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Ultrasound Imaging
Ultrasound waves contain mechanical energy that can be transformed into heat;
in which case, an increase in the temperature of the exposed tissues and cells will
result. The temperature increase depends on the energy exposure (power and time
frame) and the tissue characteristics. In specific therapeutic cases, this temperature
effect is desired for destruction of kidney and gallstones; however, for imaging purposes, no harmful temperature increase is recorded at the customary relatively low
power density levels.
Two different types of mechanical and cavitational effects can be distinguished. The
first-order effect is accomplished by strain and shear stress induced by out-of-phase acceleration of components of the same biological structure. In certain cases, the local particle acceleration can exceed 25,000 times the gravitational acceleration. In such cases,
when the focal point is relatively small compared to the structure size, the ultrasound
causes tear-and-twist effects. Examples are cellular membrane fatigue, mostly observed
in red blood cells, resulting in autolysis of the erythrocytes. Additionally ultrasound may
liquefy thixotropic structures, including mitotic and meiotic spindles. Cavitation is the
oscillatory activity of highly compressible bodies such as gas bubbles or cavities. This
feature depends largely on the pulse duration and can disrupt white and red blood cells
and epithelial cells, and may additionally cause blood coagulation dysfunction.
On the cellular and subcellular level, cells may get disrupted by the cavitation process, which produces shear stresses. Some of the reported cellular consequences are
swelling of the mitochondria and enlargement of the endoplasmic reticulum. However,
no direct effects on the functionality of the mitochondria have been observed, although
increased membrane permeability to water has been detected. Due to the relatively
long wavelength, no atomic or molecular (DNA) influence has been found.
On the biochemical level, the influence of ultrasound energy has been known to
decrease glutathione levels and increases in alanine aminotransferase (ALT) and
aspartate aminotransferase (AST) levels in the blood. Additionally an increase in
collagen synthesis has also been observed.
In terms of the influence of ultrasound on the developmental stages or organs and
systems, there is no conclusive evidence on any effects on fetal growth; however,
wound healing seems to be accelerated. Ultrasound has been shown to produce retinal damage and affect the cornea, but, at the same time, ultrasound can also be used
to treat mild cases of myopia. One significant risk is damage to the inner ear.
Despite all the evidence cited earlier, no epidemiological data can support any
seriously harmful side effect due to ultrasound exposure. This makes ultrasound the
most used diagnostic technique in hospitals.
16.15 SUMMARY
Ultrasound imaging uses a rather simple mechanism of acoustic generation by electric transducers that can produce and acquire mechanical displacement. The principle
of image formation is based on the detection of attenuation, reflection, and TOF for
reflected and transmitted pressure. There are three distinct methods of imaging in use,
attenuation tomography, reflection tomography, and TOF tomography. Each of these
methods targets a specific anatomical contrast feature. Ultrasound is used in many diagnostics and therapeutic applications due to its harmless effects on the biological tissues.
Ultrasound Imaging
Ultrasound waves contain mechanical energy that can be transformed into heat;
in which case, an increase in the temperature of the exposed tissues and cells will
result. The temperature increase depends on the energy exposure (power and time
frame) and the tissue characteristics. In specific therapeutic cases, this temperature
effect is desired for destruction of kidney and gallstones; however, for imaging purposes, no harmful temperature increase is recorded at the customary relatively low
power density levels.
Two different types of mechanical and cavitational effects can be distinguished. The
first-order effect is accomplished by strain and shear stress induced by out-of-phase acceleration of components of the same biological structure. In certain cases, the local particle acceleration can exceed 25,000 times the gravitational acceleration. In such cases,
when the focal point is relatively small compared to the structure size, the ultrasound
causes tear-and-twist effects. Examples are cellular membrane fatigue, mostly observed
in red blood cells, resulting in autolysis of the erythrocytes. Additionally ultrasound may
liquefy thixotropic structures, including mitotic and meiotic spindles. Cavitation is the
oscillatory activity of highly compressible bodies such as gas bubbles or cavities. This
feature depends largely on the pulse duration and can disrupt white and red blood cells
and epithelial cells, and may additionally cause blood coagulation dysfunction.
On the cellular and subcellular level, cells may get disrupted by the cavitation process, which produces shear stresses. Some of the reported cellular consequences are
swelling of the mitochondria and enlargement of the endoplasmic reticulum. However,
no direct effects on the functionality of the mitochondria have been observed, although
increased membrane permeability to water has been detected. Due to the relatively
long wavelength, no atomic or molecular (DNA) influence has been found.
On the biochemical level, the influence of ultrasound energy has been known to
decrease glutathione levels and increases in alanine aminotransferase (ALT) and
aspartate aminotransferase (AST) levels in the blood. Additionally an increase in
collagen synthesis has also been observed.
In terms of the influence of ultrasound on the developmental stages or organs and
systems, there is no conclusive evidence on any effects on fetal growth; however,
wound healing seems to be accelerated. Ultrasound has been shown to produce retinal damage and affect the cornea, but, at the same time, ultrasound can also be used
to treat mild cases of myopia. One significant risk is damage to the inner ear.
Despite all the evidence cited earlier, no epidemiological data can support any
seriously harmful side effect due to ultrasound exposure. This makes ultrasound the
most used diagnostic technique in hospitals.
16.15 SUMMARY
Ultrasound imaging uses a rather simple mechanism of acoustic generation by electric transducers that can produce and acquire mechanical displacement. The principle
of image formation is based on the detection of attenuation, reflection, and TOF for
reflected and transmitted pressure. There are three distinct methods of imaging in use,
attenuation tomography, reflection tomography, and TOF tomography. Each of these
methods targets a specific anatomical contrast feature. Ultrasound is used in many diagnostics and therapeutic applications due to its harmless effects on the biological tissues.