following kinds of pectoral muscles, namely, regu-
lar, convex, concave, and combinatorial. In the liter-
ature, numerous methods have been suggested to ex-
tract pectoral muscles. For example, in (Taghanaki
et al., 2017a), geometric rules with a region growing
algorithm are employed to segment pectoral muscles.
Few publicly available mammographic image anal-
ysis tools, such as LIBRA (Keller et al., 2015) and
OpenBreast (Pertuz et al., 2019), embrace the pectoral
muscle segmentation step.
Most pectoral muscle segmentation method set
a fixed parameter setting that may work with some
images and fail with others due to the variations in
the density of breasts. In this paper, we propose a
promising method to automatically segment pectoral
muscles from tomosynthesis images based on geo-
metric information of the breast and a meta-heuristic
optimization algorithm. Specifically, the proposed
method comprises the following four steps: 1) a pre-
processing step, 2) obtaining of geometric informa-
tion of pectoral muscle, 3) selection of pectoral mus-
cle pixels, and 4) finding the optimal parameters using
the grey wolf optimizer (GWO). The GWO algorithm
determines different parameters for each input image
as they depend on the visual characteristics of the im-
ages (i.e., breast density). With each input image, the
GWO optimizer determines different values of the pa-
rameters because they rely on the visual characteris-
tics of tomosynthesis images that are highly related to
breast density.
The remaining of this paper comprises the sub-
sequent sections. Section 2 presents related work.
Section 3 explains the proposed method. Section 4
presents the results. Section 5 summarizes the paper.
2 RELATED WORK
There are several pectoral muscle segmentation tech-
niques, such as thresholding methods, active contours
methods, K-means clustering methods, region grow-
ing methods, edge detection using Gabor filters meth-
ods, statistical region properties methods, and deep
learning-based methods. Each of these methods has
a different complexity that varies from highly sophis-
ticated to low complex. Below, we present examples
of these methods and explain how they address the
problem of segmenting pectoral muscle.
In (Sreedevi and Sherly, 2015) and (Unni et al.,
2018), a global threshold is used to estimate an initial
pectoral muscle boundary, and a morphology-based
boundary refinement algorithm is applied. The re-
sulting pectoral muscle region is then segmented by
a combination of global thresholding and connected
components methods. With a subset of 161 images
from the mini-MIAS mammographic images dataset,
(Sreedevi and Sherly, 2015) achieved an accuracy of
90.06% of based on visual observation of the correct-
ness of segmented images. The use of intensity only
for segmenting the pectoral muscle from the breast
region cannot produce a precise segmentation in most
cases. That happens because there is no significant
variation in texture and intensity between the pectoral
muscle tissue and the other tissues of the breast.
The authors of (Ergin et al., 2016) proposed a
region growing algorithm, in which he positions of
initial seeds for the pectoral muscle region are de-
termined based on the intensity of the region. As
the intensity cannot be effectively used for detecting
complex textures such as muscles, (Taghanaki et al.,
2017b) proposed to use a set of geometric rules and a
region growing method to segment pectoral muscles.
Using the MIAS and DDSM mammographic images
datasets, they achieved segmentation accuracy of 95%
and 94%, respectively. Based on the assumption that
pectoral muscles are near the chest wall on the upper-
right or the upper-left of mammographic images, the
authors of (Selvathi and Poornila, 2018) used only a
single seed for region growing algorithm. If the pec-
toral muscle is at right, they set the seed point into
the last 5th column in the input images; otherwise,
they set it set into (5th column of and 5th row). The
pectoral muscle boundary is then refined using mor-
phology operations.
Furthermore, line estimation methods are popular
pectoral muscle segmentation approaches, in which a
straight or a curve can represent the boundary of the
pectoral muscle. Hough transform is one of the most
popular line estimation method used for pectoral mus-
cle segmentation. Based on the Canny edge detector,
the authors of (Qayyum and Basit, 2016) proposed a
method for removing the pectoral muscle region from
mammograms. Firstly, a 3x3 median filter was used
to reduce the noise in input images. Then, they seg-
mented the initial pectoral muscle region by a com-
bination of the Canny edge detector and the inten-
sity of the region. The final pectoral muscle bound-
ary was determined using a straight-line estimation
method applied to the boundary. They achieved an ac-
curacy of 93% accuracy with the mini-MIAS dataset.
Besides, the authors of (Palkar and Agrawal, 2016)
proposed a straight-line estimation method for remov-
ing the pectoral muscle from mammograms based on
the fact that the pectoral muscle is at the upper-left
region of the breast. In the case of the original im-
ages in which the chest wall is not on the left side
of the image, they flipped them horizontally. Then,
the position of the middle-top pixel of the image was
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