Biomedical Engineering Reference
In-Depth Information
A technique for optical imaging of thick tissue is the frequency-domain technique, which
is based on photon-density waves. The governing equation of photon-density waves is the
diffusion equation
2
@Fð
r
,
t
Þ=
c
@
t
þ m
a
Fð
r
,
t
Þ
D
r
Fð
r
,
t
Þ¼
S
ð
r
,
t
Þ
ð
17
:
85
Þ
and time t [W/cm
2
], c is the speed of light in the
where
F
(
r
,t
) is the light fluence rate at point
r
m
a
is the absorption coefficient [cm
1
], and S(
medium [cm/s],
) is the source intensity
[W/cm
3
]. The frequency-domain imaging technique requires the use of inverse algorithms
for image reconstruction. Amplitude-modulated (at approximately 100 MHz) laser light is
used to illuminate the tissue at multiple sites. At each illumination site, an optical detector mea-
sures the amplitude and phase of diffuse light at multiple locations around the tissue. The
measured diffuse light can be estimated by the diffusion equation, Eq. (17.85), if the optical
properties of the tissue sample are known. Conversely, the optical properties can be calculated
by use of the measured diffuse light, which is the image reconstruction. A sample recon-
structed image is shown in Figure 17.24, which was based on a theoretically generated data
set of diffuse light.
,
t
r
17.7.2 Hybrid Optical Imaging
Several emerging imaging techniques are being developed by combining relatively trans-
parent acoustic energy with strongly scattering light, which is called hybrid optical imaging.
Ultrasound-modulated optical tomography, photoacoustic tomography, and sonolumines-
cence are briefly discussed.
20
18
16
14
12
10
8
6
4
2
2 4 6 8 101214161820
x [X0.2 cm]
0.099 0.101 0.103 0.106 0.108
Absorption coefficient
FIGURE 17.24
Sample reconstructed image using photon-density waves. A simulated tumor with elevated
absorption coefficient is visible in the lower right quadrant.
