In Vivo Molecular Fluorescence Imaging - Nanotechnology for Biomedical Imaging and Diagnostics - page 275

fluorescence concentration. Fluorescence intensity is very sensitive to the intensity
variation of excitation source, optical properties of the media, depth, changes in
system parameters, and concentration of the injected dye.

9.6.1.2 Pharmacokinetic Information uptake and clearance rate information of
fluorescent probes in biological tissues can help to overcome the limitations that
were mentioned in section “Intensity,” by using fluorescence intensity alone. In 2007,
Hillman [59] showed that temporal techniques can be used to decouple the fluores-
cence signals of each organ in a mouse body based on the differences in the accumulation
and clearance rate of fluorescence probes in different organs.
In nontargeted fluorescence dye, the temporal response (pharmacokinetics) is
usually based on monitoring the washout time of the dye from disease tissues (such
as tumors) compared to normal tissues.
In targeted fluorescent probes that can bind to specific disease biomarkers and
stay there for longer times, the fluorophore concentration can be calculated based on
the compartmental ligand-receptor model [60]. Assuming that the dissociation rate
of bound fluorophores is very low, the intensity of the fluorescence signal inside the
tumor can be written as

I = I + I + I

f free ligands in blood free ligands in thetumor bound ligand − receptor

(9.24)

t FB k F

++ −−

= α F exp −

τ βγ

[ 1 exp( )

bl T max on T

where α, β, and γ are constants; F bl , F T , and B are the concentrations of free ligands
in the blood and free and bound ligands in the tissue; and B max is the concentration
of the specific receptors in the tumor region, respectively. The clearance time of the
fluorophores from the blood circulation is τ , and ligand-receptor binding rate is k on .
In this approximation, it is assumed that after the initial time t 1 , the concentration of
local free ligands in the tumor stays constant and the intensity of the free ligands in
blood is considered to be the same as the intensity measured at the contralateral site.
Therefore, by subtracting the measurements at the contralateral site from the tumor
site, the two remaining components are the free and bound ligands to the receptors
in the tumor region, and the model time dependence of the bound ligands can be
simplified to the form

[ ]

(9.25)

yt ya bt () =+−− 1 exp( )

0

To eliminate the system variations between different experiments, the measurement
data at different time points should be normalized to the first measurement data,
when binding is almost negligible and the accumulation of the free ligands in the
tumor is stabilized. The derivative dy = ab presents the normalized rate of

dt

t = 0

accumulation (NrA). By using a fitting algorithm, NrA can be estimated from a
time series of fluorescence intensity measurements.