Biomedical Engineering Reference
In-Depth Information
These are continuing experiments that report the biological effects of dia-
gnostic ultrasound [48]. In their o
cial statements, The World Federation for
Ultrasound in Medicine and Biology (WFUMB) and the American Institute
of Ultrasound in Medicine (AIUM) have mentioned the biological effects and
adverse effects of diagnostic ultrasound [49,50]. There is some evidence sug-
gesting that the heating effect causes a biological response [51]. Ultrasound-
induced heating of the embryo or fetus should not be neglected, because a
small volume of developing bone tissue and nerve system is heated signifi-
cantly by ultrasound at diagnostic power. The lens of the human eye is highly
sensitive to heat, with the risk of the development of cataracts. Cavitation
effects on the gas/fluid border and gas-saturated fluid produce free radicals
at diagnostic power [52]. Lung tissue bleeding has been reported as a result of
cavitation-related effects. Free radicals have chemical activity such as oxida-
tion, and disrupt chemical bonds and structures of biological molecules [53].
Such effects cannot be explained by the thermal effects of ultrasound.
2.6.3
The Effect of Ultrasound on the Cell Membrane
Progress in molecular biology provides us with a lot of information that helps
us to understand machinery of cell and sub-cellular structure on a molecular
basis. Proteins and other organic compounds have their biochemical roles,
including interactions that determine biological responses. Ultrasound may
affect protein particle aggregation and may change its chemical properties.
The fluid mosaic model of the cellular membrane indicates that the lipid flui-
dity of the membrane and the membrane proteins change their compositions
due to local pressure variation such as ultrasound exposure. Phase transition
in a lipid bilayer changes membrane stability and permeability. Effects on
membrane proteins, such as receptors, can trigger a biochemical signaling
cascade followed by a variety of cellular responses. In recent years, interest
in the biological effects of ultrasound has been rekindled, as it has become a
method of manipulating living phenomena.
Biological tissue is constructed from cell and interstitial matrix and connec-
tive tissue. Each cell has a membrane at its outer boundary. The basic struc-
ture of the cell membrane is a lipid bilayer that has a hydrophobic layer
in the middle. Hydrophilic, water-soluble dye cannot penetrate the intact
membrane. Ethidium bromide is one fluorescent hydrophilic dye that does
not stain the inside of an intact cell. However, after 5 s of ultrasound expo-
sure (2 MHz, 200 mW/cm
2
, SPTA), an ethidium bromide stain was found
(Fig. 2.35).
This result shows us that ultrasound exposure at the same acoustic power
as diagnostic equipment has the effect of increasing membrane permeability.
The cell membrane works as a protective barrier when attempts are made
to transport materials into a cell. Severe disruption of the integrity of the
membrane may cause drastic changes in the internal environment of the cell
that results in immediate cell death.
