DAT Scan (Ioflupane SPECT)

The DAT scan uses single photon emission computed tomography (SPECT) with iodine-123 ioflupane to show the dopamine transporter in the striatum (caudate nucleus and putamen). The transporter sits on the presynaptic terminals of nigrostriatal neurons.

Loss of these neurons, which occurs in Parkinson disease, reduces uptake of the tracer, and the scan separates degenerative parkinsonism from conditions with an intact dopamine system.

The dose is 111 to 185 MBq intravenously, given slowly over 20 seconds. Iodine-123 ioflupane is taken up by the brain over several hours, and SPECT images are taken 3 to 6 hours (usually 4 hours) after the injection, over 30 to 45 minutes.

Thyroid uptake of free iodine is blocked with potassium iodide (or potassium perchlorate) given 30 to 60 minutes before the tracer. The effective dose is about 4 to 5 mSv.

Parkinson disease affects about 1% of people over 60. Tremor and rigidity are not always distinguishable clinically from essential tremor, drug induced parkinsonism and vascular or functional disorders, and the clinical diagnosis is wrong in 10 to 25% of early cases.

The scan has a sensitivity of about 90 to 95% and a specificity of 85 to 95% for separating degenerative parkinsonism from non-degenerative causes. It also helps to separate dementia with Lewy bodies from Alzheimer disease (sensitivity about 80 to 90%, specificity about 90%).

Indications.

• Tremor of uncertain cause: Parkinson disease against essential tremor.
• Parkinsonism with a possible drug cause (antipsychotics, antiemetics, valproate) or vascular, psychogenic or other non-degenerative cause.
• Dementia with suspected Lewy bodies against Alzheimer disease and other dementias.
• Atypical or early parkinsonism with unclear features.
• Selection for research studies.
• Differential diagnosis of gait disorders and other movement disorders in selected cases.

Not for separating Parkinson disease from multiple system atrophy and progressive supranuclear palsy, as all three show reduced uptake.

Contraindications and precautions.

• Pregnancy: contraindicated. Breast feeding: interrupt for at least 3 days (or stop), because of iodine-123 excretion in milk.
• Allergy to iodine (which differs from contrast allergy) and thyroid disease: blocking agents are used. Caution in patients with thyroid disorders.
• Drugs that bind the dopamine transporter lower uptake and can cause false positive results. These include amphetamines, methylphenidate, modafinil, cocaine, bupropion, mazindol, some antidepressants (sertraline and some others that raise serotonin and noradrenaline) and sibutramine. They are stopped for at least 5 half lives before the scan, if safe.
• Usual doses of levodopa and dopamine agonists do not need to be stopped, as they do not affect uptake.
• The patient needs to stay still for 30 to 45 minutes. Tremor and dyskinesia may degrade images, and sedation is used in severe cases.
• Recent stroke or structural brain lesions in the striatum change uptake.

Preparation and technique.

The patient takes potassium iodide (about 100 mg) or perchlorate (400 mg) 30 to 60 minutes before the injection to block the thyroid. A venous line is placed, and the tracer is injected slowly over about 20 seconds, followed by a saline flush.

The patient rests, and SPECT is done at 3 to 6 hours with a gamma camera fitted with a high resolution collimator close to the head, with a circular orbit and a 128 by 128 matrix. Head position is checked and motion is minimised. Attenuation correction is applied.

Images are reviewed visually, and semi-quantitative values are calculated. The ratio of striatal uptake to a reference region (occipital cortex) gives a specific binding ratio, compared with age matched normal values.

Normal appearances.

The two striata appear as symmetrical comma shaped structures with a large head (caudate) and a tail (putamen), with high uptake and low background. Uptake is intense, smooth and equal in the two sides. The occipital cortex and cerebellum show low uptake.

The binding ratio falls with age by about 5 to 7% each decade, so the age adjusted values are used.

Interpretation.

• Parkinson disease: reduced uptake that begins in the posterior putamen, usually asymmetric and more on the side opposite the more affected limbs. The appearance changes from a comma to an oval and then to a full stop shape as the disease advances and the caudate becomes involved.
• Multiple system atrophy and progressive supranuclear palsy: also reduced, usually more symmetrical. The scan cannot separate them from Parkinson disease.
• Dementia with Lewy bodies: reduced striatal uptake (abnormal scan) in contrast to Alzheimer disease, in which uptake is normal.
• Essential tremor: normal scan.
• Drug induced parkinsonism: normal scan, unless an underlying degenerative disease is present. Symptoms usually settle after stopping the drug.
• Vascular parkinsonism: usually normal uptake, or patchy defects that match infarcts.
• Functional (psychogenic) parkinsonism: normal scan.
• Corticobasal syndrome: asymmetric reduction.
• Normal scan with parkinsonism: strongly suggests a non-degenerative cause, but a normal scan does not exclude disease in the first few months after onset.
• Reports state the pattern (symmetric or asymmetric), the regions affected and the binding ratios compared with age matched limits.

Critical values and findings needing immediate action.

• The scan itself rarely requires emergency action.
• Structural lesions found incidentally (large tumour, haemorrhage, hydrocephalus): urgent neurology or neurosurgery review.
• Severe allergic reaction to the tracer (rare): resuscitation.
• A normal scan in a patient with progressive disability: further evaluation for other treatable causes.
• Suspected drug induced parkinsonism with distress: review and change of the causative drug.

Limitations and artefacts.

• The scan cannot separate Parkinson disease from other degenerative parkinsonian syndromes. Clinical features and MRI are needed.
• A normal scan very early in disease can be falsely reassuring, and repeat imaging or follow up is used if the clinical suspicion persists.
• Drugs that bind the transporter reduce uptake, and so do stroke, tumour or surgery in the striatum.
• Age related decline and normal variation overlap with early disease.
• Head movement, poor head positioning, and image reconstruction errors produce false asymmetry or low binding.
• Partial volume effects in small striata and atrophy reduce measured binding.
• Semi-quantitative values depend on the camera and software, and local normal ranges are needed.
• Scan results should be interpreted with clinical history, examination and MRI.
• Thyroid blockade failure causes thyroid uptake and a higher radiation dose to the thyroid.
• The scan does not show disease severity reliably, and it is not a progression marker used in routine care.

Special groups.

• Older patients: lower binding is normal for age, and age adjusted ranges are used.
• Patients with tremor or dyskinesia: sedation or repeat imaging.
• Patients with dementia: scan under supervision, with help to lie still.
• Patients on antidepressants or stimulants: review the drug list, and stop drugs when safe.
• Pregnant patients: contraindicated.
• Regions: Parkinson disease and dementia are under recognised, and the scan is available in very few centres. Clinical diagnosis, a response to levodopa, MRI of the brain and drug history are used.

Follow up tests.

• MRI of the brain for structural, vascular and atrophy findings.
• Levodopa challenge and review of response.
• Review and withdrawal of drugs that cause parkinsonism.
• Fluorodeoxyglucose PET for atypical syndromes and dementia patterns.
• Cerebrospinal fluid markers or amyloid PET if Alzheimer disease is the question.
• Neuropsychology tests and sleep assessment for dementia with Lewy bodies.
• Thyroid function if the blocking was incomplete.
• Referral to neurology, geriatrics or a movement disorder clinic.