Biomedical Engineering Reference
In-Depth Information
In ultrasound-modulated optical tomography, an ultrasonic wave is focused into a scat-
tering medium to modulate the laser light passing through the medium containing buried
objects. The modulated laser light collected by a photodetector is related to the local
mechanical and optical properties in the zone of ultrasonic modulation. If the buried objects
have optical properties that are different from those of the background scattering medium,
an image can be obtained by raster-scanning the device.
In photoacoustic tomography, a short-pulse light beam illuminates the scattering
medium. The light is diffused in the medium and partially absorbed. If there is a strong
optical absorber such as a tumor in the middle of the medium, more light will be absorbed
by this optical absorber than by its neighboring background. The absorbed optical energy is
converted into heat. Due to thermal elastic expansion, an acoustic wave is generated. Stron-
ger heat generation will produce a stronger acoustic wave. Therefore, a strong optical
absorber emanates a strong acoustic wave. If multiple acoustic transducers are used to
measure the acoustic signal around the medium, the absorber can be located based on the
temporal distribution of the acoustic signals and thus produce an image of the medium.
Functional and molecular imaging has been achieved.
The inverse of photoacoustic tomography is sonoluminescent tomography (SLT). The
ultrasonic generation of light known as sonoluminescence (SL) was first reported in 1934,
which was multiple-bubble sonoluminescence (MBSL). SL has attracted an extraordinary
amount of attention in this decade, since single-bubble sonoluminescence (SBSL) was
reported in 1990. Although the full explanation of SL is still in development, it is well
known that light is emitted when tiny bubbles driven by ultrasound collapse. The bubbles
start out with a radius of several microns and expand to about 50 microns due to a decrease
in acoustic pressure in the negative half of a sinusoidal period. After the sound wave
reaches the positive half of the period, the situation rapidly changes. The resulting pressure
difference leads to a rapid collapse of the bubbles accompanied by the emission of light.
The flash time of SL has been measured to be in the tens of picoseconds. SBSL is so bright
that it can be seen by the naked eye even in a lighted room, whereas MBSL is also visible in
a darkened room. Researchers have envisioned possible applications of SL in sonofusion,
sonochemistry, and building ultrafast lasers using the ultrafast flash of light in SL.
SLT is an application of SL that has been developed for cross-sectional imaging of strongly
scattering media noninvasively. Sonoluminescence, which is generated internally in the
medium by exposure of the medium to external ultrasound, is used to produce images of a
scattering medium by raster-scanning the medium. The spatial resolution is limited by the
focal spot size of the ultrasound and can be improved by tightening the focus.
Optical imaging techniques may also measure the optical spectra of biological tissues
noninvasively. Besides the absorption spectra that are usually measured for other materials,
the scattering spectra may also be measured. The optical spectra may be used to quantify
important physiological parameters such as the saturation of hemoglobin oxygen.
In summary, optical imaging and biomedical optics techniques in general have the
advantages of (1) the use of nonionizing radiation, (2) the capability of measuring func-
tional (physiological) parameters, (3) the potential high sensitivity to the pathologic state
of biological tissues, and (4) low cost. However, biomedical optics is a challenging research
field in its infancy, and as it grows, it will continue to require the participation of many
diverse talented physicians, scientists, and engineers.
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