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Computer-Assisted Segmentation of White Matter Lesions in 3D MR Images Us ing Support Vector Machine

Rationale and Objectives

Brain lesions, especially white matter lesions (WMLs), are associated with cardiac and vascular disease, but also with normal aging. Quantitative analysis of WML in large clinical trials is becoming more and more important.

Materials and Methods

In this article, we present a computer-assisted WML segmentation method, based on local features extracted from multiparametric magnetic resonance imaging (MRI) sequences (ie, T1-weighted, T2-weighted, proton density-weighted, and fluid attenuation inversion recovery MRI scans). A support vector machine classifier is first trained on expert-defined WMLs, and is then used to classify new scans.

Results

Postprocessing analysis further reduces false positives by using anatomic knowledge and measures of distance from the training set.

Conclusions

Cross-validation on a population of 35 patients from three different imaging sites with WMLs of varying sizes, shapes, and locations tests the robustness and accuracy of the proposed segmentation method, compared with the manual segmentation results from two experienced neuroradiologists.

Cerebrovascular disease (CVD) in elderly individuals is very important. In particular, CVD increases the likelihood of clinical dementia ( ) even in the absence of clinical stroke ( ), albeit the literature is somewhat inconclusive as to whether CVD has simply an additive role to Alzheimer’s disease (AD) or there are interactions between the two. Approximately one third of patients that meet clinical and pathologic diagnostic criteria for AD have some degree of vascular pathology ( ). The impact of CVD on mild cognitive impairment—in which the etiology of the cognitive deficit is generally less clear—is likely to be even greater. Therefore, to identify biologic markers specific to the AD process, it is critical to also identify the extent of concurrent CVD related brain injury that is often clinically silent ( ), because, at the very least, CVD increases the likelihood of clinical presentation of dementia, for the same level of AD-related pathology.

Population studies, such as the Cardiovascular Health Study or the Rotterdam Scan Study, have shown that brain lesions, especially white matter lesions (WMLs), are associated with age, clinically silent stroke, higher systolic blood pressure, lower forced expiratory volume in 1 second, hypertension, atrial fibrillation, carotid and peripheral arterioscleroses, impaired cognition, and depression ( ). Furthermore, it has been shown that stroke patients with a large WML load have an increased risk of hemorrhagic transformation, higher preoperative risk of a disabling or fatal stroke during endarterectomy, or intercerebral hemorrhage during anticoagulation therapy ( ). The increased interest in brain lesion research may improve diagnosis and prognosis possibilities for patients with cardiovascular symptoms.

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Figure 1

Summary of our computer-assisted white matter lesion segmentation protocol.

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Methods

Patients and MR Imaging

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Preprocessing

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Training

Manual segmentation

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Attribute vector

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Figure 2, Intensity overlaps between white matter lesions tissue and normal tissue in T1, T2, proton density-weighted, and fluid attenuation inversion recovery scans, respectively (histograms for normal tissue have been scaled by 0.1 for visualization purpose).

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Figure 3, Image intensities from all modalities and all voxels in the spatial neighborhood of a voxel form an attribute vector that serves as an “imaging signature” of each voxel.

Figure 4, Discrimination ability of attribute vectors (AV). Left: Fluid attenuation inversion recovery scans image with selected lesion voxel marked as white cross. Right: Distance distribution in Hilbert space from all other voxels to this selected voxel. AVs of other lesion voxels are similar (having small distance in the attribute space) to the selected voxel, indicating that this imaging signature is characteristic of lesions.

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SVMs

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Figure 5, An example of two-class (+ and −) problem showing optimal separating hyperplane (dotted line) that support vector machines use to divide two groups’ data, and the associated support vectors. Data shown by + and − represent binary class +1 and −1, respectively.

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K(x,y)=exp(∥x−y∥22α2) K

(

x

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exp

(

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where x and y are two feature vectors, and α controls the size of the Gaussian kernel.

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Training SVM via AdaBoost

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Segmenting a New Image

Voxel-wise segmentation of WML by SVM

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Figure 6, Illustration of voxel-wise segmentation by support vector machines (SVM). Left: The result of voxel-wise evaluation map showing different lesion rating for each voxel, based on generated SVM model (1: lesion; −1: normal). Right: White matter lesion segmentation results after thresholding the map on the left superimposed on fluid attenuation inversion recovery image. Threshold actually corresponds to SVM classification boundary as illustrated in Fig 7 , with two classes labeled as −1 and 1 respectively, 0.0 is selected as a threshold.

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Elimination of false-positive labels

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D2h(v1,v2)=K(v1,v1)+K(v2,v2)−2K(v1,v2), D

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where K is the Gaussian kernel function used by the SVM.

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Figure 7, Illustration of L , N , and F distribution in Hilbert space. Green and red represent attribute vectors (AVs) of healthy and lesion tissue, respectively, whereas blue represents AVs of voxels that are misclassified mostly because minor registration errors between the four different acquisitions (T1, T2, PD, and fluid attenuation inversion recovery scans) causes them to have imaging profiles that are drastically different from the training set, and hence prone to misclassification.

Figure 8, Demonstration of false positive elimination via attribute vector distance in Hilbert space. (a) Distance distribution of {dvℓi} {dviℓ} (blue, true positives), {dLvfi} {dvifL} (red, false positives), and the overlap between {dvℓi} {dviℓ} and {dLvfi} {dvifL} (violet). (b) Distance distribution of {dvni} {dvin} (blue, true negatives), {dNvfi} {dvifN} (red, false positives), and the overlap between {dvni} {dvin} and {dNvfi} {dvifN} (violet). White matter lesion segmentation results (c) before false-positive elimination and (d) after false-positive elimination via thresholding the distance map.

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Figure 9, Demonstration of orbital false positive elimination. Left: Orbital false positives (red) overlaid on fluid attenuation inversion recovery scans before false-positive elimination. Right: After orbital false-positive elimination.

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Results

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Figure 10, Comparison of white matter lesion segmentation results between gold standard and computer-assisted segmentation for two subjects. In subject 1, gold standard and computer-assisted lesion measurements are 11,714.9 mm 3 and 12,397.9 mm 3 , respectively; in subject 2, gold standard and computer-assisted lesion measures are 15,978.5 mm 3 and 17,884.9 mm 3 , respectively.

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ROC Analysis

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Figure 11, A zoomed part of receiver operating characteristic curve of our segmentation algorithm. *: Indicates the result of the second rater compared with gold standard (first rater). Other symbols on the curve denote different thresholds (ie, Δ threshold = −0.15, + threshold = 0.0, ○ threshold = 0.05, □ threshold = 0.2 (see Fig 6 for the definition of threshold).

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Statistical Analysis

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Figure 12, 95% CI (confidence intervals) for gold standard (first rater), second rater, and computer-assisted segmentation method (computer) over 35 subjects, respectively. Volume measurements are in mm 3 .

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Discussion

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Acknowledgments

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