PET Imaging of Alzheimer’s and other Dementias
Early Alzheimers Disease (AD) is often difficult to diagnose clinically. PET imaging using (F-18) FDG has been shown to be accurate in the early detection of AD. The Centers for Medicare and Medicaid Services have approved FDG-PET imaging as a routine exam for the early and differential diagnosis of AD, specifically to differentiate from Frontotemporal Dementia (FTD). This is based on evidence showing that adding PET to a clinical exam increases diagnostic sensitivity in AD.
How does FDG-PET/CT work?
One of the earliest changes in AD is altered glucose metabolism. Some researchers are now referring to AD as Type 3 Diabetes because of relative insulin resistance in the brain of AD patients resulting in less glucose uptake. FDG-PET measures cerebral metabolic rate of glucose uptake. By the time a patient has dementia symptoms, a reduction in glucose uptake has already occurred and can be measured by PET. In fact, FDG-PET is so sensitive to glucose uptake, that patients with increased risk for AD (e.g. family history or genetic susceptibility) will often show reduction in glucose uptake within specific areas of the brain before the onset of clinical symptoms.
A Quick Checklist for CMS Compliance
Patient has Mild Cognitive Impairment (MCI) or early dementia.
Cognitive decline has been documented over a 6 month period.
Other causes of dementia have been excluded such as multi-infarct dementia, metabolic disorders, and depression.
It is unclear whether the patient’s symptoms are due to AD or FTD
Billing Codes Applicable
CPT: 78608–Brain imaging, positron emission tomography (PET); metabolic evaluation.
ICD-9: Diagnosis that meet medical necessity for PET metabolic brain imaging.
290.0 – Senile dementia, uncomplicated
290.10–290.13 – Pre-senile dementia
290.20–290.21 – Senile dementia with delusional or depressive features
290.3 – Senile dementia with delirium
331.0 – Alzheimer’s disease
331.11 – Pick’s disease
331.19 – Other fronto-temporal dementia
331.2 – Senile degeneration of brain
331.9 – Cerebral degeneration, unspecified
780.93 –Memory loss BRAIN PET for Alzheimer’s Disease and Fronto-Temporal Dementia
What does AD look like on PET scan?
The FDG-PET pattern of early AD is very specific. It shows a pattern of abnormally low glucose uptake (hypometabolism) in specific areas of the brain, the posterior cingulate, precuneus, and temperoparietal regions. This can be observed on visual inspection. In addition, at Partners Imaging, we create topographic maps of the brain that quantitate the degree of hypometabolism in each region. Also, the degree of glucose uptake in each region is compared to an age-matched control group of cognitively normal individuals to give us a “z-score” or statistical measure relative to normal glucose uptake.
Arrows point to regional cerebral hypometabolism in a patient with Alzheimers disease.