Key words breast - breast tumor - mammographic density
Schlüsselwörter Mamma - Mammakarzinom - mammografische Dichte
Introduction
The development of digital image receptor systems in mammography has progressed to
such an extent over the past few years that, in addition to their conventional application,
digital mammography systems can now also be used as a platform for further, new examination
methods such as contrast mammography or tomosynthesis [1 ], [2 ], [3 ], [4 ], [5 ], [6 ].
Digital breast tomosynthesis (DBT) has the potential to remove undesirable masking
as a result of superimposed layers and thus reduces false-positive or false-negative
examination findings. Whether DBT can be regarded as an alternative to digital mammography
or whether it is just an additional examination method in assessment diagnostics is
as yet not clear [7 ], [8 ], [9 ], [10 ], [11 ], [12 ], [13 ], [14 ].
A novel method comprising 2 stereo mammography images (stereoscopy) ([Fig. 3 ]) using linearly polarised filter glasses produces a 3-dimensional (3D) overall image
(visualisation) of the breast.
Fig. 1 Wisconsin RMI Phantom, Model 152 A, Radiation Measurements, Middleton, WI, USA. 16
wax blocks with 4 round lumps, 5 calcifications, 6 threads, 1 empty, raised radiolucent
position 4.5 cm, net density 1.5.
Fig. 2 a to c Stereoscopy. The distance between the human eyes is roughly 65 mm, i.e. each eye
has a different view of the three-dimensional world. Through these 2 different viewpoints
of a virtual system we are able to determine the relative depth of different objects
in the object viewed, because the 2 independent images are combined within the brain
to determine the depth (a and b ). This process is used in photography by making 2 standard images (0° and 4° projection)
in order to simulate natural sight (c ).
Fig. 3 a to c 3D digital full field mammography. Monitoring with mammographies in the c-c view
and c-c view with an angle of plus 4° (stereoscopy), respectively (a and b ) with the use of linearly polarised filter glasses in order to obtain a holistic
3D image of the breast (c ).
The aim of the pilot study was to examine the detection of simulated mammographic
lesions (micro-calcifications and tumour-like masses) with these novel mammography
methods and to contrast these with the findings from another, already established
mammography system.
Material and Method
For the examinations the Amulet digital mammography system (FujiFilm, Tokyo, Japan)
was used. This system operates with a 24 × 30 cm2 detector which is firmly integrated into the system [15 ], [16 ]. This totally novel detector is constructed from two superimposed layers of high-purity
amorphous selenium, separated by a very thin layer of selenium that is just 1 µm thick
and has been doped in a targeted manner with foreign atoms. This detector achieves
a pixel size of 50 µm2 .
The mammography system is fitted with a bimetal x-ray tube, which offers the options
of molybdenum or tungsten as anode material in combination with molybdenum or rhodium
filters. Within the scope of the present investigation all images were made with a
manually set molybdenum/molybdenum combination and an x-ray current-time product of
100 mAs at a tube voltage of 30 kV ([Table 1 ]).
Table 1 Parameters of the mammography system during examination.
Amulet
Manufacturer
FujiFilm
Anode
Mo, W
Filter
Mo, Rh
Scattered radiation grid
linear
Conversion material
Semiconductor
Sampling process
Optically induced sampling
Pixel size
50 µm
Spatial resolution (Nyquist Frequency)
10 lp/mm
Field size
24 × 30 cm2
We initially took 3 different images (image pairs) in c-c and ml views (2D) and in
the c-c view and c-c view with an angle of plus 4° (stereoscopy), respectively.
As in earlier studies [17 ] the Wisconsin Mammographic Random Phantom (Model 152 A, Gammex Inc.) was selected
as test object ([Fig. 1 ]). This phantom contains a total of 5 elements with microcalcifications, 6 with thread-like
structures and 4 with tumour-like masses. These 15 simulated lesions are located in
15 separate wax blocks in the phantom; in addition one empty wax block without lesions
is present. The wax blocks are interchangeable, so that different distribution patterns
in the phantom can be realised. A learning effect can thus be avoided for the evaluators.
The complete phantom recreates a standard breast with a compression layer thickness
of approx. 4.5 to 5 cm.
The evaluation of the phantom image was performed on a 2.5 × 2.5 k monitor by five
radiologists with varying years of mammographic experience. Each radiologist was shown
3 image pairs (images in c-c and ml views) (2D) with different phantom compositions
and in the c-c view and c-c view with an angle of plus 4°, respectively, using linearly
polarised glasses (3D) for evaluation. In linearly polarised filter glasses the light
is polarised linearly, i.e. the light oscillates within a plane determined by the
filter. Here the filters must be positioned at a right angle to one another for the
left and right viewing position in order to enable the separation of the two views
and to achieve a 3D visualisation ([Figs. 2 ] and [3 ]). The viewing time per image was limited to a maximum of 5 minutes. As part of the
study the detection sensitivity with the new mammography method was to be evaluated
– for this reason the correct
positive rate for each individual lesion type was determined.
Findings
[Table 2 ] lists the findings of the study determined by the 5 evaluators, broken down according
to the digital mammography system used, the different examination methods (2D and
3D respectively) and the lesion type. Additionally, the number of lesions actually
present and the resulting detection rate, as well as the mean finding determined on
the basis of the three lesion types, are given.
Table 2 Number of correctly detected simulated lesions using the digital mammography system
and the detection rate in per cent averaged over the five evaluators, each three phantom
configurations (c-c and ml plane [2D] and c-c and c-c plus 4° plane, respectively
[3D]).
