Functional Brain Imaging in Dementia - The Transition from SPECT to PET

Functional Brain Imaging in Dementia - The Transition from SPECT to PET

Alan D Waxman
Published: US Neurology Review 2005
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Neurodegenerative disorders are increasing rapidly in the US as the population ages. It is estimated that six to eight million people in the US have some form of neurodementia. The most frequent etiology for dementia is Alzheimer’s disease (AD), accounting for nearly 70% of all dementia cases.1–4 Until recently, single positron emission computed tomography (SPECT) imaging of the brain using perfusion agents of Tc-99m hexamethylpropyleneamineoxime (HMPAO) or Tc- 99m ethyl cysteinate dimer (ECD) was the main functional neuroimaging test employed to make the diagnosis of either AD or frontal temporal dementia. As fluorodeoxyglucose positron emitted tomography (FDG-PET) distribution improved and more highresolution PET detectors were placed on the market, the use of FDG-PET to evaluate neurodementia began to grow. Recent approval for reimbursement for FDGPET neuroimaging in dementia subjects by government agencies and insurance companies have created a rising interest in the use of nuclear neuroimaging to evaluate patients with suspected organic dementias in order to improve diagnostic accuracy.

Justification of Functional Brain Imaging in the Assessment of Dementia
Determining the etiology of dementia has proven to be extremely difficult using clinical assessment and standard cross-sectional imaging studies of the brain. Functional brain imaging in the form of SPECT and FDG-PET have improved our diagnostic abilities, not only to determine whether abnormality exists but to subcategorize the abnormalities into different subsets of dementia.

Table 1: A Comparison of Functional Brain Imaging Data Applied to the Separation of Dementia from Normal Controls5

AD = Alzheimer’s disease; MCI = mild cognitive impairment; FDG-PET = fluorodeoxyglucose positron emitted tomography; HMPAO SPET = hexamethylpropyleneamineoxime single-photon emission tomography; MMSE = mini mental state examination.


Meta-analysis of the literature regarding functional brain imaging in the dementias concluded that functional brain imaging provided critical information in the initial diagnosis of dementia as well as aiding in the differential diagnosis of the specific dementia disorders.5,6 Devous compared functional brain imaging with clinical assessment and magnetic resonance imaging (MRI). This information is summarized in Table 1. In a review by Silverman, the sensitivity for detecting AD using FDG-PET was 91.5% ± 3.5%.7

Table 2 summarizes these results and compares the results of FDG-PET with clinical evaluation based upon the report of the Quality Standard Subcommittee of the American Academy of Neurology (AAN). Additional reports appeared to document the superiority of functional brain imaging over clinical assessment and cross-sectional imaging.

Functional brain imaging studies, particularly FDGPET, appeared to be extremely important in the early phases of the dementias, particularly AD in which mild cognitive impairment (MCI) may be an early manifestation. The separation of an early Alzheimer’s process from other causes of dementia, especially vascular and psychiatric related, are extremely important in that clinical management decisions and prognosis depend upon the correct diagnosis.8,9

PET and SPECT Comparisons
Due to the wide availability of SPECT detectors, SPECT imaging of the brain was most often used for evaluating patients with neurodementia.10,11 As more information became available with regards to the utility of FDG-PET in evaluating patients with suspected neurodementia, it became apparent that the ability to separate normal controls from patients with true dementia was superior for PET when directly compared with SPECT.12,13 The overall literature now appears to indicate FDG-PET as the most accurate assessment in the evaluation of patients with suspected AD. Silverman et al. determined a sensitivity of 93% with specificity of 58% based upon the standard of clinical diagnosis in reviewing the PET literature for diagnosis of AD.6

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