In Vivo Molecular Fluorescence Imaging - Nanotechnology for Biomedical Imaging and Diagnostics - page 270

Emission
fibers

Fiber couple
with lens

Lens

PMT

Optical
switch

Pre-amp

Filter wheel

Oscillator
100 MHz

Mixer

Optical
switch

Mixer

Computer

SYNC

Oscillator
100.02 MHz

Excitation
fibers

Diode laser with
temperature controller

T

Laser power
supply and
cooler

FIgure 9.11 Schematic of a frequency-modulated system.

In practice, implementation of the FD is usually limited to one or a few modula-
tion frequencies. Therefore, the TD data can provide more information about the
media, since a short laser pulse contains a broader range of the modulation frequencies,
including the zero-frequency component. on the other hand, instrumentation for
time-resolved imaging is considerably more expensive, and data collection time is
longer relative to a FD system. Therefore, in applications that need very fast imaging,
the FD technique is more suitable.

9.5 ModelIng oF lIght propagatIon In tIssue

This section briefly discusses forward models of light propagation in tissue followed
by light propagation of fluorescence signal in high scattering media and different
simulation algorithms to implement these models.

9.5.1 Forward Model for light propagation in tissue

In highly scattering media, the light propagates in random directions. However, the
path of photon propagation from a source to a detector can be statistically modeled
with mathematical formulas. Photon transport in scattering media, like tissues, can
be modeled with the radiative transport equation (rTe)

ˆ

1 (,,) ∂ Lrst

∫∫

+∇ . (,,) Lrsts ˆ ˆ =− ( µµ + ) Lrst (,,) ˆ + µ Lrs t f ss d Qrst (, ,) (, ˆ ′ ˆˆ ′ ) Ω + ′ (, ˆ , )

s a s

c ∂ t

4 π

(9.8)