268
T. Inagaki and S. Tsuchikawa
11.2 Measuring Apparatus
For the detection of DTOF data of samples, measuring system should contain 1. laser
source, 2. photodetector, and 3. time-resolved system. Torricelli et al. summarized
[4] the evolution history of TR spectroscopic components, i.e., in the first generation since the early 1990s, the laser from gas, dye, or solid state laser was used as
light source and detected DTOF profiles using microchannel plate photomultiplier
(PMT) with electronic chain for time-correlated single photon counting (TCSPC)
with NIM module. The TR spectroscopic components at the second generation,
2000–2010, semiconductor laser heads with external RF driver was used as light
source and detected by compact metal channel dynode PMT with TCSPC electronic
board system. Now it is possible to use the supercontinuum fiber laser with powerful
emission with broad wavelength range as light source and detected by hybrid PMT
with time-to-digital converters module with USB controller. The distance from light
injection points to receiver (ρ in Fig. 11.1) should be optimized according to the range
of μ a and μ
s of the samples. Estimated value of attenuation is in the order of 10
6 –10
8 ,
and temporal dynamic span is over a 1–10 nm range when the sample has general
optical properties in the NIR region, i.e., μ a = 0 − 0.05 cm
−1 , μ
s = 5 − 25 cm
−1
and ρ = 1 − 3 cm.
11.3 Data Analysis
After obtaining the time-resolved light intensity signal, μ a and μ
s are estimated by
fitting the TR data obtained with the analytical solution of the diffusion equation by
the nonlinear inverse algorithm. Convolution between the theoretical TR reflectance
with the IRF is calculated at first in order to take the broad shape of the IRF and then
used to fit the experimental TR reflectance curve. As mentioned in introduction, solution of diffusion equation can be applied only if the scattering is dominant compared
to absorption in media, and radiance is detected at a sufficient larger distance from
the injection point, i.e., distance should be much longer than one mean free path
1/μ
. MC simulation is also used to model the light propagation in biological tissue
because of its flexibility and simplicity to simulate photon propagation processes
in arbitrary shapes with complex boundary conditions or spatial localization. For
that simulation, the photon propagation in the turbid medium is traced until it exits
the sample surface or is absorbed. The movement of photons from one photon tissue
interaction to the next photon tissue interaction is described by a probability function
using the optical properties of the tissue. Repeat these processes for a large number
of photons to estimate the photon distribution in the tissue. Detailed explanation for
the data analysis with fitting or MC simulation can be found in book [8].
T. Inagaki and S. Tsuchikawa
11.2 Measuring Apparatus
For the detection of DTOF data of samples, measuring system should contain 1. laser
source, 2. photodetector, and 3. time-resolved system. Torricelli et al. summarized
[4] the evolution history of TR spectroscopic components, i.e., in the first generation since the early 1990s, the laser from gas, dye, or solid state laser was used as
light source and detected DTOF profiles using microchannel plate photomultiplier
(PMT) with electronic chain for time-correlated single photon counting (TCSPC)
with NIM module. The TR spectroscopic components at the second generation,
2000–2010, semiconductor laser heads with external RF driver was used as light
source and detected by compact metal channel dynode PMT with TCSPC electronic
board system. Now it is possible to use the supercontinuum fiber laser with powerful
emission with broad wavelength range as light source and detected by hybrid PMT
with time-to-digital converters module with USB controller. The distance from light
injection points to receiver (ρ in Fig. 11.1) should be optimized according to the range
of μ a and μ
s of the samples. Estimated value of attenuation is in the order of 10
6 –10
8 ,
and temporal dynamic span is over a 1–10 nm range when the sample has general
optical properties in the NIR region, i.e., μ a = 0 − 0.05 cm
−1 , μ
s = 5 − 25 cm
−1
and ρ = 1 − 3 cm.
11.3 Data Analysis
After obtaining the time-resolved light intensity signal, μ a and μ
s are estimated by
fitting the TR data obtained with the analytical solution of the diffusion equation by
the nonlinear inverse algorithm. Convolution between the theoretical TR reflectance
with the IRF is calculated at first in order to take the broad shape of the IRF and then
used to fit the experimental TR reflectance curve. As mentioned in introduction, solution of diffusion equation can be applied only if the scattering is dominant compared
to absorption in media, and radiance is detected at a sufficient larger distance from
the injection point, i.e., distance should be much longer than one mean free path
1/μ
. MC simulation is also used to model the light propagation in biological tissue
because of its flexibility and simplicity to simulate photon propagation processes
in arbitrary shapes with complex boundary conditions or spatial localization. For
that simulation, the photon propagation in the turbid medium is traced until it exits
the sample surface or is absorbed. The movement of photons from one photon tissue
interaction to the next photon tissue interaction is described by a probability function
using the optical properties of the tissue. Repeat these processes for a large number
of photons to estimate the photon distribution in the tissue. Detailed explanation for
the data analysis with fitting or MC simulation can be found in book [8].