Background
Although deep learning approaches achieve high diagnostic accuracy for detecting Alzheimer’s disease (AD) based on MRI scans, they are rarely applied in clinical research due to a lack of ...suitable methods for model comprehensibility and interpretability. Recent advances in convolutional neural networks (CNN) visualization algorithms may help to overcome these problems.
Method
We implemented a CNN model structure, trained it on 662 T1‐weighted MRI scans obtained from ADNI in a twentyfold cross‐validation procedure, and validated it on 1655 cases from three independent samples. Various CNN visualization algorithms were compared, which generated relevance maps indicating the contribution of individual image areas for detecting AD. We developed an interactive web application to display the 3D relevance maps and interactively change various visualization parameters. We assessed the clinical utility of relevance maps by comparing hippocampus relevance scores with hippocampus volume.
Result
Across the three independent validation datasets (Fig. 1), group separation showed high accuracy for AD dementia versus controls (AUC≥0.92) and moderate accuracy for amnestic mild cognitive impairment (MCI) versus controls (AUC≥0.73). Relevance maps obtained from Layer‐wise Relevance Propagation (LRP) provided a high spatial resolution and strongest focus (Fig. 2 & Fig. 3). LRP maps indicated that hippocampal atrophy was the most informative factor for AD detection (Fig. 4), with additional contributions from atrophy in other cortical and subcortical regions. Relevance scores within the hippocampus were highly correlated with hippocampal volumes (median: r=‐0.81, Fig. 5). The relevance maps of individual patients revealed additional clusters in the frontal and occipital lobe, which may be an indicator of model overfitting or potential bias of the training sample. When comparing the twenty cross‐validation models, stronger focus of the models on irrelevant areas was associated with lower accuracy to detect MCI.
Conclusion
The relevance maps highlighted atrophy in regions that we had hypothesized a priori, which strengthens the overall comprehensibility and validity of the CNN models.
Although convolutional neural networks (CNNs) achieve high diagnostic accuracy for detecting Alzheimer's disease (AD) dementia based on magnetic resonance imaging (MRI) scans, they are not yet ...applied in clinical routine. One important reason for this is a lack of model comprehensibility. Recently developed visualization methods for deriving CNN relevance maps may help to fill this gap as they allow the visualization of key input image features that drive the decision of the model. We investigated whether models with higher accuracy also rely more on discriminative brain regions predefined by prior knowledge.
We trained a CNN for the detection of AD in N = 663 T1-weighted MRI scans of patients with dementia and amnestic mild cognitive impairment (MCI) and verified the accuracy of the models via cross-validation and in three independent samples including in total N = 1655 cases. We evaluated the association of relevance scores and hippocampus volume to validate the clinical utility of this approach. To improve model comprehensibility, we implemented an interactive visualization of 3D CNN relevance maps, thereby allowing intuitive model inspection.
Across the three independent datasets, group separation showed high accuracy for AD dementia versus controls (AUC ≥ 0.91) and moderate accuracy for amnestic MCI versus controls (AUC ≈ 0.74). Relevance maps indicated that hippocampal atrophy was considered the most informative factor for AD detection, with additional contributions from atrophy in other cortical and subcortical regions. Relevance scores within the hippocampus were highly correlated with hippocampal volumes (Pearson's r ≈ -0.86, p < 0.001).
The relevance maps highlighted atrophy in regions that we had hypothesized a priori. This strengthens the comprehensibility of the CNN models, which were trained in a purely data-driven manner based on the scans and diagnosis labels. The high hippocampus relevance scores as well as the high performance achieved in independent samples support the validity of the CNN models in the detection of AD-related MRI abnormalities. The presented data-driven and hypothesis-free CNN modeling approach might provide a useful tool to automatically derive discriminative features for complex diagnostic tasks where clear clinical criteria are still missing, for instance for the differential diagnosis between various types of dementia.
Background: Although convolutional neural networks (CNN) achieve high diagnostic accuracy for detecting Alzheimer's disease (AD) dementia based on magnetic resonance imaging (MRI) scans, they are not ...yet applied in clinical routine. One important reason for this is a lack of model comprehensibility. Recently developed visualization methods for deriving CNN relevance maps may help to fill this gap. We investigated whether models with higher accuracy also rely more on discriminative brain regions predefined by prior knowledge. Methods: We trained a CNN for the detection of AD in N=663 T1-weighted MRI scans of patients with dementia and amnestic mild cognitive impairment (MCI) and verified the accuracy of the models via cross-validation and in three independent samples including N=1655 cases. We evaluated the association of relevance scores and hippocampus volume to validate the clinical utility of this approach. To improve model comprehensibility, we implemented an interactive visualization of 3D CNN relevance maps. Results: Across three independent datasets, group separation showed high accuracy for AD dementia vs. controls (AUC\(\geq\)0.92) and moderate accuracy for MCI vs. controls (AUC\(\approx\)0.75). Relevance maps indicated that hippocampal atrophy was considered as the most informative factor for AD detection, with additional contributions from atrophy in other cortical and subcortical regions. Relevance scores within the hippocampus were highly correlated with hippocampal volumes (Pearson's r\(\approx\)-0.86, p<0.001). Conclusion: The relevance maps highlighted atrophy in regions that we had hypothesized a priori. This strengthens the comprehensibility of the CNN models, which were trained in a purely data-driven manner based on the scans and diagnosis labels.
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