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Файл:Ординатура / Хирургия / Библиотека им академика М.И. Перельмана / Книга_5196_Библиотеки_им_академика_М_И_Перельмана.pdf
CHAPTER 14 X-Ray Breast Imaging: Methods and Radiation Safety Aspects
277
A
Fig. 14.1 (A) Mammography of the breast using the craniocaudal projection. (B) Mammography can be used
to detect breast cancer.
and is likely to result in many false-positive readings,
leading to unnecessary biopsies in that population. The
increased density of the breast of younger women tends
to reduce radiographic contrast in the completed image.
Digital mammography units which can enhance contrast with image gray-level manipulation, offer substantial improvement for patients with dense breasts. Such
units combined with a newer irradiation technique
known as digital tomosynthesis (discussed later in this
chapter) will lower the percentage of false-positive readings caused by very dense breasts and consequently,
permit a more effective screening of younger women.
Earlier detection will save lives in general.
Mammography Screening
The authors of this textbook support the recommendations of the American College of Radiology, the American
Cancer Society, and the American Medical Association.
These groups advocate annual mammography screening
or mammography screening at least every other year for
women age 40 to 49 years. Before the onset of menopause,
a baseline mammogram is also highly recommended for
comparison with mammograms taken at a later age. The
interested reader should contact these organizations for
their latest policy statements on this subject.
Dose Reduction in Mammography
Dose reduction in mammography can be achieved by
limiting the number of projections taken or by lowering
the dose associated with each projection. In standard
B
mammography, axillary projections should be done
only on request of the radiologist. If standard mammography is performed as a routine screening procedure, it is prudent to perform only craniocaudal and
mediolateral oblique projections of each breast with
adequate compression to demonstrate breast tissue uniformly from the nipple to the most posterior portion.
Filtration for Mammographic Equipment
Appropriate attenuation is necessary for mammographic equipment, which by design produces photons
with an energy range of 17 to 23 keV. Metallic elements
such as molybdenum (42Mo96) and rhodium (45Rh
have most commonly been employed as filters for these
low-energy x-ray beams. When the x-ray tube target is
made of molybdenum, either a 0.03-mm molybdenum
filter or a 0.025-mm rhodium filter may be selected.7
For rhodium x-ray tube targets, only rhodium filters are
used. These filtration and target materials facilitate a
satisfactory level of contrast in the obtained radiographic image over the clinical extent of compressed
breast thickness. This is accomplished by employing
specific filters that preferentially select or permit passage of a particular range or window of energies from
the emerging x-ray spectrum that is very favorable for
the photoelectric interaction. Molybdenum filters allow
a lower energy window (17 to 20 keV) than rhodium
filters (20 to 23 keV) (Fig. 14.2). Molybdenum filters are
therefore suitable for small and average breast thickness,
whereas rhodium filters used with a molybdenum or
103
)
278
Number of photons
CHAPTER 14 X-Ray Breast Imaging: Methods and Radiation Safety Aspects
Tungsten target/
aluminum filter
0 5 10 15 20 25 30 0 5 10 15 20 25 30 0 5 10 15 20 25 30
17–20 17–20 20–23
A B C
Fig. 14.2 (A and B) X-ray emission spectra for tungsten and molybdenum anodes. Note that tungsten pro-
duces a high volume of x-ray photons above the 17 to 20 keV range considered ideal for mammography. These
photons merely degrade the quality of the recorded image. The molybdenum anode, however, produces few
x-ray photons above the ideal energy range, thereby initiating a higher contrast on the finished image. (C) A
rhodium anode produces a higher average energy x-ray beam than does the molybdenum anode. The energy
range for rhodium-produced photons is 20 to 23 keV. Photons from this energy range can provide better
penetration of larger, denser breasts. (From Ballinger PW, Frank ED: Merrill’s atlas of radiographic positions
and radiologic procedures, ed 9, St. Louis, 1999, Mosby.)
