Biomedical Engineering Reference
In-Depth Information
TABLE 18.1
Common Biomedical Lasers Listed by Wavelength from Short to Long
Laser type
Wavelength
Tissue Absorber
Power c
CW/Pulsed
UV, 126 ~ 350nm
~ 0.2J/pulse
Excimer lasers a
Hb, Melanin
Pulsed
350 ~ 1100nm
Ion lasers (Ar + and Kr + )
Hb, Melanin, NIR
window
mW to 40W
CW
He-Cd laser
325nm, 442nm
Hb, Melanin
~ 100mW
CW
Semiconductor (Diode) laser
400 nm ~ microns
Hb, Melanin H 2 O
several W
CW
Cu vapor
512nm
Hb, Melanin
Pulsed
1−10 W
KTP b
532nm
Hb, Melanin
100 W
Pulsed
Nd:YAG laser
532nm 1064nm
Hb, HbO 2 , NIR
window
up to 100MW
CW or pulsed
Ti-sapphire laser
650nm ~ 1100nm
Hb, HbO 2 , NIR window
0.5 ~ 1.5W
CW or pulsed
Ruby laser
694nm
Melanin
high
Pulsed
Alexandrite laser
800nm
NIR window
up to 100W
Pulsed
Dye lasers
UV-Vis-IR,
tunable
Varies
up to kW
CW or pulsed
CO 2 laser
Far IR, ~ 10 µm
H 2 O
mW to kW
CW or pulsed
Note: The corresponding tissue absorbers, laser power level, and operating model (CW or pulsed) for each
laser are listed.
a The wavelength is excimer dependent, such as XeF, XeCl, KrF, ArF, and KrCl.
b KTP laser is pumped by Nd:YAG laser.
c The power for pulsed laser is the average over time when expressed in W.
out in the laser-tissue interaction map (Figure 18.2a). For instance,
while CW laser delivers energy continuously, pulsed laser is able
to confine the energy into short periods of time called pulses (with
duration as short as femto-seconds, 10 −15 s). Depending on a subset
of laser parameters (power and pulse duration), the mechanism of
tissue interaction can change (Judy 2000, Niemz 2004). Examples
include: photochemical (laser-triggered chemical reaction or
decomposition (Srinivasan 1986)); photothermal (heat generation
and temperature increase (Welch 1995)); photo ablative (intensive
temperature increase and tissue vaporization (Vogel 2003)); and
photomechanical (generation of tensile stress due to explosive abla-
tion (Paltauf 2003, Jacques 1993)). Photochemical and photother-
mal interactions usually require continuous low power laser, while
photo ablative and photomechanical interactions need pulsed high
power laser. However, these effects can overlap, for example, photo
ablation also has thermal and mechanical effects. Some examples
of laser treatment include photodynamic therapy, which is photo-
chemical in nature (Niemz 2004), laser interstitial thermotherapy
(LITT), which is photothermal (Müller 1995), and photo-ablation
of benign prostatic hyperplasia (BPH) (Kuntz 2007).
The introduction of GNPs increases the absorption of laser
energy (discussed in detail later in optical properties of GNPs),
thereby reducing the laser energy needed to achieve the same
effects, such as photothermal therapy, bubble formation, and
plasma formation. This can be easily visualized in Figure 18.2b
where a comparison is made with laser parameters from the lit-
erature that were used to achieve the effects described herein, in
systems with and without GNPs. The exposure time decreases as
one move toward the left (x-axis), and the laser power decreases
as one moves down (y-axis). Thus, the presence of GNPs is shown
to reduce the power and exposure time necessary to achieve the
same outcome. It is worth noting that the figure is plotted on a
logarithmic scale, and as a result noticeable shifts in the figure
represent order of magnitude change.
18.2.3 applying Laser to the tumor
Depending on the position of the tumor to be treated, appro-
priate methods of applying laser energy should be adopted. In
general, there are three ways, including: (1) direct illumination if
close to the surface, (2) interstitial laser fiber, and (3) endoscopes
or catheters. The penetration depth of all laser approaches
is wavelength dependent and in the best case can reach up to
~1 cm (Peng 2008, Niemz 2004). Most preclinical studies use
rodent systems, and the tumors are grown near the surface
(Hirsch 2003, von Maltzahn 2009). In these studies, direct laser
illumination is applied to the tumor. The laser attenuates when
penetrating in the tissue, and the collimated intensity decreases
exponentially according to Beer's law:
=− −µ+µ
(
)
*z,
2
(18.4)
Iz
()
I
(1
r
).
e
(W/m )
as
0
ref
where I is the laser intensity from the source [W/m 2 ], r ref is
the portion that is reflected at the air-tissue interface, µ is the
absorption coefficient [m −1 ], µ is the scattering coefficient [m −1 ],
 
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