Page 388 - Biomedical Engineering and Design Handbook Volume 2, Applications

366 DIAGNOSTIC EQUIPMENT DESIGN

Elastography is a technique whose goal is to characterize breast masses by measuring their elastic
properties under compression. Studies of excised breast specimens have demonstrated that while fat tis-
sue has an elastic modulus that is essentially independent of the strain level (the amount of compres-
sion), normal fibroglandular tissue has a modulus that increases by 1 to 2 orders of magnitude with
increasing strain (Krouskop et al., 1998). Furthermore, carcinomas are stiffer than normal breast tissue
at high strain level, with infiltrating ductal carcinomas being the stiffest type of carcinoma tested
(Krouskop et al., 1998). Using a specially constructed device containing a motor-driven cylindrical
specimen “indenter” and a load cell, Samani et al. measured the stress-strain curves of 169 ex vivo
breast tissue samples. They found that under conditions of small deformation, the elastic modulus of
normal breast fat and fibroglandular tissues are similar, while fibroadenomas were approximately twice
as stiff. Fibrocystic disease (a benign condition) and malignant tumours exhibited a three- to sixfold
increased stiffness, with high-grade invasive ductal carcinoma exhibiting up to a 13-fold increase in
stiffness compared to fibroglandular tissue (Samani et al., 2007).
Breast elastography can be performed with ultrasound (UE) or MRI (MRE). Hui et al. com-
pared the performance of UE, mammography, and B-mode ultrasound alone in differentiating
benign and malignant breast lesions. They found that the three modalities had approximately equal
sensitivity, but that the specificities of mammography (87 percent) and UE (96 percent) were
significantly better than that of US alone (73 percent) (Zhi et al., 2007). Other investigators have
measured increased specificity for UE compared to B-mode US, but with reduced sensitivity
(Thomas et al., 2006).

12.5 FUTURE DIRECTIONS

12.5.1 Multimodality imaging
There is a general consensus in the breast imaging community that no single imaging modality is
likely to be able to detect and classify early breast cancers, and that the most complete solution
for diagnostic breast imaging is likely to be some combination of complementary modalities.
However, again the unique properties of the breast create challenges for successfully merging the
information. In particular, the mechanically pliant nature of the breast permits optimization of
breast shape for the particular modality used (compressed for x-ray mammography, coil-shaped
for breast MRI, pendant for breast scintigraphy, etc.). The result is that multimodality image
fusion is extremely difficult. One approach to overcoming this problem is to engineer systems per-
mitting multimodality imaging of the breast in a single configuration. Toward this end, dedicated
breast scanners are being developed that integrate digital mammography and ultrasound (Sinha et
al., 2007; Surry et al., 2007), digital tomosynthesis and optical imaging (Boverman et al., 2007),
NIR spectroscopy and MRI (Carpenter et al., 2007), digital tomosynthesis and limited angle
SPECT (More et al., 2007), and breast CT and breast SPECT (Tornai et al., 2003). Figures 12.5
and 12.6 show corresponding structural and functional slices extracted from a dual modality data
set. The images were obtained on a dual modality tomographic (DMT) breast scanner developed
at the University of Virginia. The DMT scanner uses an upright mammography-style gantry arm,
with breast support and compression mechanisms that are independent of the gantry arm and sup-
port the breast near the arm’s axis of rotation (AOR). This design permits multiple-view, tomo-
graphic image acquisition for both modalities (x-ray transmission tomosynthesis and gamma
emission tomosynthesis). The DMT scanner employs full isocentric motion in which the tube and
x-ray and gamma ray detectors rotate around a common AOR. Figure 12.6 shows a series of slices
from a dual modality tomographic scan of a 7.9 cm compressed breast. The slices shown are 1 mm
thick and consecutive slices are spaced by 10 mm. The top row contains the x-ray tomosynthesis
images; the middle row contains the gamma emission tomosynthesis slices; and the bottom row
contains the merged slices. Biopsy indicated poorly differentiated carcinoma. The radiotracer was
99m Tc-sestamibi. This example illustrates both heterogeneous radiographic density and heteroge-
neous radiotracer uptake.