What is a DaT (Dopamine Transporter) scan, its indications, and the procedure itself, particularly for older adults experiencing symptoms such as tremors, rigidity, or cognitive decline?

What is a DaT Scan?

A DaT (Dopamine Transporter) scan is a specialized nuclear medicine SPECT imaging study using I-123 Ioflupane that visualizes the integrity of dopaminergic neurons in the striatum by binding to presynaptic dopamine transporters. 1

Technical Overview

• The scan uses I-123 Ioflupane (trade name DaTSCAN), a radiotracer that binds specifically to dopamine transporters on presynaptic dopaminergic nerve terminals in the striatum 2
• SPECT (Single Photon Emission Computed Tomography) imaging is performed to detect the distribution of the radiotracer in the brain 1
• A normal scan shows symmetric, comma-shaped uptake in both the caudate nuclei and putamina bilaterally 3
• An abnormal scan demonstrates loss of the normal comma shape, indicating reduced dopamine transporter density from neuronal degeneration 3

Primary Clinical Indications

Movement Disorders with Diagnostic Uncertainty

The most appropriate use of DaT scanning is when clinical diagnosis of parkinsonism remains uncertain after thorough neurological evaluation. 4, 5

Specific scenarios where DaT imaging provides diagnostic value include:

• Early subtle parkinsonian symptoms where clinical examination is equivocal 4
• Suboptimal or absent response to levodopa in patients suspected of having Parkinson's disease 4
• Prominent action tremor making it difficult to distinguish essential tremor from Parkinson's disease 4, 5
• Drug-induced parkinsonism to determine if underlying dopaminergic dysfunction exists 4, 5
• Atypical presentations such as lower extremity-predominant or other unusual parkinsonian features 4
• Patients diagnosed with Parkinson's disease for 3-5 years without apparent progression or motor fluctuations, raising diagnostic doubt 4

Dementia Syndromes

DaT scanning is a suggestive feature for diagnosing dementia with Lewy bodies (DLB) when clinical features overlap with Alzheimer's disease. 3

• Reduced striatal uptake has 78% sensitivity and 90% specificity for differentiating DLB from Alzheimer's disease 3
• The scan helps identify patients with coexistent Alzheimer's disease and DLB pathology, as DLB shows reduced striatal activity while Alzheimer's disease shows normal uptake 3
• DaT imaging can facilitate earlier diagnosis in patients with prodromal DLB symptoms 3
• However, DaT scanning is NOT a first-line test for ataxia or general dementia evaluation 3

Distinguishing Parkinsonian Syndromes from Non-Degenerative Conditions

• Normal DaT scans help identify "SWEDDs" (Subjects Without Evidence of Dopamine Deficiency), which include essential tremor, dystonic tremor, psychogenic parkinsonism, and vascular parkinsonism without basal ganglia infarction 1, 5
• The scan cannot distinguish between idiopathic Parkinson's disease and atypical parkinsonian disorders (multiple system atrophy, progressive supranuclear palsy, corticobasal degeneration), as all show reduced uptake 1, 5

The Procedure Itself

Pre-Procedure Preparation

• Thyroid blockade is administered before the scan to prevent radioactive iodine uptake by the thyroid gland 2
• Patients should inform the nuclear medicine team of all medications, as certain drugs can interfere with dopamine transporter binding 2

Imaging Protocol

• The I-123 Ioflupane radiotracer is injected intravenously 2
• SPECT imaging is typically performed 3-6 hours after injection to allow optimal tracer uptake and clearance of background activity 2
• The scan itself takes approximately 30-45 minutes, during which the patient must remain still 2
• Images are acquired using a gamma camera that rotates around the patient's head 2

Image Interpretation

• Quantitative analysis software (such as BRASS) can provide normalized uptake ratios for objective assessment 6
• Visual interpretation assesses the symmetry and shape of striatal uptake 3
• Loss of the comma shape, particularly in the putamen, indicates dopaminergic degeneration 3
• Asymmetric uptake correlates with asymmetric clinical symptoms 1

Clinical Impact and Outcomes

In a prospective series of 201 patients with clinically uncertain parkinsonism, DaT scanning changed the clinical diagnosis in 39.8% of cases and led to medication changes in 70.1% of patients. 4

• The scan was abnormal in 58.7% of patients, normal in 37.8%, and inconclusive in only 3.5% 4
• Congruence between initial clinical diagnosis and DaT imaging results increases over time, favoring the baseline DaT imaging findings 5
• Striatal uptake correlates with disease severity, particularly bradykinesia and rigidity 5

Important Caveats and Limitations

• A reduced DaT scan should be regarded as supportive rather than diagnostic of dopamine-deficient parkinsonism 1
• The scan does not replace clinical evaluation by a movement disorder specialist or cognitive disorders specialist 7
• DaT imaging cannot differentiate between different causes of dopaminergic degeneration (Parkinson's disease vs. atypical parkinsonisms) 1, 5
• For dementia evaluation, the American College of Radiology recommends FDG-PET before DaT scanning due to cost considerations and higher diagnostic yield 7
• Combining DaT imaging with FDG-PET provides superior diagnostic accuracy (88.9% correct classification) compared to either modality alone 6