Biomedical Engineering Reference
In-Depth Information
Several other techniques are under investigation for breast cancer imaging. Magnetic res-
onance imaging (MRI) offers great promise for imaging of the radiographically dense
breast. Breast MRI is superior to mammography in differentiating solid from cystic lesions
and is equivalent to mammography in providing information regarding different parenchy-
mal patterns. Injection of intravenous contrast material with MRI increases cancer detect-
ability even though breast cancer and glandular tissues have similar magnetic resonance
tissue characteristics. However, breast MRI is expensive, has inferior spatial resolution to
mammography, and cannot image microcalcifications. Breast CT has been investigated for
the differentiation of benign from malignant solid masses. Because breast CT involves the
use of intravenous injection of iodinated contrast material and is expensive, it is not suited
for routine breast cancer screening.
17.7.1 Optical Tomographic Imaging
Nonionizing optical tomography is a new and active research field, although projection
light imaging was investigated as early as 1929. The optical properties of normal and dis-
eased tissues are usually different despite the large variation of values in optical properties
of the normal tissues alone. Therefore, it is possible to detect some breast cancers based on
measurements of optical properties.
The optical difference is not surprising because cancerous tissues manifest significant
architectural changes at the cellular and subcellular levels, and the cellular components that
cause elastic scattering have dimensions typically on the order of visible to near-IR wave-
lengths. Some tumors are associated with vascularization, where blood causes increased
light absorption. The use of optical contrast agents may also be exploited to enhance the
optical contrast between normal and abnormal tissues. Because the optical information is
determined by the molecular conformations of biological tissues, optical imaging is
expected to provide sensitive signatures for early cancer detection and monitoring.
Because tissues are optically turbid media that are highly scattering, light is quickly dif-
fused inside tissues as a result of frequent scattering. The strong scattering has made optical
detection of biological tissues challenging. A typical scattering coefficient for visible light in
biological tissues is 100 cm
1
in comparison with 0.2 cm
1
for x-rays used in medical diag-
nostics. Light transmitted through tissues is classified into three categories: ballistic light,
quasi-ballistic light, and diffuse light. Ballistic light experiences no scattering by tissue
and thus travels straight through the tissue. Ballistic light carries direct imaging information
just as x-ray radiation does. Quasi-ballistic light experiences minimal scattering and carries
some imaging information. Multiply-scattered diffuse light carries little direct imaging
information and overshadows ballistic or quasi-ballistic light in thick biological tissue.
One of the techniques used for optical tomography is called “early-photon imaging.” If
diffuse light is rejected, and ballistic or quasi-ballistic light is collected, buried objects can
be detected much like x-ray projection. This technique uses a short-pulse laser (
1 ps pulse
width) to illuminate the tissue. Only the initial portion of transmitted light is allowed to
pass to a light detector, and the late-arriving light is gated off by a fast optical gate. Because
the ballistic or quasi-ballistic photons travel along the shortest path length, they arrive at
the detector sooner than diffuse photons. If only ballistic light is detected, the technique
is called ballistic imaging. It has been shown that ballistic imaging is feasible only for tissue
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