Molybdenum target/
molybdenum filter
Energy (keV)
Rhodium target/
rhodium filter
rhodium anode are better for larger (i.e., compression
thickness of 6 cm and greater) or dense breasts because
they will produce an x-ray beam with more penetrating
energy. Systemic use of such materials has the effect of
reducing the mean glandular dose (MGD)* in firm
breast tissue.
Maintaining and enhancing subject contrast are of
paramount importance in mammography. Beryllium
(4Be9) takes the place of the glass in the window of the
low-kVp x-ray producing mammographic x-ray tube to
accommodate this need.7 This light, strong alkaline
earth metal permits the relatively soft characteristic radiation important for enhancing contrast to exit the
tube without undergoing any significant attenuation.
Recently, it has been shown that newer, full-field digi-
tal mammography systems and digital tomosynthesis
systems provide better images with tungsten targets
*MGD is a convenient parameter that can be used to characterize the average absorbed dose to the breast as a result of a
mammographic x-ray exposure. It will be discussed in detail
later in this chapter.
184
(74W
) with rhodium filtration (W/Rh) for most breast
thicknesses and tungsten targets with silver (47Ag
108
) filtration (W/Ag) for thicker breasts. Digital detectors are
better able to separate out image features in mammography when presented with the broader spectra provided by
the tungsten targets.
Molybdenum x-ray tube targets with molybdenum or
rhodium filtration are being replaced in newer digital systems by tungsten targets with rhodium or silver filtration.
8,9
DIGITAL BREAST TOMOSYNTHESIS/3D MAMMOGRAPHY
Tomography
The method used for generating an in-focus twodimensional (2D) image of a slice or cross-section
through a three-dimensional (3D) object is called To-
mography. Traditional x-ray tomography achieves this
result by simply moving an x-ray source in a preselected
arc in one direction (i.e., clockwise or counterclockwise)
as a mechanically linked x-ray detector traverses in a
CHAPTER 14 X-Ray Breast Imaging: Methods and Radiation Safety Aspects
279
constant plane in the opposite direction (right to left or
left to right) during the exposure. This facilitates a
sharpening of structures in the focal plane while causing
structures in other planes above or below to appear
blurred. Typically, selected arc angles have ranged from
15 degrees up to 45 degrees. In the past this technique
was used to acquire more detail of specific structures
within the patient, such as solid tumors. This practice
has been almost totally superseded by the development
of computed tomography.
Digital Breast Tomosynthesis (DBT)
10
In standard digital mammography, the 3D breast structure is projected onto a detector plane perpendicular to
the x-ray source, and the multiple tissues and structures of
the compressed breast appear overlapped in the acquired
projection image. This has two detrimental effects on a
radiologist’s ability to detect subtle lesions from these
usual mammography images: first, malignant lesions or
tumors may be obscured by the presence of overlapping
normal glandular tissue, producing false negatives; second,
the superimposition of normal tissues might combine
into a structure that appears to be an abnormality thereby
signaling a false positive. This lowering of sensitivity and
specificity in conventional mammography caused by tissue superimposition is often called “anatomical” or “structure” noise. DBT has the ability to overcome anatomical
noise by adding depth resolution to a mammogram. This
technique was first approved by the US FDA in 2011.
Digital breast tomosynthesis is an imaging technique
which generates multiple planar images or views of the
breast from a series of low-dose x-ray projection images
acquired by a full field digital detector * while the x-ray
tube rotates within a limited arc about the patient’s breast.
*Full field digital detector historically refers to: When digital
imaging was introduced as an alternative to analog film-screen
radiography, technology enabled only small field of views in
breast imaging. This new technology was used primarily to obtain small “spot views” during stereotactic biopsies and wire locations, thus bypassing the time-consuming process of developing
film-screen images and greatly expediting these procedures.
Larger digital detectors were subsequently developed that permitted imaging an entire small breast, but multiple images were required for larger breasts, requiring added radiation and time. The
latter limitation was partially overcome by mammographic
equipment using fan-beam technology. Eventually, larger digital
detectors became available, thus enabling full-field imaging.
