One major image quality problem in digital tomosynthesis mammography (DTM) is the poor depth-resolution
caused by the inherent incomplete sampling. This problem is more pronounced if high-attenuation objects, such as
metallic markers and dense calcifications, are present in the breast. Strong ghosting artifacts will be generated in the
depth direction in the reconstructed volume. Incomplete sampling of DTM can also cause visible ghosting artifacts in
the x-ray source motion direction on the off-focus planes of the objects. These artifacts may interfere with radiologists'
visual assessment and computerized analysis of subtle mammographic features. We previously developed an artifact
reduction method by using 3D geometrical information of the objects estimated from the reconstructed slices. In this
study, we examined the effect of imaging system blur in DTM caused by the focal spot and the detector modulation
transformation function (MTF). The focal spot was simulated as a 0.3 mm square array of x-ray point sources. The
detector MTF was simulated using the Burgess model with parameters derived from published data of a GE FFDM
detector. The spatial-variant impulse responses for the DTM imaging system, which are required in our artifact
reduction method, were then computed from the DTM imaging model and a given reconstruction technique. Our results
demonstrated that inclusion of imaging system blur improved the performance of our artifact reduction method in terms
of the visual quality of the corrected objects. The detector MTF had stronger effects than focal spot blur on artifact
reduction under the imaging geometry used. Further work is underway to investigate the effects from other DTM
imaging parameters, such as x-ray scattering, different polyenergetic x-ray spectra, and different configurations of
angular range and angular sampling interval.
Digital Tomosynthesis Mammography (DTM) is an emerging technique that has the potential to
improve breast cancer detection. DTM acquires low-dose mammograms at a number of projection angles
over a limited angular range and reconstructs the 3D breast volume. Due to the limited number of projections
within a limited angular range and the finite size of the detector, DTM reconstruction contains boundary and
truncation artifacts that degrade the image quality of the tomosynthesized slices, especially that of the
boundary and truncated regions. In this work, we developed artifact reduction methods that make use of both
2D and 3D breast boundary information and local intensity-equalization and tissue-compensation techniques.
A breast phantom containing test objects and a selected DTM patient case were used to evaluate the effects
of artifact reduction. The contrast-to-noise ratio (CNR), the normalized profiles of test objects, and a non-uniformity
error index were used as performance measures. A GE prototype DTM system was used to
acquire 21 PVs in 3° increments over a ±30° angular range. The Simultaneous Algebraic Reconstruction
Technique (SART) was used for DTM reconstruction. Our results demonstrated that the proposed methods
can improve the image quality both qualitatively and quantitatively, resulting in increased CNR value,
background uniformity and an overall reconstruction quality comparable to that without truncation. For the
selected DTM patient case, the obscured breast structural information near the truncated regions was
essentially recovered. In addition, restricting SART reconstruction to be performed within the estimated 3D
breast volume increased the computation efficiency.
Digital tomosynthesis mammography (DTM) can provide quasi-3D structural information of the breast by
reconstructing the breast volume from projection views (PV) acquired in a limited angular range. Our purpose is to
design an effective classifier to distinguish breast masses from normal tissues in DTMs. A data set of 100 DTM cases
collected with a GE first generation prototype DTM system at the Massachusetts General Hospital was used. We
reconstructed the DTMs using a simultaneous algebraic reconstruction technique (SART). Mass candidates were
identified by 3D gradient field analysis. Three approaches to distinguish breast masses from normal tissues were
evaluated. In the 3D approach, we extracted morphological and run-length statistics texture features from DTM slices as
input to a linear discriminant analysis (LDA) classifier. In the 2D approach, the raw input PVs were first preprocessed
with a Laplacian pyramid multi-resolution enhancement scheme. A mass candidate was then forward-projected to the
preprocessed PVs in order to determine the corresponding regions of interest (ROIs). Spatial gray-level dependence
(SGLD) texture features were extracted from each ROI and averaged over 11 PVs. An LDA classifier was designed to
distinguish the masses from normal tissues. In the combined approach, the LDA scores from the 3D and 2D approaches
were averaged to generate a mass likelihood score for each candidate. The Az values were 0.87±0.02, 0.86±0.02, and
0.91±0.02 for the 3D, 2D, and combined approaches, respectively. The difference between the Az values of the 3D and
2D approaches did not achieve statistical significance. The performance of the combined approach was significantly
(p<0.05) better than either the 3D or 2D approach alone. The combined classifier will be useful for false-positive
reduction in computerized mass detection in DTM.
