Biological and ultrasound biological effect studies show that

Biological Effects of Diagnostic Medical   Ultrasound

 

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 and acoustic
power, may lead to significant biological effects, that are commonly
divided in thermal and non-thermal effects 5.

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The thermal mechanism refers to heating
of tissues due to absorption of acoustic energy in tissues and its transformation into heat. In contrast to the non-thermal effects of ultrasound heat is considered as a potential teratogenic factor. Risk levels due to heating
of tissues are 39.5 °C (103.1 °F) for embryonic tissues and 41.0
°C (105.8 °F)  for adult tissues. Factors that improve
heating include higher ultrasound intensity, longer exposure time, higher
frequency, higher absorption, higher thermal conductivity, and lower perfusion
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 can be thought of as that portion which changes direction. Since the
medium can absorb energy to produce heat, a temperature rise may occur as long
as the rate at which heat is produced id greater than the rate at which the heat is removed 4   .

The
non-thermal mechanism involves the mechanical phenomena of ultrasonic action, mainly cavitation_l3l Cavitation, in broad sense, refers to ultrasonical induced activity occurring
in liquid or liquidlike solid material
that contains bubbles or pockets containing 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
mechanisml4l.
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 all right about allowing myself to be scanned in studying
purposes in the lab as well as scanning other students.

 

 

References

 

1.     
2 Doody C.,
Porter H., Duck F.A., Humphrey V.F. (1999). In vitro heating of human fetal
vertebra by pulsed diagnostic ultrasound. Ultrasound Med Biol, 25: 1289-94.

2.      
3 Deane C., Lees C. (2000). Doppler obstetric ultrasound:
A graphical display of temporal changes in safety indices. Ultrasound Obstet
Gynecol, 15: 418-23.

3.      
Hlinomozova Z, Hrazdira I. (2005). ALARA- Principle and
safety problems of diagnostic ultrasound. Scripta Medica (BRNO), 78(6),
341-346

4.     
William D. O’Brien (1986). Biological effects of
ultrasound: rational for the measurement of selected ultrasonic output
quantities. FuburaPublishing Company, 3(3),
165-179.

5.     
http://www.wikiecho.org/wiki/Biologic_Effects_of_Ultrasound_and_Safety

 

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