Page 79 - Biomedical Engineering and Design Handbook Volume 1, Fundamentals

56 BIOMECHANICS OF THE HUMAN BODY

shown as a promising clinical approach for maximizing thermal damage to the targeted blood ves-
sels under the skin while minimizing injury to the epidermis (Jia et al., 2006). Laser use in ophthal-
mology has a long history. Energy absorption in the tissue or blood is largely dependent on the
wavelength of the laser used; longer wavelengths penetrate more deeply into tissue than short wave-
lengths. Most of the laser-based treatments depend upon light/tissue interactions that occur in the
superficial layers associated with the neuro-retina and retinal pigment epithelium (RPE).
Conventional laser treatment for the retinal layer uses continuous or pulse wave laser (wavelength:
527 nm) with exposure time in the range of 100 to 200 ms, and power in the range of 50 to 200 mW
(Banerjee et al., 2007). The laser is primarily absorbed by the melanin granules in the RPE tissue.
On the other hand, in laser photocoagulation of the choroidal feeder vessels, laser energy must pen-
etrate the overlying retinal layers, RPE, and choriocapillaris to reach the choroid and then be
absorbed by the targeted feeder vessel. Considering that the targeted vessels in these studies lie rel-
atively deep, it is logical that the widely used 805-nm-wavelength diode laser was selected as the
source for maximizing energy absorption. An experimental study on pigmented rabbit eyes has
shown that the photocoagulation of large choroidal arterioles can be accomplished with relatively
little concomitant retinal tissue damage (Flower, 2002), when using near-infrared wavelengths, espe-
cially when used in conjunction with an injection of a biocompatible dye that enhances absorption
of the laser energy. A recent theoretical simulation of the temperature field in the vicinity of the
choroidal vessel has illustrated the strategy to achieve thermal damage while preserving the sensitive
RPE layer (Zhu et al., 2008).
Unlike the electromagnetic heating devices mentioned above, ultrasound heating is a mechanical
hyperthermic technique. The acoustic energy, when absorbed by tissue, can lead to local temperature
rise. Ultrasound offers many advantages as an energy source for hyperthermia because of its small
wavelength and highly controllable power deposition patterns, including penetration depth control in
human soft tissue (Hariharan et al., 2007b, 2008). The depth of the resulting lesion could theoreti-
cally be increased or decreased by selecting a lower or higher ultrasound frequency, respectively. It
has been shown that scanned focused ultrasound provides the ability to achieve more uniform tem-
perature elevations inside tumors than the electromagnetic applicators. Moros and Fan (1998) have
shown that the frequency of 1 MHz is not adequate for treating chest wall recurrences, since it is too
penetrating. As for a deep-seated tumor (3 to 6 cm deep), longer penetration depth is achieved by
using relatively low frequency (1 MHz) and/or adjusting the acoustic output power of the transducer
(Moros et al., 1996). The practical problem associated with ultrasound heating is the risk of over-
heating the surrounding bone-tissue interface because of the high ultrasound absorption in bone.
Another hyperthermia approach involves microparticles or nanoparticles which can generate heat
in tissue when subjected to an alternating magnetic field. Magnetic particle hyperthermia procedure
consists of localizing magnetic particles within tumor tissue or tumor vasculature and applying an
external alternating magnetic field to agitate the particles (Gilchrist et al., 1957). In this case, mag-
netic particles function as a heat source, which generates heat due to hysteresis loss, Néel relaxation,
brownian motion, or eddy currents. Subsequently, a targeted distribution of temperature elevation
can be achieved by manipulating the particle distribution in the tumor and tuning the magnetic field
parameters. Compared to most conventional noninvasive heating approaches, this technique is capa-
ble of delivering adequate heat to tumor without necessitating heat penetration through the skin sur-
face, thus avoiding the excessive collateral thermal damage along the path of energy penetration if
the tumor is deep seated. In addition to treatment of deep seated tumor, the employment of nanopar-
ticle smaller than 100 nm is especially advantageous in generating sufficient heating at a lower mag-
netic field strength. Typically, the particle dosage in the tumor and the magnetic field strength are
carefully chosen to achieve the desired temperature elevation. Generally, the usable frequencies are
in the range of 0.05 to 1.2 MHz and the field amplitude is controlled lower than 15 kA/m. Previous
in vitro and in vivo studies have used a frequency in the 100 kHz range (Rand et al., 1981; Hase
et al., 1989; Chan et al., 1993; Jordan et al., 1997; Hilger et al., 2001). The studies of heat genera-
tion by particles suggest that the heating characteristic of magnetic particles depends strongly on
their properties, such as particle size, composition, and microstructure (Chan et al., 1993; Hergt et al.,
2004; Hilger et al., 2001; Jordan et al., 1997). In particular, as the particle size decreases, thermal
activation of reorientation processes leads to superparamagnetic (SPM) behavior that is capable of
generating impressive levels of heating at lower field strengths. The spherical nanoparticle of 10 nm