In plain English
Single-Photon Emission Computed Tomography (SPECT) and DAT Scan imaging are nuclear medicine techniques used in selected dementia cases. DAT Scan (123I-FP-CIT SPECT) is particularly valuable in suspected Dementia with Lewy Bodies.
The two types
SPECT and DAT Scan both use radiolabelled tracers and the same type of scanner, but answer different questions:
Perfusion SPECT
Measures regional cerebral blood flow using a tracer such as 99mTc-HMPAO. The pattern of reduced flow reflects underlying dementia pathology, similar in principle to FDG-PET but with lower resolution.
DAT Scan (123I-FP-CIT SPECT)
Measures dopamine transporter activity in the basal ganglia. Reduced uptake indicates loss of dopamine neurons, which occurs in Parkinson's Disease and Dementia with Lewy Bodies but not in Alzheimer's Disease. The DAT Scan is highly specific for the alpha-synuclein dementias.
When DAT Scan is recommended
NICE NG97 recommendation 1.2.20 advises 123I-FP-CIT SPECT (DAT Scan) when the diagnosis is uncertain and Dementia with Lewy Bodies is suspected. Typical situations:
- Visual hallucinations and fluctuating cognition without clear Parkinsonism;
- REM Sleep Behaviour Disorder with cognitive change;
- Atypical features in a person previously diagnosed with Alzheimer's Disease;
- Distinguishing Dementia with Lewy Bodies from Frontotemporal Dementia or atypical Alzheimer's.
What the DAT Scan shows
- Normal DAT Scan: symmetric "comma-shaped" uptake in the basal ganglia. Argues against Dementia with Lewy Bodies (although does not exclude it absolutely).
- Abnormal DAT Scan: reduced or asymmetric uptake, typical "dot" shape. Supports Dementia with Lewy Bodies, Parkinson's Disease, or related alpha-synuclein conditions.
A normal scan does not rule out Dementia with Lewy Bodies entirely; an abnormal scan in the right clinical context substantially supports the diagnosis.
What to expect on the day
For DAT Scan, a small dose of iodine-based blocker is given orally before the scan to protect the thyroid. The radioactive tracer is then injected intravenously. After 3 to 4 hours uptake time, the scan itself lasts around 30 to 45 minutes, lying still in a scanner.
Some medicines (Cholinesterase Inhibitors, antipsychotics, antidepressants, certain Parkinson's medications) can affect the scan and may need to be withheld for several days beforehand. The referring clinician advises.
What perfusion SPECT shows in dementia
Perfusion SPECT shows similar patterns to FDG-PET, with lower resolution but wider availability. It is used when FDG-PET is not available:
- Alzheimer's Disease: reduced perfusion in parietal and posterior temporal cortices;
- Frontotemporal Dementia: reduced perfusion in frontal and anterior temporal cortices;
- Vascular Dementia: patchy reductions following vascular territories.
UK availability
DAT Scan is available in most large NHS centres, though waiting times vary. Private DAT Scan is available in some UK cities for around £1,200 to £1,800. Perfusion SPECT is more widely available on the NHS.
Where The Dementia Service fits in
If Dementia with Lewy Bodies is suspected but uncertain, The Dementia Service can arrange DAT Scan or perfusion SPECT through partner providers and integrate the result into a structured letter for your GP.
Frequently asked questions
What is the difference between DAT Scan and FDG-PET?
DAT Scan looks specifically at dopamine transporter activity in the basal ganglia and is most useful for Dementia with Lewy Bodies. FDG-PET looks at brain glucose metabolism across the cortex and is most useful for distinguishing Alzheimer's from Frontotemporal Dementia.
Are SPECT and PET the same?
Both are nuclear medicine techniques. PET has higher resolution and uses different tracers. SPECT is more widely available.
Will a normal DAT Scan rule out Lewy Body Dementia?
Not absolutely. A normal scan reduces the probability but does not exclude Dementia with Lewy Bodies entirely. The clinical picture remains paramount.
Are there medicines that affect the result?
Yes. Some Cholinesterase Inhibitors, antipsychotics, antidepressants and Parkinson's medications can affect DAT Scan uptake. Your referring clinician advises on which to hold.
How long for the result?
Reports are usually available within one to three weeks. Your assessing clinician will discuss the result at follow-up.
References
- NICE NG97 recommendation 1.2.20.
- McKeith IG et al. Diagnosis and management of Dementia with Lewy Bodies. Neurology 2017.
- Walker Z et al. Differentiation of Alzheimer's Disease from Dementia with Lewy Bodies with 123I-FP-CIT SPECT. J Neurol Neurosurg Psychiatry 2007.
- Royal College of Radiologists. Nuclear medicine in dementia.