Posterior acoustic enhancement and shadowing on ultrasound (US) images are important features used by radiologists
for characterization of breast masses. We are developing new feature extraction and classification methods for
computerized characterization of posterior acoustic patterns of breast masses into shadowing, no pattern, or enhancement
categories. The sonographic mass was segmented using an automated active contour segmentation method. Three
adjacent rectangular regions of interest (ROIs) of identical sizes were automatically defined at the same depth
immediately behind the mass. Three features related to enhancement, shadowing, and no posterior pattern were designed
by comparing the image intensities within these ROIs. Artificial neural network (ANN) classifiers were trained using a
leave-one-case-out resampling method. Two radiologists provided posterior acoustic descriptors for each mass. Posterior
acoustic patterns of masses for which both radiologists were in agreement were used as the ground truth, and the
agreement of the ANN scores with the radiologists' assessment was used as the performance measure. On a data set of
339 US images containing masses, the overall agreement between the computer and the radiologists was between 86%
and 87% depending on the ANN architecture. The output score of the designed ANN classifiers may be useful in
computer-aided breast mass characterization and content-based image retrieval systems.
KEYWORDS: Breast, Mammography, Breast cancer, Cancer, Image segmentation, Received signal strength, Feature extraction, Statistical analysis, Computer aided diagnosis and therapy, Computing systems
In this study, we compared the texture features of mammographic parenchymal patterns (MPPs) of normal subjects and
breast cancer patients and evaluated whether a texture classifier can differentiate their MPPs. The breast image was first
segmented from the surrounding image background by boundary detection. Regions of interest (ROIs) were extracted
from the segmented breast area in the retroareolar region on the cranio-caudal (CC) view mammograms. A mass set
(MS) of ROIs was extracted from the mammograms with cancer, but ROIs overlapping with the mass were excluded. A
contralateral set (CS) of ROIs was extracted from the contralateral mammograms. A normal set (NS) of ROIs was
extracted from one CC view mammogram of the normal subjects. Each data set was randomly separated into two
independent subsets for 2-fold cross-validation training and testing. Texture features from run-length statistics (RLS) and
newly developed region-size statistics (RSS) were extracted to characterize the MPP of the breast. Linear discriminant
analysis (LDA) was performed to compare the MPP difference in each of the three pairs: MS-vs-NS, CS-vs-NS, and MS-vs-CS. The Az values for the three pairs were 0.79, 0.73, and 0.56, respectively. These results indicate that the MPPs of
the contralateral breast of breast cancer patients exhibit textures comparable to that of the affected breast and that the
MPPs of cancer patients are different from those of normal subjects.
We are developing computer-aided diagnosis (CADx) methods for classification of masses on digital breast
tomosynthesis mammograms (DBTs). A DBT data set containing 107 masses (56 malignant and 51 benign) collected at
the Massachusetts General Hospital was used. The DBTs were obtained with a GE prototype system which acquired 11
projection views (PVs) over a 50-degree arc. We reconstructed the DBTs at 1-mm slice interval using a simultaneous
algebraic reconstruction technique. The regions of interest (ROIs) containing the masses in the DBT volume and the
corresponding ROIs on the PVs were identified. The mass on each slice or each PV was segmented by an active contour
model. Spiculation measures, texture features, and morphological features were extracted from the segmented mass.
Four feature spaces were formed: (1) features from the central DBT slice, (2) average features from 5 DBT slices
centered at the central slice, (3) features from the central PV, and (4) average features from all 11 PVs. In each feature
space, a linear discriminant analysis classifier with stepwise feature selection was trained and tested using a two loop
leave-one-case-out procedure. The test Az of 0.91±0.03 from the 5-DBT-slice feature space was significantly (p=0.003)
higher than that of 0.84±0.04 from the 1-DBT-slice feature space. The test Az of 0.83±0.04 from the 11-PV feature
space was not significantly different (p=0.18) from that of 0.79±0.04 from the 1-PV feature space. The classification
accuracy in the 5-DBT-slice feature space was significantly better (p=0.006) than that in the 11-PV feature space. The
results demonstrate that the features of breast lesions extracted from the DBT slices may provide higher classification
accuracy than those from the PV images.
KEYWORDS: Image segmentation, Breast, Magnetic resonance imaging, 3D image processing, Magnetism, Breast cancer, 3D acquisition, 3D scanning, 3D modeling, Mathematical modeling
The goal of this study was to develop an automated method to segment breast masses on dynamic contrast-enhanced
(DCE) magnetic resonance (MR) scans that were performed to monitor breast cancer response to neoadjuvant
chemotherapy. A radiologist experienced in interpreting breast MR scans defined the mass using a cuboid volume of
interest (VOI). Our method then used the K-means clustering algorithm followed by morphological operations for initial
mass segmentation on the VOI. The initial segmentation was then refined by a three-dimensional level set (LS) method.