13
X-ray tube
Movement
Compression
paddle
Center of
Breast
No grid
Fig. 14.3 DBT imaging configuration. The vertical direction is
along the z-axis and the plane of the detector (also known as
in-plane) corresponds to the x and y axis direction. (From
Vaughan CL: Novel imaging approaches to screen for breast
cancer: recent advances and future prospects, Medical Engi-
neering & Physics 72:27–37, 2019.)
rotation
Flat panel
detector
This is physically achieved by modifying a standard
digital mammography platform so that the gantry containing the x-ray tube assembly is able to rotate about an
axis located above the breast support within the breast,
while the compressed patient’s breast and the mammography detector remain stationary. Breast positioning in DBT is the same as is used for conventional digital
mammography, with the breast compressed, but often
to a lesser degree, on a stationary support situated directly above the detector assembly to permit different
oblique views.
11,12
Fig. 14.3 is a schematic of a standard
mammographic imaging setup as used for DBT with
the x-ray tube at several angular orientations.
Digital breast tomography is often referred to as
“3D” mammography because mammographic images
can be reconstructed at arbitrary angles from the many
different x-ray projections. The DBT image creation
technique uses the same algorithms* as those for computed tomography (CT). The images are not of the
same quality, however, as CT images, since those are
acquired from a full 360-degree x-ray projection, but
the technology and algorithms are improving. In typical
*Algorithm refers to a finite sequence of computer-implementable instructions used by a computer to solve a class of
problems or to perform a computation.
280
CHAPTER 14 X-Ray Breast Imaging: Methods and Radiation Safety Aspects
0°
+20° –20°
Tube motion
Compression paddle
Compression paddle
Digital detector
Fig. 14.4 Multiple gantry angle effects and
depth resolution. (From Ikeda DM, Miyake KK:
Breast imaging: the requisites, ed 3, St. Louis,
2017, Elsevier.)
Digital Breast Tomography, the third axis (z-direction)
is stereoscopically extracted from the overall twodimensional data. At different x-ray tube angles, objects
at different heights in the breast are projected at different locations onto the detector with their detector separation increasing as the x-ray tube gantry angle increases (Fig. 14.4). The subsequent image reconstruction
process leads to a stack of synthesized non-blurred slice
images (similar in general appearance to 2D mammography planar images) of different depth layers in the
breast parallel to the detector surface. The degree of
overall resolution is predominantly determined by the
detector’s characteristics, the reconstruction algorithm
used, and the total angle of rotation of the x-ray tube.
The in-plane (x-y) sharpness, or resolution, of the image is much higher than the resolution in the z-axis or
the direction between adjacent slices (called depth resolution) due to the incomplete sampling of the object
Images seen on
digital detector
–20°0°+20°
from a relatively small angular scan range. As of 2016,
the best z-axis spatial resolution* obtained has been
2.3 mm using a 50-degree arc.
11
Effects of Tomographic Angular Scan Range
On the Depth Resolution of Structures. A standard 2D
full field digital mammogram (FFDM) gives no specific
information of the depth in the breast (z-axis location) of
a particular mass or abnormal structure, implying that it
could be at any depth. But as can be observed from
Fig.14.4 with a sufficient gantry angular displacement,
the lesion can be clearly distinguished on a detector from
*Spatial resolution refers to the ability to distinguish spatially
close objects as distinct separate objects and is given by a limiting number (e.g., in millimeters or in line pairs resolved per
millimeter) that quantifies that limit.
CHAPTER 14 X-Ray Breast Imaging: Methods and Radiation Safety Aspects
281
adjacent structures. A small tomosynthesis arc yields only
a poor separation of vertically close objects (i.e., slight zaxis separation) while a large angle tomosynthesis arc
provides a good visual separation of these objects and
therefore a more precise vertical localization of the lesion
or mass.
12
On In-Plane Image Quality. If an x-ray source moves
through a larger angular range, the obtained in-plane
(x-y) image quality increases, at least initially. A larger
angular range allows for extra view directions, which
can improve the quality of the reconstructed image
since the inside of the breast can be inspected more
thoroughly. This is of importance since for many types
of breast masses there is not much variation or differentiation from one laterally adjacent position to the next.
