Brain PET (FDG, Amyloid, DAT)
- Brain PET answers three very different questions with three very different tracers: how busy is the brain (FDG), is sticky amyloid plaque present (amyloid PET), and are the dopamine neurons intact (DAT scan).
- FDG maps glucose use, so it shows where metabolism drops — and the pattern of cold spots is what suggests a dementia type.
- Amyloid PET is a presence/absence test: a positive scan means plaque is there, but plaque alone does not equal Alzheimer's dementia.
- The DAT scan looks at the dopamine "loading dock" in the striatum and helps separate true Parkinsonian disorders from things that only mimic them.
The skull is opaque and the brain mostly looks the same shade of gray no matter how sick it is, so for a long time we could see the brain's shape but not its chemistry. Brain PET is the workaround. We feed in a tracer that follows one specific molecular errand, then watch where it goes. Pick the right tracer and you get to eavesdrop on metabolism, on plaque, or on dopamine — three completely different stories from the same machine.
If you want the engine-room version of how the scanner turns positron annihilations into pictures, that lives in PET physics & PET/CT. Here we care about what each tracer is actually telling you.
FDG: a fuel gauge for the brain
FDG is glucose wearing a tiny radioactive disguise. Cells pull it in to burn for fuel, but the disguise jams the machinery, so it gets trapped right where it was taken up — a glowing fuel-consumption map. In oncology we hunt hot tumors. In the brain, the trick flips: the brain is already the hungriest organ in the body and lights up beautifully, so what you're hunting for are the cold spots, the regions that have gone quiet.
The neat part is that different dementias starve different neighborhoods, and the pattern is a fingerprint.
| Pattern of low FDG | Suggests |
|---|---|
| Posterior temporoparietal and posterior cingulate, often both sides | Alzheimer-type degeneration |
| Frontal and anterior temporal lobes | Frontotemporal dementia |
| Occipital / visual cortex involvement | Dementia with Lewy bodies |
FDG also has a day job outside dementia: in epilepsy surgery workups, an FDG scan done between seizures often shows the seizure focus as a quiet, low-uptake patch — the opposite of what you'd guess. Hungry during the storm, exhausted afterward.
A quick honesty note: these patterns suggest, they don't prove. Real patients overlap, mix, and refuse to read the textbook. FDG is a strong supporting witness, not the judge.
Amyloid PET: is the sticky stuff there?
Amyloid tracers do one job: they stick to beta-amyloid plaque, the gunk that accumulates in the Alzheimer brain. Think of plaque as flypaper in the walls of the house; the amyloid tracer is a dye that only sticks to flypaper. If the walls light up, there's flypaper there.
The read is refreshingly binary — positive or negative — but the interpretation is where people trip.
A negative amyloid scan is the powerful result: no significant plaque makes Alzheimer pathology very unlikely, which is genuinely useful for ruling it out. A positive scan is softer than it feels — plaque accumulates with normal aging too, so a cognitively healthy older person can scan positive. Positive amyloid means "plaque is present," not "this patient's dementia is Alzheimer's."
So the grammar matters. Amyloid PET is best at saying no. When it says yes, you still have to ask whether the plaque actually explains the patient in front of you.
DAT scan: counting the dopamine docks
The DAT scan is the odd one out — it's usually a SPECT study, not a true PET, but it sits in the same molecular-imaging family and gets lumped in. DAT stands for dopamine transporter, the little protein at nerve endings in the striatum that recycles dopamine. The tracer latches onto those transporters, so the scan is essentially a headcount of healthy dopamine nerve terminals.
A normal result looks like two plump symmetric commas in the center of the brain — the caudate and putamen, lit up on both sides. In Parkinson disease and its relatives, the dopamine terminals die off, and the comma shrivels: the tail fades first, often worse on one side, until you're left with a sad little dot.
The DAT scan's real value is sorting out confusing cases. It cannot tell Parkinson disease apart from other degenerative Parkinsonian syndromes — they all kill the same neurons. What it can do is separate a true Parkinsonian disorder (abnormal scan) from mimics like essential tremor or drug-induced tremor (normal scan), where the dopamine terminals are perfectly intact.
Tying the three together
Same scanner, three errands. FDG asks how busy is the brain and where did it go quiet. Amyloid asks is the sticky plaque present. DAT asks are the dopamine neurons still alive. None of them is a one-click diagnosis machine — every one is a clue that only earns its keep next to the patient's actual story and the rest of their imaging.
If you want to see how these results sit alongside the MRI findings in a real dementia workup, that's the dementia imaging page. The molecular and structural pictures are far more convincing together than either is alone — which, conveniently, is the whole point of imaging the brain twice.