The velocity field of the LS method was formulated in terms of the mean curvature which guaranteed the smoothness of
the surface and the Sobel edge information which attracted the zero LS to the desired mass margin. We also designed a
method to reduce segmentation leak by adapting a region growing technique. Our method was evaluated on twenty
DCE-MR scans of ten patients who underwent neoadjuvant chemotherapy. Each patient had pre- and post-chemotherapy
DCE-MR scans on a 1.5 Tesla magnet. Computer segmentation was applied to coronal T1-weighted images. The in-plane
pixel size ranged from 0.546 to 0.703 mm and the slice thickness ranged from 2.5 to 4.0 mm. The flip angle was
15 degrees, repetition time ranged from 5.98 to 6.7 ms, and echo time ranged from 1.2 to 1.3 ms. The computer
segmentation results were compared to the radiologist's manual segmentation in terms of the overlap measure defined as
the ratio of the intersection of the computer and the radiologist's segmentations to the radiologist's segmentation. Pre-
and post-chemotherapy masses had overlap measures of 0.81±0.11 (mean±s.d.) and 0.70±0.21, respectively.
Breast vascular calcifications (BVCs) are calcifications that line the blood vessel walls in the breast and appear
as parallel or tubular tracks on mammograms. BVC is one of the major causes of the false positive (FP) marks from
computer-aided detection (CADe) systems for screening mammography. With the detection of BVCs and the calcified
vessels identified, these FP clusters can be excluded. Moreover, recent studies reported the increasing interests in the
correlation between mammographically visible BVCs and the risk of coronary artery diseases. In this study, we
developed an automated BVC detection method based on microcalcification prescreening and a new k-segments
clustering algorithm. The mammogram is first processed with a difference-image filtering technique designed to
enhance calcifications. The calcification candidates are selected by an iterative process that combines global
thresholding and local thresholding. A new k-segments clustering algorithm is then used to find a set of line segments
that may be caused by the presence of calcified vessels. A linear discriminant analysis (LDA) classifier was designed to
reduce false segments that are not associated with BVCs. Four features for each segment selected with stepwise feature
selection were used for this LDA classification. Finally, the neighboring segments were linked and dilated with
morphological dilation to cover the regions of calcified vessels. A data set of 16 FFDM cases with vascular
calcifications was collected for this preliminary study. Our preliminary result demonstrated that breast vascular
calcifications can be accurately detected and the calcified vessels identified. It was found that the automated method can
achieve a detection sensitivity of 65%, 70%, and 75% at 6.1 mm, 8.4 mm, and 12.6mm FP segments/image, respectively,
without any true clustered microcalcifications being falsely marked. Further work is underway to improve this method
and to incorporate it into our FFDM CADe system.
The goal of this study was to develop an automated method to identify corresponding nodules in serial CT scans for
interval change analysis. The method uses the rib centerlines as the reference for initial nodule registration. From an
automatically-identified starting point near the spine, each rib is locally tracked and segmented by expectation-maximization.
The ribs are automatically labeled, and the centerlines are estimated using skeletonization. 3D rigid affine
transformation is used to register the individual ribs in the reference and target scans. For a given nodule in the
reference scan, a search volume of interest (VOI) in the target scan is defined by using the registered ribs. Template
matching guided by the normalized cross-correlation between the nodule template and target locations within the search
VOI is used for refining the registration. The method was evaluated on 48 CT scans from 20 patients. The slice thickness
ranged from 0.625 to 7 mm, and the in-plane pixel size from 0.556 to 0.82 mm. Experienced radiologists identified 101
pairs of nodules. Two metrics were used for performance evaluation: 1) the Euclidean distance between the nodule
centers identified by the radiologist and the computer registration, and 2) a volume overlap measure defined as the
intersection of the VOIs identified by the radiologist and the computer registration relative to the radiologist's VOI. The
average Euclidean distance error was 2.7 ± 3.3 mm. Only 2 pairs had an error >10 mm. The average volume overlap
measure was 0.71 ± 0.24. Eight-three out of 101 pairs had overlap ratios > 0.5 and only 2 pairs had no overlap.
An important purpose of a CAD system is that it can serve as a second reader to alert radiologists to subtle cancers that
may be overlooked. In this study, we are developing new computer vision techniques to improve the detection
performance for subtle masses on prior mammograms. A data set of 159 patients containing 318 current mammograms
and 402 prior mammograms was collected. A new technique combining gradient field analysis with Hessian analysis
was developed to prescreen for mass candidates. A suspicious structure in each identified location was initially
segmented by seed-based region growing and then refined by using an active contour method. Morphological, gray
level histogram and run-length statistics features were extracted. Rule-based and LDA classifiers were trained to
differentiate masses from normal tissues. We randomly divided the data set into two independent sets; one set of 78
cases for training and the other set of 81 cases for testing. With our previous CAD system, the case-based sensitivities
on prior mammograms were 63%, 48% and 32% at 2, 1 and 0.5 FPs/image, respectively. With the new CAD system,
the case-based sensitivities were improved to 74%, 56% and 35%, respectively, at the same FP rates. The difference in
the FROC curves was statistically significant (p<0.05 by AFROC analysis). The performances of the two systems for
detection of masses on current mammograms were comparable. The results indicated that the new CAD system can
improve the detection performance for subtle masses without a trade-off in detection of average masses.
