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.
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.
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.
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.
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.
Cardiac CT has been reported to be an effective means for clinical diagnosis of coronary artery plaque disease.
We are investigating the feasibility of developing a computer-assisted image analysis (CAA) system to assist
radiologist in detection of coronary artery plaque disease in ECG-gated cardiac CT scans. The heart region was
first extracted using morphological operations and an adaptive EM thresholding method. Vascular structures in
the heart volume were enhanced by 3D multi-scale filtering and analysis of the eigenvalues of Hessian matrices
using a vessel enhancement response function specially designed for coronary arteries. The enhanced vascular
structures were then segmented by an EM estimation method. Finally, our newly developed 3D rolling balloon
vessel tracking method (RBVT) was used to track the segmented coronary arteries. Starting at two manually
identified points located at the origins of left and right coronary artery (LCA and RCA), the RBVT method moved
a sphere of adaptive diameter along the vessels, tracking the vessels and identifying its branches automatically to
generate the left and right coronary arterial trees. Ten cardiac CT scans that contained various degrees of coronary
artery diseases were used as test data set for our vessel segmentation and tracking method. Two experienced
thoracic radiologists visually examined the computer tracked coronary arteries on a graphical interface to count
untracked false-negative (FN) branches (segments). A total of 27 artery segments were identified to be FNs in the
10 cases, ranging from 0 to 6 FN segments in each case. No FN artery segment was found in 2 cases.
CT pulmonary angiography (CTPA) has been reported to be an effective means for clinical diagnosis of pulmonary
embolism (PE). We are developing a computer-aided detection (CAD) system to assist radiologist in PE detection in
CTPA images. 3D multiscale filters in combination with a newly designed response function derived from the
eigenvalues of Hessian matrices is used to enhance vascular structures including the vessel bifurcations and suppress
non-vessel structures such as the lymphoid tissues surrounding the vessels. A hierarchical EM estimation is then used to
segment the vessels by extracting the high response voxels at each scale. The segmented vessels are pre-screened for
suspicious PE areas using a second adaptive multiscale EM estimation. A rule-based false positive (FP) reduction
method was designed to identify the true PEs based on the features of PE and vessels. 43 CTPA scans were used as an
independent test set to evaluate the performance of PE detection. Experienced chest radiologists identified the PE
locations which were used as "gold standard". 435 PEs were identified in the artery branches, of which 172 and 263
were subsegmental and proximal to the subsegmental, respectively. The computer-detected volume was considered true
positive (TP) when it overlapped with 10% or more of the gold standard PE volume. Our preliminary test results show
that, at an average of 33 and 24 FPs/case, the sensitivities of our PE detection method were 81% and 78%, respectively,
for proximal PEs, and 79% and 73%, respectively, for subsegmental PEs. The study demonstrates the feasibility that the
automated method can identify PE accurately on CTPA images. Further study is underway to improve the sensitivity
and reduce the FPs.
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.
Digital Tomosynthesis Mammography (DTM) is a promising modality that can 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. DTM can provide depth information to separate
overlapping breast tissues occurred in conventional mammograms, thereby facilitating detection of subtle
lesions. In this work, we investigated the impact of the imaging parameters and reconstruction methods on
the Z-axis resolution in DTM systems. The Z-axis resolution represents the ability of the DTM system to
distinguish adjacent objects along the depth direction. A DTM system with variable image acquisition
parameters was modeled. In this preliminary study, a computer phantom containing a high-density point
object embedded in an air volume was used. We simulated a range of DTM conditions by generating an
appropriate number of PV images in 3° increments covering a total tomosynthesis angle from ±15° to ±30°.
