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Carotid Artery Localization and Lesion Classification on 3D-MERGE MRI using Neural Network and Object Tracking methods1University of Washington, Seattle, WA, United States, 2Radiology, Renji Hospital, School of Medicine, Shanghai Jiao Tong University, Shanghai, China, 3Department of Biomedical Engineering, Tsinghua University School of Medicine, Beijing, China
Synopsis
Carotid vessel wall imaging (VWI) with MRI provides additional prognostic value for cerebro/cardiovascular ischemic events, beyond current clinical diagnostic imaging methods. While fast 3D carotid MRI is possible, manual review of the large 3D dataset is time consuming. Automatic identification of artery locations and lesion categories are therefore required for VWI screening protocols. With neural network and object tracking methods, we developed a fully automated analysis tool to find common/internal/external carotid arteries and flag possible high-risk lesion locations. The tool achieved 0.782 Intersection over Union (IoU) for artery localization, and 0.895 sensitivity for high-risk lesion classification.
INTRODUCTION
Atherosclerosis is a major cause of cardio- and cerebrovascular disease mortality and morbidity globally 1–3. Atherosclerotic lesion progression leads to arterial stenosis and/or occlusion, which may cause downstream ischemic events due to atheroembolization or hypoperfusion 4. Several fast 3D carotid black blood MRI sequences are now available 5,6. 3D Motion Sensitized Driven Equilibrium prepared Rapid Gradient Echo (3D-MERGE) 5 black blood MRI can image the carotid vessel wall with high isotropic resolution in a short 2-minute scan thus ensuring patient compliance and diagnostic image quality (becoming available in clinical MRI scanners). However, screening for high-risk lesion is limited by manual image review time. Therefore, fully automated analysis tools on 3D-MERGE are needed.
An automated method to flag locations of atherosclerotic lesions and classify lesion types can help reduce radiologist workload 7–9 and improve screening for atherosclerotic lesions. Identification using deep learning and object tracking may provide a time-efficient solution.
Therefore, the goal of this study is to develop a fully automated analysis tool to identify the locations of the carotid vessel wall from 3D-MERGE images and then flag slices with characteristics of high-risk atherosclerotic lesions.
METHODS
The workflow for this model is shown in Figure 1.
Study sample
3D-MERGE MRI of 269 subjects were collected as part of the Carotid Atherosclerosis Risk Assessment (CARE-II) study 10 with IRB approval and informed consent.
MR imaging
Scans was performed on a 3T Philips (Best, The Netherlands) MR scanner with: TR/TE=10.35/4.87, acquired resolution =0.7mm isotropic (interpolated to 0.35mm isotropic) using bilateral carotid phased array coils.
Image pre-processing
3D-MERGE images were resampled in the axial direction with isotropic resolution of 0.4mm. Each image slice was filtered using a vessel wall enhanced model 11 emphasizing on vessel wall edges, as wall thickness is a key characteristic in determining lesion types.
Human labeling
An experienced radiologist labeled the dataset using unfiltered axial slices. For each slice, bounding boxes of each carotid artery (minimum encompassing rectangle around each vessel wall region) were drawn and classified into three categories (normal artery, early lesion and advanced lesion), based on modified AHA classification for MRI 12 defined in Table 1. An online learning method 13 was used to minimize tedious human labeling by iteratively correcting prediction results. Slices with poor image quality or where part of artery was out of the field of view were excluded (87 subjects). Ultimately, 151, 17, 14 subjects were used as training, validation and testing sets. An average of 95.9±41.1 slices were labeled per subject.
Artery localization
The Yolo V2 neural network 14 model was trained to identify bounding boxes in each slice. Predicted bounding boxes along the slices were reconstructed into tracklets (trajectory fragments) and refined using a tracking by detection method 15,16, which utilizes the neighboring information of bounding boxes to maintain the robustness and continuity of predicted artery wall location.
Lesion classification
The vessel walls in consecutive five slices were cropped using the location of refined bounding boxes and the box in the center slice was classified using a convolutional neural network (8 layers of convolutional layers followed by 3 layers of fully connected layers and softmax layer).
RESULTS
Location sensitivity is 0.985, precision is 0.998, mean Intersection over Union (IoU) is 0.782.
The percentages of artery categories labeled by the radiologist are 27.4% for normal artery, 27.5% for early lesion, and 45.1% for advanced lesion. The classification accuracy of three categories is 0.746, and the weighted kappa for agreement between the radiologist and the tool is 0.77 (0.76-0.78). For advanced lesions, the accuracy is 0.895, the sensitivity is 0.892, and the precision is 0.877. An example showing prediction on one slice is shown in Figure 2. Confusion matrix is shown in Table 2.
Slices with lesions can be flagged onto artery centerlines acquired from centers of bounding boxes in 3D space. 3D Visualization of carotid artery centerline and lesions is shown in Figure 3.
DISCUSSION
The localization and classification models were shown to accurately identify artery locations and detect lesion types with high performance. Although the training process took days, the prediction was completed within a minute. 3D visualization that flags the area of high-risk lesions will substantially reduce review burden for radiologists in clinical practice.
The most common mistakes for the classification model occur mostly near the common carotid bifurcation where complex geometry of the carotid bifurcation and bulb is a challenge.
CONCLUSION
The proposed fully automated carotid 3D-MERGE MRI image analysis tool is capable of artery localization and lesion classification with a high level of accuracy and agreement compared to findings from expert reviewers. This tool can potentially provide a rapid technique for rapid high-risk lesion screening in large populations.Acknowledgements
Li Chen and Huilin Zhao contributed equally to this work. This research is supported by grants from Natural Science Foundation of China (81271536, 61271132 and 81361120402), National Institute of Health (R01-HL103609) and Philips Healthcare. We gratefully acknowledge CARE-II investigators and support of NVIDIA Corporation for donating the Titan Xp GPU.References
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