tomosynthesis stationary motion, full isocentric motion, and

tomosynthesis provides high quality images
due to improved mass visibility, bilateral asymmetry, and architectural distortion. Furthermore, the flexibility of
the digital tomosysnthesis system facilitates adequate 3-D lesion localization, higher image quality, easy diagnosis of the system
elements, fewer recalls.

Technical Aspects of
Tomosynthesis

The digital tomosynthesis mammography employs an xc-ray system that
rotates within
a limited arc angle. The x-ray system of the digital
tomosysnthesis differs from
those of the conventional tomography which rotate 360 degrees around
the object of interest. The digital tomosyntheis acquire images whiole the breast is relatively compressed. The system
acquires series of digital images
by repeatedly exposing the area of interest
to low dosages of the x-ray beams.

Motion Geometry

The motion geometry of the digital
tomosynthesis varies from
one manufacturer to another.
The main motion geometries include stationary motion,
full isocentric motion,
and partial isocentric motion. The
stationary motion involves the use of multi-beam x-ray which
allows the acquisition of all projection- view images. The stationary motion
geometry is characterized by a stationary detector which provides the users
with a number of benefits
such as a high degree
of positioning patients
due to the minimal
thickness of the detector.
Furthermore, the architecture of the stationary geometry allows the x-ray beams to hit the detector at an oblique
angle which is essential for minimizing or eliminating the spatial
resolution. The efficacy of the stationary geometry is dependent on the lateral
length of the
detector. A longer detector is preferred due
to its ability
to absorb x-ray energy
emitted by the peripheral regions
of the breast which
consequently reduces the intensity of the artifacts in the processes digital image. On the
other hand, the full isocentric motion both the detector and the x-ray source
are permanently fixed
and use the same axis to rotate.
The permanent fixing
of both the x-ray
source and the detector in the
isocentric geometry increases
its susceptibility to the formation
of the blurring artifacts. The anomaly is often minimized by utilizing a small ratio
between the tilting
angles of the
x-ray source and the detector. Again, the formation of the blurred
artifacts on images
acquired using isocentric geometry by moving
the center of rotation towards the plane
of the detector which leads
to the formation of the detector tilt relative to the angle
of the x-ray source.
The architecture of the partial
isocentric motion allows
the x-ray source to rotate within
a limited arc of tomosysnthesis angle while the detector remains
stationary. The partial isocentric motion is preferred
due to its efficiency and ability to provide optimal
images of the breast (42).
The spatial isocentric geometry provides high
quality digital images
due to its possession of high quality properties for rejecting x-ray scattering.

System Geometry

The geometry of the digital
tomosynthesis is described in terms of the location of the x-ray source.
The geometry defined by the location
of the x-ray source
takes into account the focal spot
location, the distance of the
detector from the x-ray source, and
the path of the x-ray beam relative
to the tomography angle and detector. Additionally, the system geometry
is also defined
by the disalignment of the detector
relative to the beam of the x-ray.

Tomography Angle

The angle of tomography refers to the range
of angles swept
by the x-ray source
while using the digital
tomosynthesis
to acquire the digital images
of the female
breast. The angle
of tomography plays
a critical

role in determining the level of slice separation for the reconstructed images and the resolution of the
final image. Retrospectively, a wide range
of the tomography angle provides
a high resolution image and an increased slice separation for the reconstructed images. In addition, the combination of a wide angle
of tomography and high x-ray dosage
tends to improve
the quality of the image
acquired using the digital
tomosysnthesis. Furthermore, the use of a wide tomography angle
minimizes the intensity of the out-of- plane artifacts that occur during
the acquisition of the digital
image. However,
the reduction of the
tomography angle leads
to the distortion of the image
resolution on z-plane
(42).

Number of Projections

The quality of the images
acquired using digital
tomosynthesis is dependent on the number
of projection-views employed
for a given range of tomography angle.
A high number
of projection-view mammograms facilitate adequate angular sampling which eliminates all form of artifacts from
the acquired image. However, an increment in the number
of projection-views does
not guarantee an improvement in the performance of the digital
tomosynthesis.

Dose

There exist a direct correlation between the quality
of the image acquired using the digital tomosynthesis and
the amount of x-ray dose used
(42). The digital
tomosysnthesis employ a series of low-dose of x-ray which are
run through a limited arc of tomography angle to acquire
the image of the
required part of the female
breast. The incorporation of the 3-D technology into
the architecture of the
digital tomosynthesis allows
the use of lower doses
of the x-ray without
compromising the quality
and resolution of the breast image.
However, the use of the low x-ray doses for image acquisition in digital
tomosynthsis makes it susceptible to effects of noise and the out-of-plane artifacts. The selection of detectors which
possesses high detective quantum efficiency and able to achieve rapid
readout tend to eliminate both noise and artifacts from the final image.

Reconstruction
Algorithms

The utilization of small tomography angle causes the
digital tomosynthesis to acquire images
using limitred projection-views which affects the amount of information
captured in the image. The inadequate information gathered by the digital images
requires corrective measures to provide optimal data to the radiologists. The above enumerated limitation of the
digital tomosysnthesis images
are often alleviated using
special reconstruction algorithms (42). The reconstruction of the raw images assist
in the removal of artifacts while
at the same improving the quality of the image
acquired.

Artifacts

Image artifacts in digital tomosynthesius mammograms is attributed to a number
of factors which include limited projection-views, the need for image reconstruction, and a small
range of angle
of tomography (43). The presence
of the artifacts on the digital image
tends to obscure
critical details of the breast tissue close
to its edges which may cause a patient to be misdiagnosed. In addition, the presence of pronounced artefacts may
interfere with the visual evaluation of the image by the radiologist and the ability
to detect minor
features of the mammogram when using computer-aided detection. The presence
of image artifacts in digital tomosynthesis mammograms is caused
by several factors. Artifacts which occur during
the process of image acquisition are mainly caused
by both the small sizes of the x-ray beam
and the detector. The artifacts under
this category include
the bright area artifacts and the staircase artifact. The bright
area artifact sis
caused by the
inability of the
detector to

absorb x-ray beams originating from parts of the breast
beyond its region
of effectiveness. On the other hand, the staircase artifact
occurs due to the inability of the x-ray beam
to penetrate through
some breast tissue. In addition, the process of image reconstruction
contributes to the formation of the artifacts.
The artifacts caused
by image reconstruction include blurr and ripple artifact
and the halo artifact. The blurr and ripple artifact
is caused by the occurrence of a dense
structure in several
sections of the image
of the breast. The halo artifact occurs due to the reconstruction of faint signal
around dense sections of the breast
image.