Thus, image contrast is improved. However, if the total
allowable breast dose is fixed while the number of views
is increased, there is a deterioration in the overall image
quality in each view because of increased image noise
due to a lessening of the x-ray exposure per view (i.e.,
too low a signal-to-noise ratio [SNR]). The best perfor-
mance for all tomosynthesis angles occurred when from
one x-ray projection to the next, the angular increment
was about 2.75 degrees.
14,15
Effects Summary. The following quantities depend on
acquisition geometry, scan parameters, and system
hardware components12:
• Resolution.
• Detector Noise.
• Artifact level.
• Patient Dose.
• Accessible image volume.
• Examination time.
Optimal image acquisition geometry therefore
should be a compromise that accounts for the effects of
total tomosynthesis angle, angular increment, and the
number and distribution of projections on14:
• correctly identifying architectural distortions and
soft tissue lesions
• the perception of calcifications
• increased motion blur with increased scan time.
Image Reconstruction (IR)
Each DBT arc acquisition sequence may typically consist
of from 15 to 25 separate x-ray projections. From the data
generated by these, many well-defined planar views (their
number depends on the compressed breast thickness)
can be obtained after computer processing. These synthesized slices can subsequently be viewed either individually
or in a continuous sequence thereby generating a “movie”.
This latter option, called cine mode, offers a clinician an
approximate three-dimensional scan of the breast which
can yield an improved examination of suspicious architecture. Because only a limited arc angle inspection is
available (e.g. from 15 up to 50 degrees depending on the
mammographic unit), depth resolution through the
breast of various objects or structures will also be dependent upon the depth range (z-axis extent) spanned by
them. Thus, for structures such as microcalcifications
that typically only persist across a few tomosynthesis
planes, the image fidelity will be high.
Due to the breast’s highly inhomogeneous volume,
typical required viewing voxel* dimensions in tomosynthesis are 100 mm 3 100 mm pixels in-plane (x-y plane)
by 1 mm in depth (z direction) to adequately resolve
variations of interest in the breast. As a result of this
resolution, the 3D distribution of attenuation that is
entirely projected into just one image in conventional
mammography can in DBT be separated into many
1 mm thick layers, thereby increasing the visualization
of features that are often obscured by overlapping structures in a 2D single projection view.
As the x-ray projections are obtained, the raw image
digital data is exported into the data processing computer’s reconstruction algorithm. The conventional
method of image reconstruction used in digital tomosynthesis is the Shift and Add Algorithm (SAA)14. As
shown in Fig. 14.4, objects within the image sample that
are at different heights above the detector will be projected to different positions on the detector as the x-ray
tube moves. It then becomes possible for the computer by
properly registering** these images to both shift and add
them so that similarly catalogued structures in a particular plane are all lined up and are thus reinforced and
those non-similarly registered structures are distributed
over the image (misaligned) and are thereby blurred. As a
*Voxel is a term used for a discrete image volume element (a 3D
measure), and pixel is a term used for a discrete image area
element.
**
Registration of imaged structures is the procedure of precisely aligning two or more images of the same object. One
image of the object always is classified as the reference image.
Then, geometric transformations or local displacements (i.e.,
a shifting) are applied to all of the other images similar to the
reference image so that they align with the reference image.
14
282
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CHAPTER 14 X-Ray Breast Imaging: Methods and Radiation Safety Aspects
result, for each reconstructed or synthesized plane, there
is in sharp relief only the structures belonging to that
plane, while other structures located in adjacent planes
are out of focus in the plane of interest. Fig. 14.5 gives a
pictorial summation of the acquisition and reconstruction processes. This combined method of acquisition and
reconstruction significantly reduces anatomical noise pro-
moting easier and more reliable lesion detection. All of
1. Acquisition
A
2. Projections 3. Tomographic sections
the projection images are included in every synthesized
planar image and all of these “planes”, each parallel to the
detector surface, are generated through the entire breast
from just a small number of x-ray projection images.
