Page 196 - Bio Engineering Approaches to Cancer Diagnosis and Treatment

8.2 Approved modes for ultrasound therapy 195

the amount or the time of impact, more than the amount of tolerance, causes serious
damage (e.g., in the red blood cells). In some cases, the forces caused by ultrasound
radiation can also move a cell or nerve line. An example of neuronal disorientation
has been observed in the mouse brain [47]. One way to reduce the negative effects
of tension or displacement is to shift the transmitter over short periods of time. Van
Balo [49] reported the effect of shear stress caused by ultrasonic field on cells. He
also stated that ultrasound waves may be several times more tolerable than cells.
This excessive tension may result in the destruction of the cell. Shear stress due to
ultrasonic waves may have destructive biological effects.
8.2.3.4 Nonlinear effects of ultrasound on blood
The ultrasound field has secondary effects on the tissue and blood. Nonlinear terms
only reinforce the pressure field in tissue. But the vortex flow creates by them in
blood and remains until the end of the radiation time. Also, the acoustic force is
applied to the particles. The acoustic radiation force and acoustic streaming are dis-
cussed. Due to the high influence of ultrasound on microparticles, acoustic modes
can be considered as one of the most effective noncontact methods in biological
application [50, 51]. Today, researchers in the field of acoustics call acoustic force
as one of the methods for dealing with micronutrients [51]. Settnes and Bruus [51]
investigated the forces on a small particle in an ultrasonic field in a viscous flow.
In this study, the forces applied to a spherical, compressible, submerged particle in
a viscous fluid are analytically calculated by Prandtl-Schlichting boundary theory.
It is also claimed that these results are true for any particle diameter and boundary
layer thickness smaller than wavelength. Destgeer et al. [52] proposed a method for
the stable separation of particles in a microchannel under the influence of moving
surface ultrasonic waves. In this method, particles with diameters of 3 and 10 µm,
continuously and without contact, are separated by 100% accuracy. Turning off the
ultrasonic transmitter will remove all particles from the left path. As the transmitter
power increases, the gap between 3 and 10 µm particles increases, and eventually
at 151 mW all larger particles are ejected from another path. Muller and et al. [53]
investigated the 3D motion of spherical microparticles in a rectangular cross-section
channel under the influence of ultrasound waves. They simplified the equations for
an isothermal medium and then compared their analytical solution with experimental
measurements, which observed a 20% error for a particle with a diameter of half a
micrometer.
The steady stream that is created by the absorption of sound in the viscous fluid
in the presence of ultrasonic waves is called acoustic streaming [54]. Depending
on the mechanism generating the acoustic streaming, different speeds, characteris-
tic lengths and profiles are obtained. Flow velocity range is from about 1 µm/s to
1 cm/s. The flow length scale range is varied from 1 µm to 1 cm [55]. The absorp-
tion of acoustic energy in the boundary layer results in acoustic streaming which
is much higher than the energy absorption in the other section of the fluid due to
the type of acoustic excitation and the aspect ratio Fig. 8.5. Unlike the acoustic
flow induced by the boundary layer, Bulk-driven acoustic (Eckart) streaming is the