We have previously developed a breast boundary detection method by using a gradient-based method to search for
the breast boundary (GBB). In this study, we developed a new dynamic multiple thresholding based breast boundary
detection system (MTBB). The initial breast boundary (MTBB-Initial) is obtained based on the analysis of multiple
thresholds on the image. The final breast boundary (MTBB-Final) is obtained based on the initial breast boundary and
the gradient information from horizontal and the vertical Sobel filtering. In this way, it is possible to accurately segment
the breast area from the background region. The accuracy of the breast boundary detection algorithm was evaluated by
comparison with an experienced radiologist's manual segmentation using three performance metrics: the Hausdorff
distance (HDist), the average minimum Euclidean distance (AMinDist), and the area overlap (AOM). It was found
that 68%, 85%, and 90% of images have HDist errors less than 6 mm for GBB, MTBB-Initial, and MTBB-Final,
respectively. Ninety-five percent, 96%, and 97% of the images have AMinDist errors less than 1.5 mm for GBB,
MTBB-Initial, and MTBB-Final, respectively. Ninety-six percent, 97%, and 99% of the images have AOM values
larger than 0.9 for GBB, MTBB-Initial, and MTBB-Final, respectively. It was found that the performance of the
proposed method was improved in comparison to our previous method.
In computer-aided detection (CAD) applications, an important step is to design a classifier for the differentiation of the abnormal from the normal structures. We have previously developed a stepwise linear discriminant analysis (LDA) method with simplex optimization for this purpose. In this study, our goal was to investigate the performance of a regularized discriminant analysis (RDA) classifier in combination with a feature selection method for classification of the masses and normal tissues detected on full field digital mammograms (FFDM). The feature selection scheme combined a forward stepwise feature selection process and a backward stepwise feature elimination process to obtain the best feature subset. An RDA classifier and an LDA classifier in combination with this new feature selection method were compared to an LDA classifier with stepwise feature selection. A data set of 130 patients containing 260 mammograms with 130 biopsy-proven masses was used. All cases had two mammographic views. The true locations of the masses were identified by experienced radiologists. To evaluate the performance of the classifiers, we randomly divided the data set into two independent sets of approximately equal size for training and testing. The training and testing were performed using the 2-fold cross validation method. The detection performance of the CAD system was assessed by free response receiver operating characteristic (FROC) analysis. The average test FROC curve was obtained by averaging the FP rates at the same sensitivity along the two corresponding test FROC curves from the 2-fold cross validation. At the case-based sensitivities of 90%, 80% and 70% on the test set, our RDA classifier with the new feature selection scheme achieved an FP rate of 1.8, 1.1, and 0.6 FPs/image, respectively, compared to 2.1, 1.4, and 0.8 FPs/image with stepwise LDA with simplex optimization. Our results indicate that RDA in combination with the sequential forward inclusion-backward elimination feature selection method can improve the performance of mass detection on mammograms. Further work is underway to optimize the feature selection and classification scheme and to evaluate if this approach can be generalized to other CAD classification tasks.
In this study, our purpose was to develop a false positive (FP) reduction method for computerized mass detection systems based on the analysis of bilateral mammograms. We first detect the mass candidates on each view by utilizing our unilateral computer-aided detection (CAD) system. For each detected object, the regional registration technique is used to define a region of interest (ROI) that is "symmetrical" to the object location on the contralateral mammogram. Spatial gray level dependence matrices (SGLD) texture features and morphological features are extracted from both the ROI containing the detected object on a mammogram and its corresponding ROI on the contralateral mammogram. Bilateral features are then generated from the extracted unilateral features and a final bilateral score is formed as a new feature to differentiate symmetric from asymmetric ROIs. By incorporating the unilateral features of the mass candidates and their bilateral scores, a bilateral classifier was trained to reduce the FPs. It was found that our bilateral CAD system achieved a case-based sensitivity of 70%, 80%, and 85% at 0.52, 0.83, and 1.05 FPs/image on the test data set. In comparison to the FP rates for the unilateral CAD system of 0.67, 1.11, and 1.69, respectively, at the corresponding sensitivities, the FP rates were reduced by 22%, 25%, and 37% with the bilateral symmetry information.
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