vascular disease, ruling out treatable causes, and establishing a safe baseline before certain therapies.
What Is a PET Scan?
PET (Positron Emission Tomography) examines brain function or brain proteins, not structure. PET uses a small amount of tracer injected into a vein. Different tracers answer different questions.
PET answers the question: "What is happening biologically in the brain?"
There are several types of PET scans used in cognitive care.
FDG-PET: Brain Activity and Metabolism
FDG-PET measures how brain cells use glucose (energy). Active brain regions use more energy; affected regions use less.
FDG-PET can:
- Show patterns of reduced brain activity
- Help distinguish between different neurodegenerative conditions
- Clarify diagnosis when symptoms overlap
This scan is especially useful for understanding network dysfunction - how brain systems are working together (or not).
Amyloid PET: Detecting Amyloid Protein
Amyloid PET looks for amyloid plaques, a protein associated with Alzheimer's disease.
Important points:
- Amyloid can be present years before symptoms
- A positive scan supports Alzheimer's biology
- A negative scan makes Alzheimer's disease very unlikely
Amyloid PET does not measure severity or predict timing. It helps determine whether Alzheimer's-type pathology is present.
Tau PET: Mapping Disease Progression
Tau PET detects tau protein, which correlates more closely with symptoms and disease stage.
Tau PET can:
- Show where disease is actively affecting brain networks
- Help explain symptom patterns (memory, language, vision, behavior)
- Provide insight into disease burden
Tau imaging is primarily used in specialized or research settings and is not always necessary for routine care.
Why Imaging Results Must Be Interpreted Carefully
Imaging findings are never interpreted alone.
Brain scans must be understood alongside:
- Symptoms and daily function
- Cognitive testing
- Medical history and medications
- Blood or spinal fluid biomarkers
A scan can show changes without symptoms, and symptoms can occur with subtle imaging findings. The goal is context, not labels.
Why Imaging Is Repeated Over Time
In some situations, repeat imaging helps:
- Monitor disease progression
- Assess treatment safety (such as with certain Alzheimer's therapies)
- Track vascular or inflammatory changes
Not everyone needs repeat scans. Imaging plans are individualized.
What This Means
MRI and PET imaging provide different but complementary information. MRI shows structure and safety, while PET reveals function and biology. Together, they help clarify diagnosis, guide treatment decisions, and support thoughtful long-term planning.
Imaging is not about predicting the future - it is about understanding the present more clearly so care can be tailored to the person, not just the scan.
If you have questions about which imaging tests are recommended and why, your care team can explain the reasoning and how results fit into the larger picture of brain health.