Biomedical Engineering Reference
In-Depth Information
M
ir
ror
Light
source
Splitter
Detector
Signal
processer
Computer
display
Tissue
Figure 8.12
Schematic diagram of the optic system of OCT.
An entire image is obtained by scanning the laser light over the entire area and
computing the above quantities at every location. The result is two images con-
taining reflectance information and distance (range) information. The reflectivity
profile, called an A-scan contains information about the spatial dimensions and
location of structures within the object of interest. A cross-sectional tomograph
(B-scan) is achieved by laterally combining a series of these axial A-scans. Enface
imaging at an acquired depth is possible depending on the imaging mode used. A
continuous beam is used, which is modulated in amplitude by a signal of much
larger wavelength than the laser radiation.
Instead of lasers, the use of infrared light waves enables improved penetration
through highly scattering structures, improved resolution by 10-100 (less than 100
μ
m) times, and imaging deeper tissues located several centimeters beneath trans-
parent structures and of a few millimeters in highly scattering media or tissue. Light
in the near-infrared range penetrates tissue and interacts with it in different ways;
the predominant effects are absorption and scattering. Many of the substances of
interest, such as hemoglobin and cytochromes, exhibit characteristic absorption
spectra that depend on whether the molecule is in its oxidized or reduced state.
An early tool of medical optical imaging was the
oximeter
devised in the 1930s to
detect the amount of oxygen in blood by measuring the ratio of the light absorbed
at two wavelengths. Great improvements to this concept came in the 1970s with
the advent of microprocessors and light-emitting diodes that permitted the use of
many more wavelengths, thus allowing measurement of the absolute amount of
oxygen and elimination of background effects. Assessment of the oxygen content
of arterial blood through such methods is a major diagnostic tool for monitoring
acutely ill patients. The potential of imaging with light was reinforced with the suc-
cessful application of optical tools to determine the levels of oxygen in the brain
of a cat. Later, this concept was used in monitoring brain and muscle oxygenation
in humans, as well as in other applications. Substances such as NAD/nicotina-
mide adenosine diphosphate (NADH) exhibit fluorescence properties that allow
for their detection after excitation by light. As these substances play crucial roles
in metabolic processes at the cell level, the ability to discern them through indirect
measurements has many medical implications.
