Page 186 - Bio Engineering Approaches to Cancer Diagnosis and Treatment

8.1 Ultrasound in biomedical engineering 185

ultrasound. The ultrasound probes (transducers) almost are placed on the skin (probes
may be placed inside the body) and operate at megahertz (MHz) range frequencies.
The functional and anatomical ultrasound are two categories of diagnostic ultra-
sound. The anatomical ultrasound generates images of internal organs or other struc-
tures, but the functional ultrasound creates information maps by combining data
about the movement and velocity of tissue or blood, softness or hardness of tissue,
and other physical characteristics, with anatomical images. Another functional form
of ultrasound is elastography, a method for measuring and displaying the relative
stiffness of tissues, which can be used to differentiate tumors from healthy tissue.
Elastography can be used to test for liver fibrosis, a condition in which excessive
scar tissue builds up in the liver due to inflammation. Diagnostic ultrasound is gener-
ally regarded as safe and does not produce ionizing radiation like that produced by
X-rays. Still, ultrasound is capable of producing some biological effects in the body
under specific settings and conditions.
The purpose of the therapeutic ultrasound, as a noninvasive method, is to interact
with tissues in the body such that they are either modified or destroyed without pro-
duces images. Some modifications such as moving or pushing tissue, heating tissue,
dissolving blood clots, or delivering drugs to specific locations in the body are made
possible by the use of very high-intensity beams that can destroy diseased or abnor-
mal tissues such as tumors.
Therapeutic ultrasound produces high levels of acoustic output that can be
focused on specific targets for the purpose of heating, ablating, or breaking up tis-
sue. One type of therapeutic ultrasound uses high-intensity beams of sound that are
highly targeted and is called high intensity focused ultrasound (HIFU). HIFU is
being investigated as a method for modifying or destroying diseased or abnormal
tissues inside the body without having to open or tear the skin or cause damage to
the surrounding tissue.
Using ultrasound waves in the body can lead to heat, bubbles, tension, and vibra-
tion. In the same intensity, the heat, tension, and vibration increases with increasing
frequency of waves, while the possibility of bubble formation decreases. Therefore,
determining the appropriate frequency for medical application of ultrasound waves
requires careful examination. These four essential effects can be beneficial or harm-
ful. In the lithotripsy, cancer treatment and drug delivery to brain, the tension and
vibration, generated heat and formation of bubbles are important, respectively
[10, 11].
As the pressure amplitude, the frequency, or the propagation length is increased,
the ultrasound wave could ultimately lead to a discontinuity or shock in the wave-
form, increasing frequency, nonlinear acoustic distortion, or pulse length can
increase heating and enhance some nonthermal mechanisms, for example, radiation
force. But decreasing frequency increases the chance of cavitation. Also Increasing
power or intensity tends to increase the magnitude of the bioeffect mechanisms.
Therapeutic ultrasound devices use continuous waves to deliver effective ultrasonic
energy to tissues. Some devices operate at higher amplitude and therefore, tend to
produce shocked or distorted waves. Ultrasound-induced heating is the result of the