Biomedical Engineering Reference
In-Depth Information
of the concentration of a particular blood analyte. These indirect methods of measurement
are also simplified, and the actual final design to obtain an accurate and sensitive signal can
be quite complicated. Overall, the indirect fiber optic measurements typically have higher
specificity over direct measurement approaches but at the expense of requiring a more com-
plicated probe.
17.7 BIOMEDICAL OPTICAL IMAGING
Medical imaging has revolutionized the practice of medicine in the past century. Physi-
cians are empowered to "see" through the human body for abnormalities noninvasively and
to make diagnostic decisions rapidly, which has impacted the therapeutic outcomes of the
detected diseases.
Medical imaging dates back to 1895 when x-rays were discovered serendipitously by
German physicist W. C. Roentgen, who received the first Nobel Prize in physics in 1901 for
this important work. Human anatomy can be easily imaged by simple x-ray projections, such
as chest x-rays, that are still used today. Contemporary medical imaging began with the
invention of computerized tomography (CT) in the 1970s. G. N. Hounsfield in England pro-
duced the first computer reconstructed images experimentally, and A. M. Cormack in the
United States laid the theoretical foundation. Both of them were awarded the Nobel Prize
in medicine in 1979. The essence of CT is that if an object is viewed from a number of different
angles, then a cross-sectional image of it can be computed—that is, “reconstructed.”
The advent of CT has inspired other new tomographic and even 3D imaging techniques.
The application of reconstruction to conventional nuclear medicine imaging led to positron
emission tomography (PET) and single photon emission computed tomography (SPECT). A
similar application to the technique of nuclear magnetic resonance led to magnetic reso-
nance imaging (MRI).
Different imaging modalities are used to detect different aspects of biological tissues
through a variety of contrast mechanisms. X-ray imaging senses primarily electron density
and atomic number. In MRI, proton density and its associated relaxation properties are
detected. Ultrasonography images use acoustic impedance mismatches for contrast. In
nuclear imaging, nuclear radiation emitted from the body is detected after introducing a
radiopharmaceutical inside the body to tag a specific biochemical function.
The advantages and disadvantages of different imaging modalities may be illustrated
using breast cancer detection. Breast cancer is the most common malignant neoplasm and
the leading cause of cancer deaths in women in the United States. A means for prevention
of breast cancer has not been found, and early detection and treatment are the best solutions
to improving the cure rate. At present, x-ray mammography and ultrasonography are clini-
cally used for breast cancer detection. Mammography is currently the only reliable means of
detecting nonpalpable breast cancers. As a supplementary tool, ultrasound is used to eval-
uate the internal matrix of circumscribed masses found using mammography or of palpable
masses that are obscured by radiographically dense parenchyma using mammography.
However, x-ray mammography is ionizing radiation, and imaging of radiographically
dense breasts is difficult. Ultrasonography cannot detect many of the nonpalpable cancers
that are not visible on mammograms of good quality.
