Validity of cingulate–precuneus–temporo-parietal hypometabolism for single-subject diagnosis of biomarker-proven atypical variants of Alzheimer’s Disease

A ROI-based quantitative pipeline for 18F-FDG PET metabolism and pCASL perfusion joint analysis: Validation of the 18F-FDG PET line

In Mild Cognitive Impairment (MCI), the study of brain metabolism, provided by 18F-FluoroDeoxyGlucose Positron Emission Tomography (18F-FDG PET) can be integrated with brain perfusion through pseudo-Continuous Arterial Spin Labeling Magnetic Resonance sequences (MR pCASL). Cortical hypometabolism identification generally relies on wide control group datasets; pCASL control groups are instead not publicly available yet, due to lack of standardization in the acquisition parameters. This study presents a quantitative pipeline to be applied to PET and pCASL data to coherently analyze metabolism and perfusion inside 16 matching cortical regions of interest (ROIs) derived from the AAL3 atlas. The PET line is tuned on 36 MCI patients and 107 healthy control subjects, to agree in identifying hypometabolic regions with clinical reference methods (visual analysis supported by a vendor tool and Statistical Parametric Mapping, SPM, with two parametrizations here identified as SPM-A and SPM-B). The analysis was conducted for each ROI separately. The proposed PET analysis pipeline obtained accuracy 78 % and Cohen's к 60 % vs visual analysis, accuracy 79 % and Cohen's к 58 % vs SPM-A, accuracy 77 % and Cohen's к 54 % vs SPM-B. Cohen's к resulted not significantly different from SPM-A and SPM-B Cohen's к when assuming visual analysis as reference method (p-value 0.61 and 0.31 respectively). Considering SPM-A as reference method, Cohen's к is not significantly different from SPM-B Cohen's к as well (p-value = 1.00). The complete PET-pCASL pipeline was then preliminarily applied on 5 MCI patients and metabolism-perfusion regional correlations were assessed. The proposed approach can be considered as a promising tool for PET-pCASL joint analyses in MCI, even in the absence of a pCASL control group, to perform metabolism-perfusion regional correlation studies, and to assess and compare perfusion in hypometabolic or normo-metabolic areas.

Atypical forms of Alzheimer's disease: patients not to forget

PURPOSE OF REVIEW

The aim of this paper is to summarize the latest work on neuroimaging in atypical Alzheimer's disease (AD) patients and to emphasize innovative aspects in the clinic and research. The paper will mostly cover language (logopenic variant of primary progressive aphasia; lvPPA), visual (posterior cortical atrophy; PCA), behavioral (bvAD) and dysexecutive (dAD) variants of AD.

RECENT FINDINGS

MRI and PET can detect and differentiate typical and atypical AD variants, and novel imaging markers like brain iron deposition, white matter hyperintensities (WMH), cortical mean diffusivity, and brain total creatine can also contribute. Together, these approaches have helped to characterize variant-specific distinct imaging profiles. Even within each variant, various subtypes that capture the heterogeneity of cases have been revealed. Finally, in-vivo pathology markers have led to significant advances in the atypical AD neuroimaging field.

SUMMARY

Overall, the recent neuroimaging literature on atypical AD variants contribute to increase knowledge of these lesser-known AD variants and are key to generate atypical variant-specific clinical trial endpoints, which are required for inclusion of these patients in clinical trials assessing treatments. In return, studying these patients can inform the neurobiology of various cognitive functions, such as language, executive, memory, and visuospatial abilities.