Classification of small lesions in dynamic breast MRI: Eliminating the need for precise lesion segmentation through spatio-temporal analysis of contrast enhancement
Department of Imaging Sciences and Biomedical Engineering, University of Rochester, 207 Robert B. Goergen Hall, Rochester NY 14627, USA. Machine Vision and Applications
(Impact Factor: 1.35).
10/2013; 24(7). DOI: 10.1007/s00138-012-0456-y
Characterizing the dignity of breast lesions as benign or malignant is specifically difficult for small lesions; they don't exhibit typical characteristics of malignancy and are harder to segment since margins are harder to visualize. Previous attempts at using dynamic or morphologic criteria to classify small lesions (mean lesion diameter of about 1 cm) have not yielded satisfactory results. The goal of this work was to improve the classification performance in such small diagnostically challenging lesions while concurrently eliminating the need for precise lesion segmentation. To this end, we introduce a method for topological characterization of lesion enhancement patterns over time. Three Minkowski Functionals were extracted from all five post-contrast images of sixty annotated lesions on dynamic breast MRI exams. For each Minkowski Functional, topological features extracted from each post-contrast image of the lesions were combined into a high-dimensional texture feature vector. These feature vectors were classified in a machine learning task with support vector regression. For comparison, conventional Haralick texture features derived from gray-level co-occurrence matrices (GLCM) were also used. A new method for extracting thresholded GLCM features was also introduced and investigated here. The best classification performance was observed with Minkowski Functionals area and perimeter, thresholded GLCM features f8 and f9, and conventional GLCM features f4 and f6. However, both Minkowski Functionals and thresholded GLCM achieved such results without lesion segmentation while the performance of GLCM features significantly deteriorated when lesions were not segmented (p < 0.05). This suggests that such advanced spatio-temporal characterization can improve the classification performance achieved in such small lesions, while simultaneously eliminating the need for precise segmentation.
Available from: Xixi Wang
- "For this purpose, texture features derived from Minkowski Functionals were used to characterize both lesion and non-lesion areas by capturing local topological properties. Such Minkowski Functionals have been previously used for pattern recognition problems in medical imaging, such as classifying between healthy and pathological lung tissue on CT , estimating the bone strength through analysis of trabecular micro-architecture , classifying benign and malignant lesions on dynamic breast MRI , characterizing healthy and osteoarthritic patellar cartilage on phase contrast x-ray computed tomography , etc. "
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ABSTRACT: Cone beam computed tomography (CBCT) has found use in mammography for imaging the entire breast with sufficient spatial resolution at a radiation dose within the range of that of conventional mammography. Recently, enhancement of lesion tissue through the use of contrast agents has been proposed for cone beam CT. This study investigates whether the use of such contrast agents improves the ability of texture features to differentiate lesion texture from healthy tissue on CBCT in an automated manner. For this purpose, 9 lesions were annotated by an experienced radiologist on both regular and contrast-enhanced CBCT images using two-dimensional (2D) square ROIs. These lesions were then segmented, and each pixel within the lesion ROI was assigned a label - lesion or non-lesion, based on the segmentation mask. On both sets of CBCT images, four three-dimensional (3D) Minkowski Functionals were used to characterize the local topology at each pixel. The resulting feature vectors were then used in a machine learning task involving support vector regression with a linear kernel (SVRlin) to classify each pixel as belonging to the lesion or non-lesion region of the ROI. Classification performance was assessed using the area under the receiver-operating characteristic (ROC) curve (AUC). Minkowski Functionals derived from contrastenhanced CBCT images were found to exhibit significantly better performance at distinguishing between lesion and non-lesion areas within the ROI when compared to those extracted from CBCT images without contrast enhancement (p < 0.05). Thus, contrast enhancement in CBCT can improve the ability of texture features to distinguish lesions from surrounding healthy tissue.
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While dimension reduction has been previously explored in computer aided diagnosis (CADx) as an alternative to feature selection, previous implementations of its integration into CADx do not ensure strict separation between training and test data required for the machine learning task. This compromises the integrity of the independent test set, which serves as the basis for evaluating classifier performance.
Methods and materials
We propose, implement and evaluate an improved CADx methodology where strict separation is maintained. This is achieved by subjecting the training data alone to dimension reduction; the test data is subsequently processed with out-of-sample extension methods. Our approach is demonstrated in the research context of classifying small diagnostically challenging lesions annotated on dynamic breast magnetic resonance imaging (MRI) studies. The lesions were dynamically characterized through topological feature vectors derived from Minkowski functionals. These feature vectors were then subject to dimension reduction with different linear and non-linear algorithms applied in conjunction with out-of-sample extension techniques. This was followed by classification through supervised learning with support vector regression. Area under the receiver-operating characteristic curve (AUC) was evaluated as the metric of classifier performance.
Of the feature vectors investigated, the best performance was observed with Minkowski functional ’perimeter’ while comparable performance was observed with ’area’. Of the dimension reduction algorithms tested with ’perimeter’, the best performance was observed with Sammon's mapping (0.84 ± 0.10) while comparable performance was achieved with exploratory observation machine (0.82 ± 0.09) and principal component analysis (0.80 ± 0.10).
The results reported in this study with the proposed CADx methodology present a significant improvement over previous results reported with such small lesions on dynamic breast MRI. In particular, non-linear algorithms for dimension reduction exhibited better classification performance than linear approaches, when integrated into our CADx methodology. We also note that while dimension reduction techniques may not necessarily provide an improvement in classification performance over feature selection, they do allow for a higher degree of feature compaction.
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ABSTRACT: The ability of Minkowski Functionals to characterize local structure in
different biological tissue types has been demonstrated in a variety of
medical image processing tasks. We introduce anisotropic Minkowski
Functionals (AMFs) as a novel variant that captures the inherent
anisotropy of the underlying gray-level structures. To quantify the
anisotropy characterized by our approach, we further introduce a method
to compute a quantitative measure motivated by a technique utilized in
MR diffusion tensor imaging, namely fractional anisotropy. We showcase
the applicability of our method in the research context of
characterizing the local structure properties of trabecular bone
micro-architecture in the proximal femur as visualized on multi-detector
CT. To this end, AMFs were computed locally for each pixel of ROIs
extracted from the head, neck and trochanter regions. Fractional
anisotropy was then used to quantify the local anisotropy of the
trabecular structures found in these ROIs and to compare its
distribution in different anatomical regions. Our results suggest a
significantly greater concentration of anisotropic trabecular structures
in the head and neck regions when compared to the trochanter region (p
< 10-4). We also evaluated the ability of such AMFs to predict bone
strength in the femoral head of proximal femur specimens obtained from
50 donors. Our results suggest that such AMFs, when used in conjunction
with multi-regression models, can outperform more conventional features
such as BMD in predicting failure load. We conclude that such
anisotropic Minkowski Functionals can capture valuable information
regarding directional attributes of local structure, which may be useful
in a wide scope of biomedical imaging applications.
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