Page 183 - Bio Engineering Approaches to Cancer Diagnosis and Treatment

182 CHAPTER 8 Ultrasound applications in cancer therapy

be similar to that in saline. The mathematical representation of the relationship is
V = F·λ
Each material, as well as, tissue has a specific impedance to the passage of sound
waves. The specific impedance of a tissue will be determined by its density and elasticity.
Clearly, in the case of US passing from the generator to the tissues and then through
the different tissue types, this cannot actually be achieved. The greater the difference
in impedance at a boundary, the greater the reflection that will occur, and therefore,
the smaller the amount of energy that will be transferred. To minimize this difference,
a suitable coupling medium has to be utilized. If even a small air gap exists between
the transducer and the skin the proportion of US which will be reflected approaches
99.998% which in effect means that there will be no transmission. The coupling media
used in this context include water, various oils, creams, and gels. Ideally, the coupling
medium should be fluid so as to fill all available spaces, and should allow transmission
of US with minimal absorption, attenuation, or disturbance. At the present time, the
gel-based media appear to be preferable to the oils and creams. Also, water is a good
media and can be used as an alternative but clearly, it fails to meet the above criteria
in terms of its viscosity. In addition to the reflection that occurs at a boundary due to
differences in impedance, there will also be some refraction if the wave does not strike
the boundary surface at 90 degrees. Essentially, the direction of the US beam through
the second medium will not be the same as its path through the original medium. The
critical angle for US at the skin interface appears to be about 15 degrees.
The absorption of US energy follows an exponential pattern and more energy is
absorbed in the superficial tissues than in the deep tissues. In order to achieve certain
effect, the US dosages must be considered at some point. The penetration of ultra-
sound waves ranges from kHz to MHz frequency levels, depending on the type of tis-
sue and plan of treatment for the disease. By increasing the US beam penetration into
the tissues, the less energy is available to achieve therapeutic effects. The half-value
depth will be different for each tissue and also for different US frequencies. Table 8.1
gives some indication of average half value depths for therapeutic ultrasound [1].
It is not possible to know the thickness of each tissue layer in an individual
patient. To achieve a particular US intensity at depth, account must be taken of the
proportion of energy which has been absorbed by the tissues in the more superficial
layers. For example, the energy level at 3 and 1 MHz frequencies is decreased about
25% in depth of 4 and 8 cm of typical tissue respectively. Ultrasound frequencies are
used for medical application is summarized in Fig. 8.1.
The penetration (or transmission) of US is not the same in each tissue type.
Generally, the tissues with the higher protein content will absorb US to a greater
extent, thus tissues with high water content and low protein content such as blood
and fat, absorb little of the US energy while the tissue with a lower water content and

Table 8.1 Indication of average half value depths for therapeutic ultrasound.

Frequency Fat Muscle Tendon
1 MHz 50 mm 9 mm 6.2 mm
3 MHz 16.5 mm 3 mm 2 mm