DOI:10.2214/AJR.07.2403
AJR 2007; 189:1312-1315
© American Roentgen Ray Society
Dedicated Cone-Beam Breast CT: Feasibility Study with Surgical Mastectomy Specimens
Wei Tse Yang1,
Selin Carkaci1,
Lingyun Chen2,
Chao-Jen Lai2,
Aysegul Sahin3,
Gary J. Whitman1 and
Chris C. Shaw2
1 Department of Diagnostic Radiology, The University of Texas M. D. Anderson
Cancer Center, 1515 Holcombe Blvd., Unit 1350, Houston, TX 77030.
2 Department of Imaging Physics, The University of Texas M. D. Anderson Cancer
Center, Houston, TX 77030.
3 Department of Pathology, The University of Texas M. D. Anderson Cancer Center,
Houston, TX 77030.
Received April 12, 2007;
accepted after revision June 24, 2007.
Address correspondence to W. T. Yang
(wyang{at}di.mdacc.tmc.edu).
Presented in part at the 2005 San Antonio Breast Cancer Symposium and the
2007 annual meeting of the American Roentgen Ray Society, Orlando, FL.
Supported by research grants EB-00117 from the National Institute of
Biomedical Imaging and Bioengineering and CA104759 from the National Cancer
Institute.
Abstract
OBJECTIVE. The purpose of this study was to investigate the
feasibility of diagnostic breast imaging using a flat-panel detector-based
cone-beam CT system.
CONCLUSION. Imaging of 12 mastectomy specimens was performed at
50–80 kVp with a voxel size of 145 or 290 µm. Our study shows that
cone-beam breast CT images have exceptional tissue contrast and can
potentially reduce examination time with comparable radiation dose.
Keywords: breast neoplasm • CT radiography • mastectomy • radiation dose • specimen radiography
Introduction
Mammography is an important tool for the screening, diagnosis, and
management of breast cancers. The effectiveness of mammography is compromised
by fundamental problems including radiation scatter, noise, and the
overlapping of cancers with breast anatomy. Tomosynthesis may partially
overcome this limitation, but its image quality suffers from crude depth
resolution and associated artifacts
[1]. Cone-beam CT, on the other
hand, can provide true 3D breast images with isotropic resolution (145 µm
or smaller) and radiation dose comparable to two-view mammography
[2]. Conventional fan-beam CT
applied to breast cancer imaging in the 1970s suffered from limitations
including high patient dose, low spatial resolution, long scanning time, large
slice thickness, and cardiac and respiratory motion
[3,
4]. The advantages of cone-beam
CT include a flat-panel digital detector, true 3D images with isotropic
resolution, reduced motion artifacts, breast-only exposure to radiation,
greater efficiency in use of the X-ray beam, no overlapping structures in the
breast, and high contrast resolution. We constructed a flat-panel
detector-based cone-beam CT system to investigate the feasibility for
dedicated breast imaging
[5].
Materials and Methods
The experimental system consists of a general radiography tube pointing at
a 30 x 40 cm amorphous silicon (a-Si/CsI) flat-panel digital detector
(Paxscan 4030CB, Varian Medical Systems). A motor-driven rotation stage is
used to position and rotate the specimen to simulate dedicated breast CT in
which the patient would lie on a table in the prone position with one breast
drawn downward through an opening to allow the X-ray tube and detector to
rotate around and scan the breast beneath the table (Fig.
1A,
1B).
A total of 12 mastectomy specimens were acquired fresh from the pathology
laboratory in our institution with institutional review board approval. Each
specimen was placed in a receptacle formulated from an inverted soda bottle,
and the holder was placed on the rotation stage. Scanning was performed at
50–80 kVp with reconstructed voxel size of 145 or 290 µm. The
exposures in air at the isocenter of the cone-beam CT system were measured
with a pencil-probe ion chamber, resulting in calculated dose levels
equivalent to 6–24 mGy for the breast, which correspond to 1–4
times the mean glandular dose limits for two-view mammograms of a 5-cm-thick
compressed breast.
Results
The mean scanning time was 12 seconds for low-resolution (binning) mode,
which was adequate for visualizing tissue structures, and 48 seconds for
high-resolution (nonbinning) mode, necessary for visualizing small
calcifications. Artifacts encountered from high-density metallic tumor marker
coils and surgical clips were removed or reduced by postprocessing techniques
without compromising tissue detail (Fig.
2A,
2B). Structured noise was
minimal because of the absence of overlapping tissue. Breast anatomy was well
resolved on all images as skin, adipose, and glandular regions. Image noise
was visible but low compared with the tissue contrast. Microcalcifications
within cancers were clearly shown. The detection of cancers based on
morphologic assessment of tissue structures may be improved compared with
mammography because of the lack of overlapping glandular tissue. Cancer was
visualized as an irregular spiculated mass with associated microcalcifications
or overlying skin thickening (Figs.
3A,
3B,
3C and
4) or an area of focal
asymmetry. Multifocal breast cancer was shown on coronal slices of one of the
mastectomy specimens as small irregular masses. Benign lesions including
simple cysts were identified on CT as circumscribed oval isodense masses.
