Biomedical Engineering Reference
In-Depth Information
cross-section area is the characteristic area of the particle that
absorbs the energy passing through.
QCI
ab
=⋅
(W).
(18. 2)
nano
With GNP of a certain concentration (
N
− # NPs/ml), SAR is the
sum of heat generated by all NPs,
3
SAR
=⋅ =⋅ ⋅=α⋅
NQ
NC
I
I
(W/m).
(18. 3)
nano
abs
This equation illustrates the important fundamental terms
necessary for laser photothermal therapy with GNPs. First,
the laser fluence (
I
= W/m
2
) should be known in order to esti-
mate the heat generation and temperature change. Second, the
optical properties (
C
abs
) of GNP (and tissue as well) should be
well characterized. Third, and finally, a sufficient number of
GNPs (
N
) have to be delivered to the tumor.
This chapter discusses the important terms in photothermal
GNP therapy and their application. First, laser-tissue interac-
tions (Section 18.2) and laser fluence estimation in the tissue
with and without GNPs are discussed (Section 18.3) (
I
). hen
the optical properties of GNP (Section 18.4) are reviewed to
provide an understanding of absorption cross section (
C
abs
).
With the heat generated by GNP, the thermal response of GNP
laser heating (Section 18.5) will then be analyzed at the sin-
gle NP (nano-), cellular (micro-), and tissue (macro- or bulk)
levels. The physical response around single GNP and biological
responses of
in vitro
and
in vivo
systems (Section 18.6) are then
reviewed with brief discussion on GNP biodistribution (
N
in
Equation 18.3). Finally, existing clinical trials on GNP for pho-
tothermal therapy are reviewed (Section 18.7), and this chapter
will end with several suggestions for future studies (Section
18.8).
FIGURE 18.1
Optical absorption of tissue components including
deoxy-hemoglobin (Hb), oxy-hemoglobin (HbO
2
), water (H
2
O), and
melanin. Visible (VIS) and near infrared (NIR) wavelength regimes are
noted. (Data source: the absorption coefficients of hemoglobin, mela-
nin, and water are plotted based on the data compiled by Scott Prahl at
Oregon Medical Laser Center [http://omlc.ogi.edu/spectra/].)
and melanin. The absorption of each component is dependent
on the wavelength, and the overall absorption depends on the
tissue composition. For instance, hemoglobin and melanin
strongly absorb in the visible region, while water absorbs more
in the infrared, as shown in Figure 18.1. The wavelength window
between visible and infrared, which is around 700~1100 nm, is
often referred to as the therapeutic window (Weissleder 2001,
Jöbsis-vanderVliet 1999), or the near infrared (NIR) window.
The NIR window is the gap between the absorbers in the vis-
ible and infrared regions, and as a result these wavelengths allow
deeper light penetration. In addition to absorption, tissue is also
highly scattering, with an average path length of 0.05~0.2 mm
between two scattering events (Welch 1995). Both the absorp-
tion and scattering determine how tissue interacts with light.
For more detailed discussion of tissue optical properties, their
wavelength dependence, and the measurement techniques, one
can refer to previous literature (Welch 1995, Patterson 1989,
Farrell 1992, Cheong 1990).
18.2 Overview of Laser-tissue
Interactions
18.2.1 tissue Optical properties and
the therapeutic Window
Tissue is comprised of various structures that combine to cre-
ate a complicated optical property landscape. For instance, the
presence of multiple compartments such as cells, vasculature,
and interstitial space can each affect the optical properties of tis-
sue and make the overall absorption and scattering far different
from other bulk materials. Absorbers (i.e., chromophores) and
scatterers in the tissue structure determine the light interaction
with the tissue. It is often convenient to treat tissue as an absorb-
ing matrix with randomly distributed scatterers and then assess
bulk optical properties (Welch 1995). Experimental characteriza-
tion of the tissue then yields a bulk absorption coefficient, scat-
tering coefficient, and scattering phase function. The common
chromophores that absorb in the tissue are water, hemoglobin,
18.2.2 Biomedical Lasers and Laser tissue
Interactions with and without GNps
Lasers that can be used to either image or heat tissue can be classified
into different categories depending on the lasing medium within
the laser (solid, gas, dye, etc.), operating mode (pulsed or continu-
ous wave [CW]), and wavelength (visible, IR, etc.). Table 18.1 lists
several common biomedical lasers and their operating parameters
(including lasing wavelength and corresponding tissue absorber,
power level, and operating mode). The lasing medium and oper-
ating mode tend to determine the spectrum and stability of the
laser. Further, the irradiance (W/m
2
) and interaction time define
an area upon which mechanisms of laser tissue interactions spread
