Deciphering Metabolic Patterns of Primary Progressive Aphasia with FDG-PET

Introduction: Primary Progressive Aphasia (PPA) represents a spectrum of neurodegenerative disorders characterized by insidious language impairment and which have significant clinical overlap with Frontotemporal lobar degeneration (FTLD) and Alzheimer's disease (AD). Accurate diagnosis and classification of PPA subtypes are often challenging on clinical grounds alone. This educational exhibit will showcase the utility of Fluorodeoxyglucose (FDG) Positron Emission Tomography (PET) in elucidating distinct patterns of cerebral glucose metabolism, enabling more precise differentiation of PPA subtypes by interpreting nuclear medicine radiologists.

Methods: We examined FDG-PET brain studies performed at our instution for evaluation of dementia with a focus on metabolic patterns consistent with primary progressive aphasia. Images were analyzed using MIM software which generated 3-D SSP z score maps comparing image sets from individual patient to a database of age-similar cognitively normal subjects. Anatomic MRI images were also obtained for comparison.

Results: This exhibit will first summarize the strengths and drawbacks of FDG PET in the evaluation of dementia. We will review the clinical and imaging presentations of the subtypes of Primary Progressive Aphasia—namely, semantic variant (svPPA), nonfluent/agrammatic variant (nfvPPA), and logopenic variant (lvPPA) with a focus on FDG PET imaging. We will present the unique FDG metabolism patterns associated with these subtypes and provide comparison to patterns more typical of behavioral variant FTLD and AD. Specific examples will be provided that demonstrate the capability of metabolic imaging with FDG-PET combined with statistical mapping to clarify diagnoses in patients where anatomic imaging was nonspecific. The role of proteomic-specific imaging markers will be briefly discussed.

Conclusions: Our educational exhibit hopes to showcase the distinct metabolic patterns associated with subtypes of PPA to familiarize nuclear medicine and radiology trainees and facilitate nuanced interpretation of FDG PET beyond FTLD, DLB, and AD. A more accurate imaging-supported diagnosis would help to guide early interventions, inform care planning, and enable initiation of therapies. Early diagnosis with metabolic imaging, which would not be possible with conventional anatomic imaging, may become particularly relevant with the potential of new proteomic-targeting immunotherapies where earlier interventions are likely to be most fruitful.

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