Typically, the generated view planes are spaced 1.0 mm
center to center apart. A 4.5-cm thick compressed breast
reconstructed at 1 mm spacing will therefore, yield a
group of 45 images.
1 mm slices
5mm slab
Calcification cluster Isolated microcalcifications
B
Fig. 14.5 (A) Shift and add method for tomographic image reconstruction. (B) By combining several thin
slices together like a pancake, the existence of a calcification cluster whose components were spread out
over several adjacent 1 mm reconstructed slices is now apparent. Consequently, the production of a slab
composed from adjacent thin slices has led to a diagnostic result indicative of the potential presence of a
nearby cancer. (A, From Blum A, Noel A, Regent D: Tomosynthesis in musculoskeletal pathology. Diagn Interven
Imaging 99(7–8): 423–441, 2018.)
CHAPTER 14 X-Ray Breast Imaging: Methods and Radiation Safety Aspects
283
In the acquisition process shown in Fig. 14.5A, an
x-ray tube moves to five discrete positions.
Objects situated at two different levels are projected
onto the detector at different locations due to parallax.
The five projection images are shifted and added together to cause either of the objects in one of the two
levels to be in sharp focus, depending on the magnitude
of the shift for that object. Other types of images which
can be reconstructed from tomosynthesis are slabs, i.e.,
broader slices obtained by merging together a multiple
number of adjacent tomographic reconstructed planes
(see Fig. 14.5B). Slabs, therefore, have a substantial
thickness, typically 1 cm or more, and are particularly
useful for detecting microcalcification bunches. In fact,
microcalcifications usually seen grouped in clusters in
standard 2D mammography may not be as well visualized along the z-axis direction (depth) in the thin tomographic reconstructions and so not be obvious as making up a cluster. Furthermore, if some type of potentially
pathological feature is detected in cine mode, then image slabs, because of their much fewer number, allow a
quick spot review of the entire breast volume, prior to
getting into the fine details of reviewing the numerous
1-mm synthesized tomographic planes.
11
Advantages of DBT
DBT mammographic examinations may be used to diagnose breast cancer in people who have no overt signs
or symptoms. Also, it can more sensitively investigate
the cause of some breast problems such as localized or
distributed pain and nipple discharge. When a DBT
series is combined with 2D standard mammogram
projections, the combination may be able to yield the
following benefits.
Reduce the Need for Follow-Up Imaging. Should ab-
normalities be detected on standard mammogram images, additional imaging will usually be recommended.
This can be very stressful besides taking extra time and
leading to additional costs. Performing a 3D mammogram along with a standard 2D mammogram at the
same appointment, however, will reduce the need for
follow-up imaging.
Detect More Cancers Than a Standard Mammogram Alone.
Multiple studies indicate that performing a 3D
mammogram along with a standard mammogram can
result in about one more breast cancer found for every
15
1000 women screened when compared with a standard
mammogram alone. At the expense of some increased
patient radiation dosage, this amounts to finding a substantial number of otherwise undiscovered cancers
when one considers the number of women annually
screened.
Improve Breast Cancer Detection in Dense Breast Tissue.
A DBT mammogram offers advantages in detecting breast cancer in patients with dense breast tissue.
It has been noted that about 45% of all women receiving
screening mammography are found to have either het-
erogeneously dense or very dense breasts. Breasts are
normally considered to be dense if there is present a
much greater concentration of fibrous or glandular tissue than fatty tissue. With standard 2D mammograms,
non-fatty breasts pose challenges to accurately identifying the presence of cancer since irregularities can often
be efficiently concealed or masked within the dense tissue. With a DBT mammogram, however, breast tissue
can be scrutinized slab by slab or even thin layer by thin
layer, overcoming the obscurity effect and making it
easier to detect cancer in an early stage. It should be
noted that data collected on many patients has shown
that the degree of breast density is not a major indicator
for risk of breast cancer.
