Synopsis
Mild cognitive impairment
(MCI) is considered a transitional state between healthy controls (HC) and
Alzheimer’s disease (AD). This study compares
the predictive value of neuropsychological
evaluation, structural magnetic resonance imaging, diffusion tensor imaging and
resting state functional MRI indices able to identify MCI conversion. AD versus HC and converted
MCI (cMCI) versus non-converted MCI (ncMCI) presented different features of
differentiation. This result suggests adopting
advanced MRI techniques to
investigate early alterations. Due to the clinical heterogeneity of MCI patients,
considering cMCI as AD-like and ncMCI as HC might be inappropriate when
attempting to distinguishing between cMCI and non-converted MCI.Purpose
Mild Cognitive
Impairment (MCI) clinically is considered as a transitional state between healthy
controls (HC) and Alzheimer Disease (AD)
1. However, the annual conversion
rate of MCI to AD is approximately 12%
2. Therefore, the recognition
of MCI who will develop (converters MCI, cMCI) or not (non-converters MCI, ncMCI)
to AD and identification of optimal markers of
disease progression is of undeniable importance.
To the best of our
knowledge no one has compared, on a single dataset, the predictive value of
neuropsychological evaluation (NPS), structural magnetic resonance imaging (sMRI),
diffusion tensor imaging (DTI) and resting state functional MRI (rsfMRI). Therefore the
aim of this study is to define
the predictive value of each metric in identifying MCI that converted to AD within
2 years.
Methods
98
subjects: 36 AD (19 f, 71.8±5.8 years, 7.0±2.6 years of
education, MMSE 21.7±3.1), 31 MCI
(19 f, 72.5±5.7 years, 7.9±4.2 years of education, MMSE
24.8±2.7), and 31 HC (21 f, 68.2±6.8 years, 7.9±3.9
years of education, MMSE 28.5±1.3) underwent NPS and MRI on a Philips
Intera 1.5T scanner with an 8-channel head coil. Volumetric T1-weighted images (3DT1 FFE, TR/TE = 8.6/4 ms, 170 sagittal slices, slice thickness = 1.2 mm, in-plane resolution = 1.25×1.25 mm
2), DTI (SE-EPI, TR/TE = 11800/70 ms, 2.5 mm isotropic voxel, 15
non-collinear directions, b-values = 900 s/mm
2) and rsfMRI (FEEPI, TR/TE = 3000/60
ms, voxel size = 2.2x2.2x4 mm
3, 100 volumes) sequences were
acquired. At follow-up (12±8 months) 12 MCI converted to AD. Grey
and white matter brain areas showing significant differences among groups were
determined using Voxel Based Morphometry
(sMRI) – VBM
3, Tract-based spatial statistics (DTI) – TBSS
4 and dual regression (rsfMRI)
5
analysis. Metrics for grey matter volumes, diffusivity and anisotropy, and
functional connectivity (FC) values from the highlighted areas were used as input features for automatic classification implemented
in Orange
6. Using a leave
one out cross validation approach and testing different classification algorithms (Logistic Regression, K Nearest Neighbours, Support Vector Machine, and Classification
Tree) we assessed the
predictive power of each index in differentiating the pathological groups by
testing the classifier accuracy (ACC) on NPS, sMRI, DTI and rsfMRI
separately. The relationship among the obtained features and NPS was then verified
using linear regression analysis with SPSS
21.0
7.
Results
Our
findings confirmed previous literature on AD, highlighting that the best
classification performance results from NPS and sMRI data
[Fig.1]. With regard
to the cMCI group, the best classification performance was obtained when
considering rsfMRI and DTI metrics
[Fig.2]. In classifying AD versus HC the
combined use of 6 features from NPS and sMRI
[Tab.1] reached ACC of 97%,
sensitivity (SEN) of 94%, specificity (SPE) of 100% and area under curve (AUC)
of 98%. Furthermore, linear regression analysis revealed statistically
significant relationships between right
hippocampal volume and several NPS: memory
prose (p<0.001, R
2=0.42), Rey-Osterrieth complex figure
delayed recall (p<0.001, R
2=0.39), TMT-A (p<0.001, R
2=0.19), TMT-B (p<0.001, R
2=0.32), and phonemic verbal fluency (p<0.006, R
2=0.12). On the other hand, when classifying cMCI
versus ncMCI 9 features
[Tab.2] from all investigated techniques, were needed to
reach ACC of 77% (SE = 66%, SPE = 84% and AUC = 74%). Finally, in MCI linear
regression analysis showed statistically significant relationship between Corsi block task and two volumetric
measures: right superior parietal lobe
volume (p=0.025, R
2=0.18), and right medial temporal lobe
volume (p=0.031, R
2=0.17).
Discussion and
conclusion
Our
study is in line with previous work emphasizing the importance of NPS and
conventional sMRI data in confirming AD. The
final AD classification model resulted highly accurate, sensitive and specific
indeed. The observed relationship between atrophy and cognitive deficits in
both AD and cMCI stresses the specificity of verbal memory and attention system
deficits in AD, as well as an impaired visuospatial memory system in cMCI. Since AD versus HC and cMCI
versus ncMCI presented different features of differentiation,
our results suggests the need for adopting advanced techniques (fMRI and DTI) to properly investigate early
cerebral alterations that wouldn’t be detected using conventional approaches
only. We argue that, considering cMCI as AD-like patients and ncMCI as HC-like
subjects might be inappropriate when attempting to distinguish between cMCI and
ncMCI. Further
investigations are needed to assess the heterogeneity of this diagnostic entity;
bigger and more homogeneous cohort of patients could guarantee the improvement
of the classifier’s sensitivity in detecting the signs of MCI conversion.
Acknowledgements
Brain Connectivity Center (BCC) - C. Mondino National
Neurological Institute, University of Pavia and Italian Ministry of Health (GR-2009-1575236) for
fundings.References
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