Amulet
Amulet
Really existing lesions
Threads
18.0 (100 %)
18.0 (100 %)
18
Microcalcifications
14.3 (95.6 %)
14.3 (95.6 %)
15
Tumour-like masses
12.0 (100 %)
12.0 (100 %)
12
All lesions
44.4 (98.7 %)
44.4 (98.7 %)
45
With the digital mammography system and the dual layer selenium detector a detection
rate of 97.7 % was computed both for each of the 3 image pairs (images in the c-c
and ml views) (2D) with differing phantom composition and the 3 image pairs in the
c-c view and c-c plane with an angle of plus 4° (stereoscopy), respectively, using
linearly polarised filter glasses for visualisation (3D).
Discussion
The most important image-producing method for the early diagnosis of breast cancer
remains x-ray mammography. It is the only method with proven use as a quality-assured
screening method to lower the breast cancer mortality rate. Full-field digital mammography,
or FFDM, is today regarded as the standard mammographic method, both in curative mammography
and in the preventive mammographic screening of women without any symptoms and, in
particular, is more effective for the detection of pathological findings in women
with dense breasts than standard film foil mammography [1 ], [2 ], [3 ], [4 ], [5 ], [6 ]. Nonetheless, as with all radiological projection methods, digital mammography suffers
from the fact that it depicts three-dimensional information as a two-dimensional image.
Superimposed structures are projected onto one
image plane, so that lesions of clinical relevance can easily be covered and their
viewing obstructed by overlapping tissue. This increases the frequency of false-negative
examination findings, i.e. existing carcinomas are overlooked. Such overlapping of
normal breast tissue may also result in false-positive examination results, in that
they mock malign lesions which then lead to the patient having to undergo an unnecessary
repeat examination and may even result in an unnecessary biopsy.
The development of digital image receptor systems in mammography has progressed to
such an extent over the past few years that, in addition to their conventional application,
digital mammography systems can now also be used as a platform for further, new examination
methods such as contrast mammography or tomosynthesis. Digital breast tomosynthesis,
or DBT, is an imaging technology which can deliver layered images free from overlaps
on the basis of a limited number of individual images taken at different projection
angles. The exposure parameters for each individual layer are selected in such a way
that the radiation exposure resulting from all images taken is preferably lower than
the radiation dose from a 2-plane mammography. With the aid of different reconstruction
algorithms the breast is subsequently visualised in the layers of interest at various
depths parallel to the detector surface.
Digital breast tomosynthesis has the potential to remove undesirable masking as a
result of superimposed layers and thus reduces false-positive or false-negative examination
findings. Whether DBT can be regarded as an alternative to digital mammography or
whether it is just an additional examination method in assessment diagnostics is as
yet not clear [7 ], [8 ], [9 ], [10 ], [11 ], [12 ], [13 ], [14 ].
A novel method on the basis of 2 mammography images (c-c and c-c plus 4°) (stereoscopy)
([Fig. 2 ]) has been developed in order to obtain a three-dimensional overall image of the
breast (3D) ([Fig. 3 ]) with the use of linearly polarised filter glasses.
The present study is the first phantom study based on a digital mammography system
([Table 1 ]) to compare the detection of simulated microcalcifications, thread-like structures
and tumour-like masses in 2D and 3D visualisation ([Fig. 1 ]) [15 ], [16 ]. The detected findings listed in [Table 2 ] do not, however, show any differences. While the simulated microcalcifications could
be detected with an efficiency of up to 95.6 % with both visualisation methods – 100 %
of the thread-like structures and tumour-type masses were detected. This shows that
the phantom used is at least sufficient for a rough orientation, but is not sufficient
for a differentiated evaluation of powerful digital imaging systems. This is also
confirmed by the fact that, in terms of the phantom examination, no great differences
in the detection rates occurred despite the
great discrepancy in experience between the 5 evaluators (from 3 months to 5 years).
Despite all limitations, the present findings with the Wisconsin Mammographic Random
Phantom ([Fig. 1 ]) [17 ] must be evaluated as meeting all necessary minimum requirements. They cannot furnish
an adequate basis for unlimited clinical application of the new system. Such conclusive
findings could only be furnished by a comprehensive study on the contrast resolution
capability with the currently valid national and international standard phantom CDMAM
Phantom [18 ], [19 ]. However, even with this test specimen, disregarding any anatomical noise only objects
in front of a homogenous background can be detected. Furthermore, newly developed
phantom systems exist [20 ], [21 ], the final findings of which are yet to be determined.
The new method for the three-dimensional representation of the breast in one holistic
image with the use of linearly polarised filter glasses represents an important innovation,
in particular in comparison with tomosynthesis [22 ]. In addition, at the same time it is the deciding step towards a real, so to speak,
online 3D visualisation of the breast in future made available at any desired workplace
(PACS). To this end, besides the respective software, monitors with a resolution of
at least 3.5 × 3.5 k are necessary, making additional aids (e.g. glasses) obsolete.
Developments in this respect are still in the experimental stage. Whether an improvement
in the detection rate of examination findings to be clarified (correct positive findings)
while at the same time reducing false-negative findings – also by using computer-aided
diagnostic systems (CAD) – can actually be achieved, remains to be seen. However,
the prospect of real 3D-controlled
intervention and planning oncology management, including plastic surgery, appears
to be promising.