15
In Fig. 14.6B are shown planar images that might
be reconstructed from an obtained DBT data set as
displayed in Fig. 14.6A. The image planes depicted in
Fig. 14.6A, B are at different distances above the breast
support plate. As contrasted with the compacted 2D
image shown in Fig. 14.6C, each of these tomographic
planes clearly demonstrates a separate structure,
whereas the 2D projection in Fig. 14.6C only exhibits
a composite compacted image.
This ability to select multiple specific individual tissue planes in which various structures of interest are
visually enhanced can significantly increase both the
detection rate and the level of diagnostic confidence
with respect to apparent abnormalities.12 To summarize,
DBT images may be reconstructed as thin planes or as
slabs depending on the desired fineness of z-axis resolution. Another significant advantage of DBT is shown in
Fig. 14.7.
*Microcalcifications are small calcium deposits in breast tissue
that on a mammogram look like white specks.
284
CHAPTER 14 X-Ray Breast Imaging: Methods and Radiation Safety Aspects
2D Projection
A B C
Fig. 14.6 DBT acquired breast slice views at three different depths. (From Abrahams RB, Huda W, Sensakovic WF:
Imaging physics: case review series, St. Louis, 2020, Elsevier.)
A B C
Fig. 14.7 Comparison of positive conventional mammogram and DBT images. The DBT slices in (A) and (B)
readily demonstrate the presence of 2 distinct abnormalities, while the standard digital mammogram pictured
in (C) offers a poorer or less clear indication. (From Chan HP, Helvie MA, Hadjiiski L, et al: Characterization of
breast masses in digital breast tomosynthesis and digital mammograms: an observer performance study.
AcadRadiol 24(11): 1372–1379, 2017.)
CHAPTER 14 X-Ray Breast Imaging: Methods and Radiation Safety Aspects
A
Fig. 14.8 Incompleteness in DBT imaging
scan. The area marked (a) attenuates the
beam when the tube is in position (A) and is
projected to area (a9) on the detector. The at-
Part of the breast
a
a’
which is reconstructed
tenuation caused by this area is only added to
the most peripheral part of the breast tissue
included by the detector. As seen, not all of
the breast is captured by the detector. (From
Rangarajan K, Hari S: Artefacts in digital breast
tomosynthesis, ECR 2013 Poster C-1711,
2013. © European Society of Radiology.)
285
Artifacts in Digital Breast Tomography
16,17
Artifacts Due to Motion. The x-ray tube can move in
either a continuous or a step-and-shoot motion.* With
continuous motion, x-ray exposures must be short
enough to avoid image blurring due to focal spot motion.
If step-and-shoot motion is employed, the gantry must
come to a complete stop at each angular location before
delivering x-rays, otherwise vibrations will blur the image. It is also important that the total scan time be not too
prolonged, so as to reduce the possibility of patient motion that can degrade the visibility of microcalcifications
and small spiculations (lumps of tissue with spikes or
points on the surface). A grid is generally not used during
tomographic acquisitions since the varying irradiation
angles essentially do the work of a grid.
Artifacts Due to Method of Acquisition. Acquisition
artifacts are primarily truncation artifacts that arise due
to the limited physical size of the detector and the
greater extent of oblique x-ray beams causing incomplete coverage of the breast in multiple views (Fig. 14.8).
Some major examples of these are:
a) Bright area artifact – Appearing at extreme ends of
the image where the detector cannot see.
b) Staircase artifact – Presence of multiple lines at one
of the ends of the image creating a staircase-like appearance or artifact where the beam cannot see. At
*In step-and-shoot mode, the x-ray tube moves to a predetermined position (angle), makes an exposure, and then proceeds to the next position and so forth.
each tube position there is a corresponding part of
the breast tissue that would not be projected back by
the detector. The breast seemingly ends at different
places for different tube positions leading to the pro-
duction of lines at the edges of the image. This is
schematically shown in Fig. 14.8.
Artifacts Due to Z-Axis Resolution Deficiency. For
digital breast tomosynthesis, the attainable in-plane
(x-y) resolution for a particular cross-section or breast
slice is very good but the limited angle of rotation of
the x-ray tube (up to about a maximum of 60 degrees)
and the limited number of x-ray projections restricts
the z-axis resolution. This produces an incomplete
cancellation of objects outside of the plane of interest
and is the source of most artifacts.16 Consequently, the
precise localization of an extended depth object will be
less than perfect and some elongation of the suspicious object may also be present.
