Page 195 - Bio Engineering Approaches to Cancer Diagnosis and Treatment

194 CHAPTER 8 Ultrasound applications in cancer therapy

special time that is different for each member. If the temperature rise time due to the
presence of ultrasound is less than the time it takes for the body to transmit heat, the
damage will intensify.
In high intensity focal ultrasonic waves, transmitters with a convergent emission
level are used to concentrate the waves to raise the temperature in a particular region.
The dimension of this region is proportional to the wavelength (λ). The geometric
shape of the focal region is an elliptic and dimensions are obtained from relation
(8.1), where K is the geometric characteristic of the audio transmitter [17].

1λ×1λ×Kλ 1 λ ×1 λ × K λ (8.1)
The change in the blood and tissue temperatures during ultrasonic emission are
dependent on the characteristics of the transmitter, such as the intensity, duration,
and working cycle, as well as tissue characteristics such as the absorption coefficient
and perfusion rate, which is modeled by Pennes equation [43]. The thermal index
(EMT ) is calculated from the equation relation (8.2) [44]. In Hyperthermia therapy,
43
the radiation is continued until the cancerous thermal index is reached.

EMT43=∫0tRT−43dt EMT 43 = ∫ 0 t R ( −T 43 ) dt (8.2)

8.2.3.2 Bubble formation
One of the causes of bubble formation is the susceptibility to steam or water in
the presence of the ultrasonic field [45]. However, bubbles form at frequencies of
20–200 kHz but bubble formation has also been reported at frequencies of about
1 MHz. The bubbles are divided into two categories: stable and unstable. Movement
of the stable bubbles creates stresses to adjacent cells, but do not have any harmful
effects. In contrast, unstable bubbles that explode and disappear suddenly after several
changes in size can damage the adjacent cells [46]. The proper frequency and ampli-
tude of the waves are selected to prevent this adverse event. In other words, in using
ultrasonic waves, bubbles do not damage the cells [47]. The mechanical index (MI)
for ensuring tissue and blood health and the absence of bubble formation is obtained
from Eq. (8.3), which should be less than 0.7 for prevention of bubble formation [44].
p
MI=pf0.5 MI = 0.5 (8.3)
f
The high-intensity ultrasound waves, the range of 100–10,000 W per square cen-
timeter, the therapeutic and subtherapeutic effects are produced depending on the
2
exposure duration [43]. For example, at intensity levels of the order of 1000 W/cm
and exposure durations of the order of 1 s, HIFU produces thermal coagulation
within the small volumes determined by the size of the focal spot.

8.2.3.3 Mechanical tension
The ultrasonic beam can cause tension or displacement [48]. The limit of tolerance of
tension in any cell or tissue before irreparable damage is clear. If the stress in terms of