Biomedical Engineering Reference
In-Depth Information
TABLE 18.5
Noninvasive Thermal Imaging Techniques for Thermal Therapy Monitoring
Method
Mapping
Spatial Resolution
Sensitivity
Speed
Ref.
IR
a
Surface
~18 µm
~0.1°C
60Hz
(Childs 2000)
MRTI
b
Ye s
~0.16mm
-0.01 ppm/°C
0.2Hz
(Hirsch 2003, Schwartz 2009)
Photoacoustic
Ye s
~50 µm
~0.16°C
c
>10Hz
(Shah 2008b)
Ultrasound
Ye s
~0.3mm
>10Hz
(Shah 2008a, Shah 2008b, Liu 2010)
~0.05°C
Note:
The spatial resolution, temperature sensitivity, and speed of each technique are estimated from the references
listed.
a
Infrared thermometry
b
Magnetic resonance temperature imaging
c
The resolution of photoacoustic temperature imaging depends on the stability of pulsed laser used.
temperature is much easier than measuring the fluence rate dis-
tribution. The SAR can then be obtained by calculating the ini-
tial slope of the local temperature change, usually over a matter
of seconds:
18.6.1 Nanoscale Effects
High intensity CW and pulsed laser can transiently heat up a
GNP at the nanoscale (Zharov 2006, Keblinski 2006, Carlson
2011). Figure 18.5b shows several degrees of temperature increase
for a gold nanoshell and sphere under laser fluence of 10
4
W/cm
2
.
With even higher power pulsed lasers, the temperature of the
GNP can increase over thousands of degrees within a very short
period of time (1-100 ns scaled above). Depending on the energy
input, different responses in the surrounding medium and GNP
occur. These include the phase change of the medium (melt-
ing ice and polymer) (Richardson 2006, Govorov 2006), selec-
tive protein denaturation around the NP (Huettmann 2003,
Pitsillides 2003), acoustic wave formation because of the particle
expansion (Zharov 2007), vaporization of the water around the
particle (Zharov 2005b), melting of the particle itself, vaporiza-
tion of GNP (Letfullin 2006), optical breakdown and plasma
formation (Takeda 2006), and eventually particle fragmentation
and degradation (Letfullin 2006). These processes have been
recently summarized by Pustovalov et al. (Pustovalov 2008),
and are shown schematically in Figure 18.7, along with the phase
diagrams of water and gold (refer to phase change properties in
Table 18.6). Both theoretical (Pustovalov 2008, Merabia 2009)
and experimental efforts have been undertaken to understand
these processes, however, the complexity of the problem and
difficulty of measurements at small time and length scales have
limited our complete understanding. Each of these processes
have important applications, for example, acoustic wave forma-
tion for photoacoustic imaging (Shah 2008b), and bubble forma-
tion around the GNP and/or GNP fragmentation for enhanced
tumor treatment (Zharov 2005a). While bubble generation is
promising for both diagnostic and therapeutic purposes, the
fragmentation of GNPs may be difficult to control and requires
high laser energy input (Lapotko 2009).
SARC
dT
dt
=ρ
3
(W/cm).
(18.16)
initial
Recently, noninvasive or minimally invasive methods,
such as infrared thermography, magnetic resonance temper-
ature imaging (MRTI), ultrasound thermometry, and pho-
toacoustic thermometry, have been proposed to guide and
monitor thermal therapy. Typical properties of these meth-
ods, including the 3D mapping capability, spatial resolution,
temperature sensitivity, and speed, are listed in Table 18.5.
Among these methods, Shah et al. (Shah 2008b) showed that
simultaneous ultrasound and photoacoustic temperature
measurements give results within less than 0.5°C difference,
with photoacoustic thermal imaging showing higher sig-
nal to noise than ultrasound. Magnetic resonance tempera-
ture imaging (MRTI) is also a promising technique that is
undergoing several clinical trials to monitor laser thermal
treatment (Clinicaltrials.gov study number NCT00392119,
NCT00720837, and NCT00787982). These techniques offer
exciting opportunities for
in vivo
treatment monitoring, SAR,
and injury assessment based on delivered GNPs.
18.6 Laser GNp Effects II
—
physical
and Biological responses
at Multiple Scales
The heating of GNP with laser can induce some physical effects,
for example, phase change of the surrounding medium, and bio-
logical responses when present in cellular and tissue systems. In
this section, these effects are reviewed at the nanoscale, cellular,
and tissue levels similar to the previous section. For the
in vitro
(cel lu la r) a nd
in vivo
(tissue) effects, fundamental questions,
including GNP biodistribution, mechanism of injury, and treat-
ment outcome, are discussed.
18.6.2
In Vitro
Cellular Level Effects
The introduction of GNP to the cell increases the absorption and
thus heat generation within the cell. Selective destruction can be
achieved by targeting GNPs to the cancer cells and then apply-
ing laser irradiation. However, successful thermal destruction is
only possible with specific conditions (NP and laser parameters).