The Simultaneous Algebraic Reconstruction Technique (SART) was used for reconstruction of the imaged
volume from the noise-free projection data and the results were compared to those of back-projection
method. Vertical line profiles along the Z-axis and through the object center were extracted from the
reconstructed volume and the full-width-at-half-maximum (FWHM) of the normalized intensity profile was
used to evaluate the Z-axis resolution. Preliminary results demonstrated that while the Z-axis resolution
remains almost constant as a function of depth within a 5-cm-thick volume, it is strongly affected by the PV
angular range such that the depth resolution improves with increasing total tomosynthesis angle. The depth
resolution also depends on the reconstruction algorithm employed; the SART method is superior to the
simple back-projection method in terms of depth resolution.
KEYWORDS: Digital mammography, Breast, Reconstruction algorithms, Signal to noise ratio, 3D image processing, Sensors, Metals, Iterative methods, Prototyping, Mammography
We are developing a computerized technique to reduce intra- and interplane ghosting artifacts caused by high-contrast
objects such as dense microcalcifications (MCs) or metal markers on the reconstructed slices of digital
tomosynthesis mammography (DTM). In this study, we designed a constrained iterative artifact reduction method
based on a priori 3D information of individual MCs. We first segmented individual MCs on projection views (PVs)
using an automated MC detection system. The centroid and the contrast profile of the individual MCs in the 3D breast
volume were estimated from the backprojection of the segmented individual MCs on high-resolution (0.1 mm isotropic
voxel size) reconstructed DTM slices. An isolated volume of interest (VOI) containing one or a few MCs is then
modeled as a high-contrast object embedded in a local homogeneous background. A shift-variant 3D impulse response
matrix (IRM) of the projection-reconstruction (PR) system for the extracted VOI was calculated using the DTM
geometry and the reconstruction algorithm. The PR system for this VOI is characterized by a system of linear equations.
A constrained iterative method was used to solve these equations for the effective linear attenuation coefficients (eLACs)
within the isolated VOI. Spatial constraint and positivity constraint were used in this method. Finally, the intra- and
interplane artifacts on the whole breast volume resulting from the MC were calculated using the corresponding impulse
responses and subsequently subtracted from the original reconstructed slices.
The performance of our artifact-reduction method was evaluated using a computer-simulated MC phantom, as
well as phantom images and patient DTMs obtained with IRB approval. A GE prototype DTM system that acquires 21
PVs in 3º increments over a ±30º range was used for image acquisition in this study. For the computer-simulated MC
phantom, the eLACs can be estimated accurately, thus the interplane artifacts were effectively removed. For MCs in
phantom and patient DTMs, our method reduced the artifacts but also created small over-corrected areas in some cases.
Potential reasons for this may include: the simplified mathematical modeling of the forward projection process, and the
amplified noise in the solution of the system of linear equations.
KEYWORDS: Mammography, CAD systems, Information fusion, Breast, Computer aided design, Image segmentation, Feature extraction, Computer aided diagnosis and therapy, Signal to noise ratio
We are developing new techniques to improve the performance of our computer-aided detection (CAD) system for clustered microcalcifications on full-field digital mammograms (FFDMs). In this study, we designed an information fusion scheme by using joint two-view information on craniocaudal (CC) and mediolateral-oblique (MLO) views. After cluster candidates were detected using a single-view detection technique, candidates on CC and MLO views were paired using their geometrical information. Candidate pairs were classified as true and false pairs with a similarity classifier that used the joint information from both views. Each cluster candidate was also characterized by its single-view features. The outputs of the similarity classifier and the single-view classifier were fused and the cluster candidate was classified as a true microcalcification cluster or a false-positive (FP) using the fused two-view information. A data set of 192 FFDM images was collected from 96 patients at the University of Michigan. All patients had two mammographic views. This data set contained 96 microcalcification clusters, of which 28 clusters were proven by biopsy to be malignant and 68 were proven to be benign. For training and testing the classifiers, the data set was partitioned into two independent subsets with the malignant cases equally distributed to the two subsets. One subset was used for training and the other subset was used for testing. We compared three computerized methods for geometrically pairing cluster candidates on two mammographic views. The areas under the fitted ROC curves were 0.75±0.01, 0.74±0.01, and 0.76±0.01 for the three methods, respectively. The difference between any two methods measured by the area under the fitted ROC curve, Az, was not statistically significant (p > 0.05). We also evaluated a new hybrid pairing scheme that used two different sensitivity levels for defining cluster pairs based on the single-view scores. The single-view CAD system achieved cluster-based sensitivities of 75%, 80%, and 85% at 0.48, 0.86, and 1.05 FPs/image, respectively. The joint two-view CAD system achieved the same sensitivity levels at 0.29, 0.46, and 0.89 FPs/image. When the hybrid pairing was used in the joint two-view CAD system, the same cluster-based sensitivities were achieved at 0.26, 0.37, and 0.88 FPs/image. Our results indicate that correspondence of cluster candidates on two different views provides valuable additional information for distinguishing FPs from true microcalcification clusters.
