The Dopamine Transporter Scan (DAT Scan) is a specialized nuclear medicine procedure that visualizes the density of dopamine transporters (DAT) in the brain. This imaging technique provides information on the function of the dopaminergic system, which controls movement. Physicians introduce a small amount of a radioactive tracer to image the DAT proteins. The resulting images help healthcare providers assess the health of the neurons responsible for producing and transporting dopamine in specific brain regions.
The Purpose of the DAT Scan
The primary function of the DAT scan is to help physicians differentiate between various movement disorders. The scan determines if symptoms like tremor or slowness are caused by the loss of dopamine-producing neurons, a condition known as neurodegenerative parkinsonism. This loss of neurons is a hallmark feature of disorders including Parkinson’s disease.
The procedure is particularly helpful when a clinical diagnosis is uncertain, such as distinguishing between a neurodegenerative disorder and a non-dopaminergic condition like Essential Tremor. The scan effectively distinguishes between these two groups; in cases of Essential Tremor, which does not involve dopamine neuron loss, the scan results are normal.
An abnormal scan indicates a reduction in dopamine transporters, suggesting a neurodegenerative process. This is also useful for ruling out conditions such as psychogenic tremor or drug-induced parkinsonism, where the dopamine system remains intact. A DAT scan confirms the presence or absence of dopamine transporter loss, but it cannot distinguish between different subtypes of parkinsonism, such as Parkinson’s disease and Multiple System Atrophy.
Preparing for the Procedure
Preparation for the DAT scan ensures the radioactive tracer can bind effectively without interference. Patients are instructed to stop taking certain medications that interfere with dopamine uptake. The decision to discontinue any medication must be made in consultation with the prescribing physician, who weighs the benefits against the risks of temporarily stopping treatment.
Interfering drugs can include:
- Specific antidepressants
- Stimulant medications for Attention-Deficit/Hyperactivity Disorder (ADHD)
- Cough suppressants containing phentermine
- Cough suppressants containing bupropion
Preparation also involves protecting the thyroid gland from the radioactive component of the tracer. Since the thyroid naturally absorbs iodine and the tracer contains Iodine-123, patients are given a thyroid-blocking agent, such as potassium iodide tablets or Lugol’s solution, before the injection. This preparatory step prevents the thyroid from accumulating radioactive material. Patients should drink plenty of fluids before and after the procedure to help the body eliminate the tracer quickly and minimize radiation exposure.
The Imaging Process
The DAT scan procedure begins with the slow injection of the radiotracer, Ioflupane I 123, into a vein, usually in the arm. This specialized tracer binds specifically to the dopamine transporter proteins in the brain. Following the injection, there is a mandatory waiting period, typically three to six hours, before imaging begins.
This waiting time allows the tracer to circulate through the bloodstream, cross the blood-brain barrier, and bind to dopamine transporters in the striatum, the brain region of interest. During this period, patients are usually free to move around, eat, and drink normally. Once the tracer has accumulated sufficiently, the patient is positioned on a table for the imaging phase, which uses Single-Photon Emission Computed Tomography (SPECT).
The SPECT camera, a large machine containing detectors, rotates around the patient’s head to capture images of the gamma rays emitted by the radioactive tracer. The patient must lie as still as possible during this process, which usually takes between 30 and 45 minutes, to ensure clear, high-quality images. The camera moves in close proximity to the head to accurately map the distribution of the radiotracer.
Interpreting the Scan Results
Interpretation is based on the visual appearance of tracer uptake in the striatum, which includes the caudate nucleus and the putamen. A normal DAT scan shows a characteristic pattern of symmetrical, intense radiotracer binding that resembles two commas or crescents in the brain image. The putamen and the caudate nucleus both show high levels of activity, creating this distinct visual signature.
An abnormal scan indicates a loss of dopamine transporters, showing a reduced or absent signal in the putamen. This often results in an image that looks like a period or a single dot on one or both sides. This loss of the comma shape confirms a reduction in the density of functioning dopamine neurons. The loss of binding often appears asymmetrical, meaning it is more pronounced on one side of the brain, which correlates with the side of the body where the patient’s symptoms are most severe.
An abnormal result confirms a neurodegenerative process affecting the dopaminergic system, but it does not specify the exact disorder. The physician must integrate this finding with the patient’s neurological examination and clinical history to arrive at a final diagnosis. While an abnormal scan is consistent with Parkinson’s disease, it can also be seen in other neurodegenerative conditions like Dementia with Lewy Bodies. A normal scan is strong evidence against a diagnosis of a neurodegenerative parkinsonian syndrome.