1.1.   MOTIVATION FOR OPTICAL IMAGING

The most common medical imaging modalities include X-ray radiography, ultrasound
imaging (ultrasonography), X-ray computed tomography (CT), and magnetic
resonance imaging (MRI). The discovery of X rays in 1895, for which
Roentgen received the first Nobel Prize in Physics in 1901, marked the advent of
medical imaging. Ultrasonography, which is based on sonar, was introduced into
medicine in the 1940s after World War II. The invention of CT in the 1970s, for
which Cormack and Hounsfield received the Nobel Prize in Medicine in 1979,
initiated digital cross-sectional imaging (tomography). The invention of MRI,
also in the 1970s, for which Lauterbur and Mansfield received the Nobel Prize
in Medicine in 2003, enabled functional imaging with high spatial resolution.
Optical imaging, which is compared with the other modalities in Table 1.1, is
currently emerging as a promising new addition to medical imaging.

Reasons for optical imaging of biological tissue include

1. Optical photons provide nonionizing and safe radiation for medical applications.
2. Optical spectra—based on absorption, fluorescence, or Raman scattering—
   provide biochemical information because they are related to molecular
   conformation.
3. Optical absorption, in particular, reveals angiogenesis and hypermetabolism,
   both of which are hallmarks of cancer; the former is related
   to the concentration of hemoglobin and the latter, to the oxygen saturation
   of hemoglobin. Therefore, optical absorption provides contrast for
   functional imaging.
4. Optical scattering spectra provide information about the size distribution
   of optical scatterers, such as cell nuclei.
5. Optical polarization provides information about structurally anisotropic
   tissue components, such as collagen and muscle fiber.
6. Optical frequency shifts due to the optical Doppler effect provide information
   about blood flow.
7. Optical properties of targeted contrast agents provide contrast for the
   molecular imaging of biomarkers.
8. Optical properties or bioluminescence of products from gene expression
   provide contrast for the molecular imaging of gene activities.
9. Optical spectroscopy permits simultaneous detection of multiple contrast
   agents.
10. Optical transparency in the eye provides a unique opportunity for high-
    resolution imaging of the retina.