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Fig. 3A —51-year-old woman with invasive ductal carcinoma of left
breast occupying area of 6 x 5 cm. Coronal (A) and axial
(B) CT scans of left breast show irregular mass with spiculated margins
(arrows).
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Fig. 3B —51-year-old woman with invasive ductal carcinoma of left
breast occupying area of 6 x 5 cm. Coronal (A) and axial
(B) CT scans of left breast show irregular mass with spiculated margins
(arrows).
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Fig. 3C —51-year-old woman with invasive ductal carcinoma of left
breast occupying area of 6 x 5 cm. Transverse sonogram shows irregular
solid hypoechoic mass in left retroareolar position with angular margins and
dense posterior acoustic shadowing (arrowheads).
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Fig. 4 —63-year-old woman with invasive ductal carcinoma of left
breast. Coronal CT image shows microcalcifications within area of
architectural distortion representing known cancer (arrows).
Pathology showed ductal carcinoma in situ associated with
microcalcifications.
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Discussion
Conventional screening mammography produces 2D images with limited contrast
and spatial resolution. Therefore, superimposed parenchymal structures can
obscure and prevent the detection of small cancers. Cone-beam CT overcomes the
problem of overlapping tissue by generating 3D images and is a promising
technique in diagnosing tumors at an early stage
[6]. Studies are ongoing that
focus on improving this new technology in breast imaging and investigating its
potential role in screening women with a family history of breast cancer or a
genetic predisposition, most of whom have dense breasts
[7].
The rationale in choosing a specific scanning technique is governed by
general radiologic imaging principles. Higher resolution and greater tissue
detail are achieved at higher patient dose, whereas lower radiation dose leads
to lower resolution images. The selection of exposure technique is a
compromise between patient dose and resolution. Boone et al.
[7] showed that high tissue
contrast can be achieved with an isocenter dose of 6 mGy for a 10-cm-diameter
breast. Lai et al. [8] reported
that the visibility of microcalcifications with a size of 348, 288, and 257
µm can be achieved with a radiation dose of 3, 6, and 12 mGy, respectively,
for a 10-cm-diameter breast. The spatial resolution of the images obtained
with our cone-beam CT system is 140 µm, providing potentially greater
detail and higher resolution when compared with tomosynthesis. This reflects
inherent differences between both imaging techniques. Cone-beam CT renders a
true 3D acquisition, whereas tomosynthesis provides a projection image. A
direct comparison between cone-beam CT and tomosynthesis would be useful in
evaluating the relative merits of each method.
Imaging a single breast with each acquisition may not be efficient use of
patient and scanning time. However, dedicated breast CT includes more tissue
and better optimizes the resulting image quality when imaging one breast.
Imaging both breasts simultaneously may compromise the image quality and
ability of the system to accommodate more tissue.
There is no theoretic limit in imaging large breasts. However, larger
breasts would require higher radiation dose (mAs and kVp) to penetrate thicker
tissue. The ability of the current design of dedicated cone-beam CT to
visualize and evaluate deeper portions of the breast including the axilla may
be limited because of the prone nature of the study and patient positioning.
Various different designs and positioning techniques are being investigated to
resolve this issue. A possibility is to use the "arm through hole"
technique, which is widely practiced when performing stereotactic biopsies of
lesions that are close to the chest wall.
An expected advantage of cone-beam breast CT is in imaging the
postoperative breast with deformity, in which it is expected that a true 3D
acquisition will overcome problems of overlapping structures, overlying skin
thickening, scarring with resultant obscuration of masses, or the simulation
of pseudomasses.
Loss of data with beam hardening from clips in patients with prior surgery,
after biopsy, and with tumor markers is a problem with the current technique
and may require modification with a different algorithm to correct beam
hardening artifacts from clips. Such algorithms have been developed by our
institution [9] and other
groups to minimize the artifacts and obtain slice images of reasonable
quality.
The expected radiation dose to structures within the chest as a result of
scatter when using cone-beam breast CT has not been studied but is expected to
be significant near the breast, with a sharp decreasing gradient within the
first 2 cm from the breast because of attenuation by tissues in the chest
immediately outside the field of the X-ray beam. We expect that the potential
value of MDCT in breast imaging is limited because the entire chest is
included in the field of view when using this technique, resulting in image
quality degradation from higher scatter and lower resolution. Furthermore,
structures within the chest will be subjected to unnecessary exposure and
increased dose from direct exposure and scattered radiation.
The anticipated number of images that the cone-beam CT mammogram will
contain is dependent on breast size. There may be between 700 and 1,200 slice
images available for review of a breast ranging in size from 10 to 17 cm thick
(nipple to chest wall). Although this number may seem daunting, it is expected
that current monitor viewing facilities and the opportunity to use multiplanar
reconstructions and displays will overcome the potential negative impact on
radiologist productivity.
Our preliminary study, concordant with the preliminary results in the
literature, shows that cone-beam CT of the breast can potentially reduce
examination time with comparable radiation dose, eliminates the need for
compression and additional workup views that are routine with mammography, and
shows exceptional tissue contrast. The potential uses of cone-beam CT can be
extended to aid imaging-guided surgery and interventional procedures including
biopsy or needle localization
[7]. Further study of cone-beam
breast CT with larger series is needed to better understand its potential role
in screening, diagnosis, and management of breast cancers.
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Abstract
Introduction
