Biophysical is removed [4] . The non-thermal

Biophysical research and ultrasound
biological effect studies show that ultrasound can produce changes in living
systems 4. The biological effects of ultrasound depend on the total energy
applied to a given tissue. Therefore, duration variation of exposure to wave
emission, frequency and intensity of the
ultrasound beam, pulsed
or continuous emission modality, may lead
to significant biological effects, that are usually divided in thermal and non-thermal effects 5.

The thermal effect refers to heating of
tissues due to absorption of acoustic energy in medium and its transformation into heat. In contrast to the non-thermal effects of ultrasound heat is considered as a potential teratogenic factor. Teratogen is an agent that may cause defects in development
of an embryo or fetus 6. Due to tissues heating the risk levels are 39.5 °C (103.1 °F) for
embryonic tissues and 41.0 °C (105.8 °F)  for adult tissues. Higher ultrasound
intensity, longer exposure time, higher frequency, higher absorption, higher
thermal conductivity, and lower perfusion – factors that improve heating 3. As much as 70% of the total temperature increase associated
with ultrasound occurs within the first minute
of exposure 1, but temperature continues
to rise as exposure time is prolonged. Minimizing the exposure time is
probably the single most important factor for ensuring
patient safety from
thermal injury 2. Whenever ultrasonic energy is propagated
into anattenuating material such as tissue, the amplitude of the wave decreases with distance. This attenuation is due to either absorption or scattering. Absorption is a mechanism that represents that portion of the wave energy that is converted into heat, and
scattering represent waves which changed their direction. Since the tissue can
absorb energy to produce heat, a temperature rise may occur as long as the rate
at which heat is produced is greater than the rate at which the heat is removed 4   .

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The
non-thermal effest involves the mechanical phenomena of ultrasound action, mainly cavitation 3. Cavitation,
in broad sense,
refers
to ultrasound induced activity
occurring in liquid or liquidlike solid material that
contains bubbles of gas or vapor. These bubbles originate within materials at
locations termed “nucleation sites,” the exact nature and source of which are not well understood in a complex
medium such as tissue. Cavitation
can affect a biological
system by virtue of temperature rise, a mechanical stress, and/or free radical
production. Even so, this is traditionally referred to as a nonthermal
mechanism 4. This mechanism is responsible for the possible mechanical damage
to biological structures. Actors that
improve cavitation include
higher negative amplitude
of acoustic pressure, lower frequency, longer duration of acoustic impulses,
higher repetition frequency, and lower viscosity. In general, biological risks
depend on physical characteristics of the ultrasound wave (mode, intensity, and
frequency) and sensitivity of the tissue examined to ultrasonic action (size,
structure, and attenuation). For the assessment of possible biological risks there exist
three main approaches. The first one is theoretical approach which is based on
production of simplified models of biological systems and calculation of
physical parameters responsible for biological effects. Second, experimental
approach, comprises investigation of the experimental influence of biomedical
ultrasound on different levels of biological organization (biomolecules, cells
and tissues, and whole organisms). And the third one is epidemiological approach
that comprises retrospective and prospective
studies of ultrasound diagnostic exposures on human population, especially
during pregnancy. This approach
has major
importance for safety assessment
3.

The biological
effects discussed in this paper do not appear to represent a hazard if you know the limits on the acoustic
output and transfers the responsibility for a safe examination from the manufacturer to the examiner. I
personally feel confident to be scanned in studying purposes
in the lab as well as scanning
other students.