Page 187 - Bio Engineering Approaches to Cancer Diagnosis and Treatment

186 CHAPTER 8 Ultrasound applications in cancer therapy

absorption of ultrasonic energy in biological tissue. For diagnostic ultrasound, tem-
perature raises are kept relatively low or negligible by applying the limited temporal
average intensities, and generally short exposure durations. Therapeutic applications
of ultrasonic heating, utilize longer durations of heating with unfocused beams, or
utilize higher intensity (than diagnostic) focused ultrasound. The physical therapy is
a good example for the use of unfocused heating, to produce enhanced healing with-
out injury in bone or tendon. Depending on the temperature gradients in ultrasound
therapy, the effects of mild heating, coagulative necrosis, tissue vaporization, or all
three are reported.
Ultrasonic cavitation and gas body activation are closely related mechanisms
which depend on the rarefactional pressure amplitude (about several MPa) of ultra-
sound waves. This tensile stress is supported by the medium and, for example, a
2-MPa rarefactional pressure, which is common even for diagnostic ultrasound. This
high rarefactional pressure can act to initiate cavitation activity in tissue when suit-
able cavitation nuclei are present, or directly induce pulsation of preexisting gas bod-
ies, such as occur in lung, intestine, or with ultrasound contrast agents.
For high-power or high-amplitude ultrasound for therapy, several different mech-
anisms may be contributing concurrently to the total biological impact of the treat-
ment. In addition to direct physical mechanisms for bioeffects, there are secondary
effect such as vasoconstriction, extravasation, ischemia, and immune responses [12],
that are greater than the direct insult from the ultrasound.

8.2 Approved modes for ultrasound therapy
Ultrasound has been used for therapy since the 1930s. Early applications were
applied various mechanisms and conditions for tissue heating [13]. The use of thera-
peutic ultrasound was established for physiotherapy, applications in neurosurgery
and for cancer treatment, in the 1970s, 1977, and 1979, respectively [12]. The appli-
cation of ultrasound for therapeutic efficacy also carries the risk of unintentional
adverse bioeffects so standardization, ultrasound dosimetry, benefits assurance, and
side-effects risk minimization are necessary.
Therapeutic applications of ultrasound may be used clinically after government
approval for marketing suitable treatment devices. A list of therapy applications with
FDA approved devices in clinical use is provided in Table 8.3. The therapeutic effects
of US are generally divided into thermal which include physical therapy, hyperther-
mia and high-intensity focused ultrasound and nonthermal including extracorporeal
shock wave lithotripsy, intracorporeal lithotripsy, and lower power kilohertz fre-
quency ultrasound devices. Also new methods of therapeutic ultrasound are includ-
ing new microbubble- or cavitation-based treatment methods.

8.2.1 Thermal ultrasound therapeutic applications
The important thermal ultrasound therapeutic applications are included as physical
therapy, hyperthermia, and HIFU.