The DaTscan, or Dopamine Transporter Scan, is a specialized medical imaging test used to visualize the health of the brain’s dopamine system. It is typically ordered when a person presents with a movement disorder, such as a tremor, but the diagnosis of Parkinson’s Disease (PD) remains uncertain. The scan provides objective evidence of the brain’s dopamine-producing cells, helping clinicians differentiate a Parkinsonian syndrome from other conditions that mimic its symptoms. It acts as an adjunct to the standard neurological examination, clarifying the underlying pathology in complex cases.
The Scientific Mechanism of DaTscan
The effectiveness of the DaTscan relies on mapping the density of dopamine transporters (DaT) located on presynaptic nerve terminals in the striatum. These transporters are proteins responsible for recycling dopamine, a process essential for controlled movement. In Parkinson’s disease, the progressive death of dopamine-producing neurons leads to a corresponding loss of these transporters in the striatum.
The test uses a radioactive tracer called Ioflupane I-123, which is designed to bind specifically to the DaT proteins. Once injected intravenously, the tracer crosses the blood-brain barrier and accumulates in the striatum based on the number of available transporters. Because Ioflupane I-123 contains the gamma-emitting radioisotope Iodine-123, its location is detected by a Single-Photon Emission Computed Tomography (SPECT) scanner. The resulting images provide a visual representation of the integrity of the presynaptic dopamine pathway.
Patient Preparation and the Scan Procedure
Preparation for a DaTscan ensures the tracer binds correctly and the images are clear. Patients must temporarily discontinue certain medications that could interfere with the tracer’s binding, such as some antidepressants, antipsychotics, and ADHD drugs. This temporary medication hold is determined by the prescribing physician based on the specific drug’s half-life.
A required step is the administration of a thyroid-blocking agent, such as Lugol’s solution or potassium iodide, given at least an hour before the tracer injection. This agent prevents the thyroid gland from absorbing the Iodine-123 isotope, which improves image quality and minimizes radiation exposure. The tracer is then injected slowly into an arm vein, followed by a waiting period of three to six hours to allow the Ioflupane I-123 to fully bind to the DaT in the brain.
The SPECT imaging typically takes 30 to 45 minutes. During the scan, the patient lies still on a table with their head positioned in a headrest. The scanner rotates around the head to capture data from various angles, detecting the gamma rays emitted by the tracer. Afterward, patients are encouraged to drink extra fluids to help flush the remaining tracer from the body.
Understanding the Scan Results
The visual interpretation of a DaTscan image focuses on the striatum, which is composed of the caudate nucleus and the putamen. In a normal scan, the tracer uptake creates two crescent or “comma-shaped” signals. This shape confirms a high, uniform concentration of dopamine transporters across both the putamen and the caudate nucleus.
An abnormal scan, consistent with a Parkinsonian syndrome, shows a reduction in tracer binding that begins in the putamen, often asymmetrically. As the disease progresses, the putamen’s signal fades significantly. This causes the characteristic “comma” shape to transform into a “period” or “oval” shape, representing preserved binding only in the caudate nucleus. This visual loss indicates a reduction in the density of presynaptic dopamine neurons. Results are generally interpreted visually, though quantitative analysis can measure the severity of the DaT loss.
Integrating DaTscan into Parkinson’s Diagnosis
The DaTscan is not a standalone test for Parkinson’s disease; it is a tool used to support diagnosis when symptoms are ambiguous. The primary utility of the DaTscan is differentiating between movement disorders that involve a loss of dopamine neurons and those that do not.
For conditions like essential tremor, drug-induced parkinsonism, or psychogenic disorders, the scan will typically be normal because they do not involve degeneration of the presynaptic dopamine system. A normal DaTscan effectively rules out a neurodegenerative Parkinsonian syndrome. Conversely, an abnormal scan confirms a presynaptic dopaminergic deficit, a hallmark of Parkinsonian syndromes.
A limitation of the DaTscan is its inability to distinguish between Parkinson’s Disease and other atypical Parkinsonian syndromes, such as Multiple System Atrophy (MSA) or Progressive Supranuclear Palsy (PSP). This is because all these neurodegenerative disorders involve the loss of dopamine transporters, resulting in an abnormal scan. Therefore, the final diagnosis remains clinical, relying on the neurologist’s interpretation of the scan results alongside the patient’s history, physical examination, and symptom progression.