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.
Automatic and accurate segmentation of the pulmonary vessels in 3D computed tomographic angiographic images (CTPA) is an essential step for computerized detection of pulmonary embolism (PE) because PEs only occur inside the pulmonary arteries. We are developing an automated method to segment the pulmonary vessels in 3D CTPA images. The lung region is first extracted using thresholding and morphological operations. 3D multiscale filters in combination with a newly developed response function derived from the eigenvalues of Hessian matrices are used to enhance all vascular structures including the vessel bifurcations and suppress non-vessel structures such as the lymphoid tissues surrounding the vessels. At each scale, a volume of interest (VOI) containing the response function value at each voxel is defined. The voxels with a high response indicate that there is an enhanced vessel whose size matches the given filter scale. A hierarchical expectation-maximization (EM) estimation is then applied to the VOI to segment the vessel by extracting the high response voxels at this single scale. The vessel tree is finally reconstructed by combining the segmented vessels at all scales based on a "connected component" analysis. Two experienced thoracic radiologists provided the gold standard of pulmonary arteries by manually tracking the arterial tree and marking the center of the vessels using a computer graphical user interface. Two CTPA cases containing PEs were used to evaluate the performance. One of these two cases also contained other lung diseases. The accuracy of vessel tree segmentation was evaluated by the percentage of the "gold standard" vessel center points overlapping with the segmented vessels. The result shows that 97.3% (1868/1920) and 92.0% (2277/2476) of the manually marked center points overlapped with the segmented vessels for the cases without and with other lung disease, respectively. The results demonstrate that vessel segmentation using our method is not degraded by PE occlusion and the vessels can be accurately extracted.
We are developing a two-view information fusion method to improve the performance of our CAD system for mass detection. Mass candidates on each mammogram were first detected with our single-view CAD system. Potential object pairs on the two-view mammograms were then identified by using the distance between the object and the nipple. Morphological features, Hessian feature, correlation coefficients between the two paired objects and texture features were used as input to train a similarity classifier that estimated a similarity scores for each pair. Finally, a linear discriminant analysis (LDA) classifier was used to fuse the score from the single-view CAD system and the similarity score. A data set of 475 patients containing 972 mammograms with 475 biopsy-proven masses was used to train and test the CAD system. All cases contained the CC view and the MLO or LM view. We randomly divided the data set into two independent sets of 243 cases and 232 cases. 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 from 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%, 85% and 80% on the test set, the single-view CAD system achieved an FP rate of 2.0, 1.5, and 1.2 FPs/image, respectively. With the two-view fusion system, the FP rates were reduced to 1.7, 1.3, and 1.0 FPs/image, respectively, at the corresponding sensitivities. The improvement was found to be statistically significant (p<0.05) by the AFROC method. Our results indicate that the two-view fusion scheme can improve the performance of mass detection on mammograms.
