Optical Imaging - Correction Techniques in Emission Tomography - page 244

Using hybrid three-dimensional visualization, different contrast mechanisms
can be accurately superimposed, improving the information content available.
An additional important benefit of the hybrid approach is that it enables the
improvement of the performance and image quality of the optical method
by utilizing the anatomical information from X-ray CT to form a map of
tissue optical properties, or a map of tissue structural information, for input
in the physical model. This strategy can improve the accuracy of the optical
reconstruction problem [22, 1, 25].
This chapter summarizes several concepts important in fluorescence to-
mography. Section 12.2 outlines a common theoretical description of the phys-
ical model of light propagation through tissue, which is the basis for the re-
construction of the fluorochrome distribution. Section 12.3 gives an overview
of several existing fluorescence tomography systems and expected progress of
hybrid systems.

12.2 Fluorescence molecular tomography (FMT)
Optical tomography is generally based on a physical model of photon
propagation and therefore not only yields three-dimensional imaging but also
quantification of optical contrast. The different propagation regimes associ-
ated with optical imaging range from the ballistic regime (at low scattering
conditions), where light propagation is described by the laws of geometrical
optics, to the diffusive regime where the directivity is lost due to multiple
scattering events [33]. Photon propagation in scattering media can be accu-
rately described by the radiative transfer model [53, 49, 44, 5, 8, 7]. However,
instead of the radiative transfer model, the diffusion approximation is more
often used for biomedical imaging, due to the smaller degree of complexity
and less intensive computational efforts associated with it. In the following,
the diffusion model [33], as well as an approach for the reconstruction of a
fluorescent source inside a diffuse medium [31], is outlined.

12.2.1 Light propagation model

12.2.1.1 Photon interaction with biological tissue

Two interactions of light with tissue play a major role in optical imag-
ing: absorption and scattering of photons [13, 18, 26, 55, 57]. Absorption
occurs when a photon incident on a molecule has an energy corresponding to
the energy difference between two molecular orbital levels. The radiation field
transfers its energy to the molecule and an electron is excited from the ground
level to a higher-energy state; see Figure 12.3. Several secondary processes can
follow absorption, such as the emission of new photons or heat production.