The purpose of this study was to compare the performances of two recently-developed image retrieval methods for
mammographic masses, and to investigate the inter- and intra-observer variability in radiologists' assessment of mass
similarity. Method 1 retrieved masses that are similar to a query mass from a reference library based on radiologists'
margin and shape descriptions and the mass size. Method 2 used computer-extracted features. Two MQSA radiologists
participated in an observer study in which they rated the similarity between 100 query masses and the retrieved lesions
based on margins, shape, and size. For each query mass, three masses retrieved using Method 1 and three masses
retrieved using Method 2 were displayed in random order using a graphical user interface. A nine-point similarity rating
scale was used, with a rating of 1 indicating lowest similarity. Each radiologist repeated the readings twice, separated by
more than three months, so that intra-observer variability could be studied. Averaged over the two radiologists, two
readings, and all masses, the mean similarity ratings were 5.59 and 5.57 for Methods 1 and 2, respectively. The
difference between the two methods did not reach significance (p>0.20) for either radiologist. The intra-observer
variability was significantly lower than the inter-observer variability, which may indicate that each radiologist may have
their image similarity criteria, and the criteria may vary from radiologist to radiologist. The understanding of the trends
in radiologists' assessment of mass similarity may guide the development of decision support systems that make use of
mass similarity to aid radiologists in mammographic interpretation.
A computer-aided diagnosis (CADx) system with the ability to predict the probability of malignancy (PM) of a mass can
potentially assist radiologists in making correct diagnostic decisions. In this study, we designed a CADx system using
logistic regression (LR) as the feature classifier which could estimate the PM of a mass. Our data set included 488
ultrasound (US) images from 250 biopsy-proven breast masses (100 malignant and 150 benign). The data set was
divided into two subsets T1 and T2. Two experienced radiologists, R1 and R2, independently provided Breast Imaging
Reporting and Data System (BI-RADS) assessments and PM ratings for data subsets T2 and T1, respectively. An LR
classifier was designed to estimate the PM of a mass using two-fold cross validation, in which the data subsets T1 and
T2 served once as the training and once as the test set. To evaluate the performance of the system, we compared the PM
estimated by the CADx system with radiologists' PM ratings (12-point scale) and BI-RADS assessments (6-point scale).
The correlation coefficients between the PM ratings estimated by the radiologists and by the CADx system were 0.71
and 0.72 for data subsets T1 and T2, respectively. For the BI-RADS assessments provided by the radiologists and
estimated by the CADx system, the correlation coefficients were 0.60 and 0.67 for data subsets T1 and T2, respectively.
Our results indicate that the CADx system may be able to provide not only a malignancy score, but also a more
quantitative estimate for the PM of a breast mass.
Segmentation of masses is the first step in most computer-aided diagnosis (CAD) systems for characterization of breast
masses as malignant or benign. In this study, we designed an automated method for segmentation of masses on
ultrasound (US) images. The method automatically estimated an initial contour based on a manually-identified point
approximately at the mass center. A two-stage active contour (AC) method iteratively refined the initial contour and
performed self-examination and correction on the segmentation result. To evaluate our method, we compared it with
manual segmentation by an experienced radiologists (R1) on a data set of 226 US images containing biopsy-proven
masses from 121 patients (44 malignant and 77 benign). Four performance measures were used to evaluate the
segmentation accuracy; two measures were related to the overlap between the computer and radiologist segmentation,
and two were related to the area difference between the two segmentation results. To compare the difference between the
segmentation results by the computer and R1 to inter-observer variation, a second radiologist (R2) also manually
segmented all masses. The two overlap measures between the segmentation results by the computer and R1 were
0.87+
0.16 and 0.73+ 0.17 respectively, indicating a high agreement. However, the segmentation results between two
radiologists were more consistent. To evaluate the effect of the segmentation method on classification accuracy, three
feature spaces were formed by extracting texture, width-to-height, and posterior shadowing features using the computer
segmentation, R1's manual segmentation, and R2's manual segmentation. A linear discriminant analysis classifier using
stepwise feature selection was tested and trained by a leave-one-case-out method to characterize the masses as malignant
or benign. For case-based classification, the area Az under the test receiver operating characteristic (ROC) curve was
0.90±0.03, 0.87±0.03 and 0.87±0.03 for the feature sets based on computer segmentation, R1's manual segmentation,
and R2's manual segmentation, respectively.
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.
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.
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