Digital tomosynthesis mammography (DTM) is a promising approach to breast cancer detection. DTM can provide 3D structural information of the breast tissue by reconstructing the imaged volume from 2D projections acquired at different angles in a limited angular range. In this work, we investigate the application of the Simultaneous Algebraic Reconstruction Technique (SART) to this limited-angle cone-beam tomographic problem. A second generation GE prototype tomosynthesis mammography system was used in this study. Projection-view images of different breast phantoms were acquired from 21 angles in 3° increments over a ±30° angular range. The digital detector is stationary during image acquisition. We used an ACR phantom and two additional phantoms to evaluate the image quality and reconstruction artifacts. The Back-Projection (BP) method was also implemented for comparison to SART. The contrast-to-noise ratio (CNR), line profile of features and an artifact spread function (ASF) were used to quantitatively evaluate the reconstruction results. Preliminary results show that both BP and SART can separate superimposed phantom structures along the Z direction, but SART is more effective in improving the conspicuity of tissue-mimicking details and suppressing interplane blurring. For the phantoms with homogeneous background, the BP method resulted in less noisy reconstruction and higher CNR values for masses than SART, but SART provided greater enhancement in the contrast of calcification clusters and the edge sharpness of masses and fibrils. It was shown that acceptable reconstruction can be achieved by SART after only one iteration.
We are developing a computer-aided detection (CAD) system to detect microcalcification clusters automatically on full field digital mammograms (FFDMs). The CAD system includes five stages: preprocessing, image enhancement and/or box-rim filtering, segmentation of microcalcification candidates, false positive (FP) reduction, and clustering. In this study, we investigated the performance of a nonlinear multiscale Laplacian pyramid enhancement method in comparison with a box-rim filter at the image enhancement stage and the use of a new error metric to improve the efficiency and robustness of the training of a convolution neural network (CNN) at the FP reduction stage of our CAD system. A data set of 96 cases with 200 images was collected at the University of Michigan. This data set contained 215 microcalcification clusters, of which 64 clusters were proven by biopsy to be malignant and 151 were proven to be benign. The data set was separated into two independent data sets. One data set was used to train and validate the CNN in our CAD system. The other data set was used to evaluate the detection performance. For this data set, Laplacian pyramid multiscale enhancement did not improve the performance of the microcalcification detection system in comparison with our box-rim filter previously optimized for digitized screen-film mammograms. With the new error metric, the training of CNN could be accelerated and the classification performance in validation was improved from an Az value of 0.94 to 0.97 on average. The CNN in combination with rule-based classifiers could reduce FPs with a small tradeoff in sensitivity. By using the free-response receiver operating characteristic (FROC) methodology, it was found that our CAD system can achieve a cluster-based sensitivity of 70%, 80%, and 88% at 0.23, 0.39, and 0.71 FP marks/image, respectively. For case-based performance evaluation, a sensitivity of 80%, 90%, and 98% can be achieved at 0.17, 0.27, and 0.51 FP marks/image, respectively.
We have developed a computer-aided detection (CAD) system for breast masses on mammograms. In this study, our purpose was to improve the performance of our mass detection system by using a new dual system approach which combines a CAD system optimized with "average" masses with another CAD system optimized with subtle masses. The latter system is trained to provide high sensitivity in detecting subtle masses. For an unknown mammogram, the two systems are used in parallel to detect suspicious objects. A feed-forward backpropagation neural network trained to merge the scores of the two linear discriminant analysis (LDA) classifiers from the two systems makes the final decision in differentiation of true masses from normal tissue. A data set of 86 patients containing 172 mammograms with biopsy-proven masses was partitioned into a training set and an independent test set. This data set is referred to as the average data set. A second data set of 214 prior mammograms was used for training the second CAD system for detection of subtle masses. When the single CAD system trained on the average data set was applied to the test set, the Az for false positive (FP) classification was 0.81 and the FP rates were 2.1, 1.5 and 1.3 FPs/image at the case-based sensitivities of 95%, 90% and 85%, respectively. With the dual CAD system, the Az was 0.85 and the FP rates were improved to 1.7, 1.2 and 0.8 FPs/image at the same case-based sensitivities. Our results indicate that the dual CAD system can improve the performance of mass detection on mammograms.
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