Biomedical Engineering Reference
In-Depth Information
tissues and cannot propagate over a long distance in a straight line. However,
the light interacts with the atoms and molecules, of which the tissue consists
through the energy level and/or the vibration energy level, and informs us
about the live state of the tissues. By combining both of these advantageous
characteristics, new techniques for medical diagnostics, surgical operations,
and biopsy can be pioneered.
In aiming at these applications, optoacoustic tomography and sonolumi-
nescence imaging have been applied to the observation of tissue for the past
several decades. In optoacoustic tomography, the modulating light irradiated
to the tissues is converted to ultrasound waves at the location of the opti-
cal absorbers, because the heated regions due to light absorption produce
expansion and shrinking of the local volumes, which generates compression
waves. Observing these waves, the optical information, such as the complex
refractive index, in the tissue can be obtained. In contrast with tomogra-
phy, light is generated locally at the tissues by means of irradiating with
focused ultrasound in sonoluminescence, and the resultant light intensity is
observed. Among the research projects using ultrasound wave and light, new
ultrasound-assisted optical imaging has been developed recently as a good
candidate observing the inside of the tissues [31-35]. In this section, one of
these new imaging methods is described [31].
2.4.2
Ultrasound-Assisted Optical Imaging
In ultrasound-assisted optical imaging, the light scattered in the tissues is
tagged locally with an ultrasound wave to distinguish it from the other ma-
terials. A converging pulsed-ultrasound wave and a continuous light are used
to irradiate a tissue. The ultrasound wave, which interacts with light via the
density change in the tissue, modulates the density of the material at the in-
stantaneous location: meanwhile, the light is strongly scattered by the tissue,
consisting of the sum of small components in a cell. The modulation changes
the optical scattering eciency according to the dependence of the optical
property, because the path of the scattering for the incident light is changed
by the change of the eciency. As the pulsed ultrasound wave propagates
through the sample, the temporal intensity of the light transmitted through
the tissues is changed. Through the observation of this change, optical infor-
mation such as the complex refractive index can be obtained.
2.4.3
The Experimental Setup
Figure 2.24 shows the experimental setup of the ultrasound-assisted cross-
sectional imaging system. A specially shaped ceramic ultrasound transducer
generates converging pulsed ultrasound waves. A laser beam from a He-
Ne laser (632.8 nm wavelength, 0.5 mW output power, and 1.0 mm beam
diameter) is irradiated to the sample in the focal region of the converging
ultrasound wave, which means that the ultrasound wave and the light are