Fig. 14.9 depicts this situation.
Artifacts Due to Reconstruction Process.
16
a) Ripple – This arises from a poorly but still visualized
structure of high density (e.g., a calcification) being
observed in different slices from those in which it is
actually located.
b) Halo artifact – Manifestation of a very-low-intensity
signal (dark) appearing to circle around high-
intensity objects such as calcifications (white) leads
to the creation of a halo or crown. Halos are par-
ticularly pronounced along the sweep direction of
the x-ray tube.
286
CHAPTER 14 X-Ray Breast Imaging: Methods and Radiation Safety Aspects
Breast
abnormality
or mass
Fig. 14.9 Location uncertainty of a mass due to DBT limitations. (From Rangarajan K, Hari S: Artefacts in
digital breast tomosynthesis, ECR 2013 Poster C-1711, 2013. © European Society of Radiology.)
Properties of DBT Summarized
18,19
Expanding the Angular Sweep of the X-Ray Tube.
• Improves z-axis or depth resolution and the blurring
of out of plane objects.
• Permits finer or thinner reconstruction slices.
• At larger angles can begin to degrade in-plane or x-y
sharpness but allows better out-of-plane resolution
for larger objects such as masses that occupy multiple
planes.
Increasing the Number of Projections for a Given Angular Range.
• Lowers the visibility of artifacts.
• Increases the patient’s absorbed dose if the same
exposure settings (i.e., technique factors) are used.
• Increases the relative importance of detector noise if,
however, the total dose is held constant.
Number of Projections Required Depends On:
• The angular extent of x-ray tube rotation.
• The number of image pixels or individual pixel size.
• The characteristics of the full field detector.
• The degree of differences in contrast of absorbing
objects within the breast.
DBT Imaging Unit Characteristics
18
Shown below in Fig. 14.10 are preparatory and operational
pictures of a typical 3D mammography system. Table 14.1
is an inventory of the physical characteristics and operational parameters of several available commercial DBT units.
Potential depth
uncertainty from
DBT reconstruction
typically the axis of rotation will be positioned in the
center of the compressed breast. The scan itself takes less
than two to three seconds per x-ray projection and there
can be as many as 25 views taken. The entire procedure
from the patient entering the examination room up to
leaving it will usually take about 20 minutes. Box 14.1
lists the various steps during the radiation exposures
process for a typical commercial DBT unit.
18
With DBT alone, a compression force only great
enough to securely retain the breast in a stable position
during the procedure may suffice. For this situation, it is
more feasible to use a flexible compression paddle, which
will be less uncomfortable for the patient. In fact, it is
possible that some DBT examinations could be performed using only half of the compression force employed currently in standard 2D digital mammography,
leading to a substantial reduction in perceived and/or
actual patient pain while incurring no clinically significant change in breast imaging fidelity and tissue cover-
19,20
age.
The effects of decreased compression were extensively examined in a study done with phantom images
generated by using computer modeling methods that
simulated three dissimilar breasts, each having two different compressed thicknesses (4 cm and 6 cm) as compared with a lesser compressed pair (4.5 and 6.75 cm
compressions). Using lesion conspicuity* as the metric of
choice for masses and calcifications, the authors found
no significant observational difference with the lesser
compressions when the exposure parameters were varied
to maintain a constant dose to the breasts.
19–21
DBT Procedure: Steps and Details
For a DBT examination, the patient’s breast is positioned
the same way as it is in a conventional mammogram
procedure. The radiologist may specify what the focal
distance or location of the axis of rotation will be but
*The concept of lesion conspicuity is used to objectively quan-
tify radiographic observational error. It is defined as a ratio between lesion contrast and the surroundings. Experiments have
been done to determine whether this ratio or measure can be
well correlated with the probability of detecting faint lesions.
22