## Sound propagate as a series of compressions and

Sound is a mechanical disturbance set up in a medium,
such that small parts of the medium, i.e., particles, execute oscillatory
movements. This process does not involve any mass transfer, and originates from
a local change in the stress or pressure field within the medium. Mechanical
energy is ’embedded’ in the medium, in the form of elastic strains and
vibrations of the molecules. However, due to the medium’s elasticity, when a
particle is displaced elastic forces that tend to restore it to its original
position are developed. Through this oscillation and the interaction
between  different particles, acoustic
energy can propagate across the medium in the form of a wave. In the case of
sound, these waves are called acoustic waves. Therefore, sound requires a
medium to propagate, be it a gas, liquid, or solid, but cannot propagate in
vacuum.

Acoustic waves propagate as a series of compressions
and rarefactions of the medium. Acoustic waves of frequencies between 20 Hz and
20 kHz are called audible while acoustic waves of higher frequencies are
referred to as ultrasound or ultrasonic waves. Depending on the direction of
the particle motion, acoustic waves can be categorized as longitudinal or
transverse waves.

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Acoustic Medium Properties: Density: The density ? (in
kg/m3) of a medium is the ratio of the medium’s mass per unit volume.  Density plays a major role in the behavior of
acoustic waves, as it influences a medium’s characteristic acoustic impedance
Z. Speed of Sound : A medium’s speed of sound c (in m/s), is the speed at which
an acoustic wave propagates through that medium. It is known from theoretical
acoustics, that a medium’s c is related to that medium’s compressibility ? and
density ?.

Compressibility is a measure of the relative volume
change of a medium as a response to a given pressure. Alternatively, c can be
calculated through the medium’s elastic bulk modulus B (measured in Pa), which
is the reciprocal of ?, and acts as a measure of the medium’s tendency to be
deformed elastically, i.e., non-permanently, when a force is applied to it.

Absorption Mechanisms & Coefficient: When an
acoustic wave is propagating through a medium such as tissue, it experiences a
loss of kinetic energy through conversion to thermal energy by a phenomenon
called absorption. The absorption coefficient ? constitutes the sum of all the
aforementioned losses and is a frequency dependent medium property, as
discussed above. Characteristic
Acoustic Impedance: The characteristic acoustic impedance Z) is an inherent
property of a medium.

Acoustic Wave Properties: Acoustic Intensity: A
definitive parameter associated with an acoustic wave is its intensity. An
ultrasound wave carries kinetic energy as it propagates. Intensity, defined as
the energy propagating through unit area per unit time. Reflection and Refraction are
other properties of acoustic wave Ref. Diffraction:
When an incident wave impinges upon a barrier with finite length, and therefore
edges, the wave tends to spread and/or bend around those edges. A similar
phenomenon is observed when that barrier exhibits small openings. This
phenomenon is referred to as diffraction, and causes the wave  trajectories to bend and propagate in zones
that would have been shadowed otherwise. Scattering: Scattering is a direct
consequence of reflection and is the cornerstone of diagnostic ultrasound.
Scattering occurs when an acoustic wave travels through an inhomogeneous
medium. 5.  Attenuation: When an acoustic
wave is propagating through a medium, the amplitude of the acoustic pressure,
and all related quantities such as intensity, are reduced exponentially as the
wave progresses. This phenomenon is called attenuation. Attenuation is a result
of absorption by the medium, as well as scattering, and determines the extent
of penetration of an acoustic wave in a material or tissue. Physical Effects of
Ultrasound: Acoustic waves interact with the
medium in which they are propagating through the particle motion and
pressure variations. This interaction yields a number of different physical
effects, which can be classified into thermal effects and nonthermal effects. Thermal
effects are mostly related to the medium’s temperature increase, due to the
conversion of acoustic energy into heat. The nonthermal effects are mechanical
in nature and include radiation force, acoustic streaming and the formation and
cavitation of microbubbles.

Nonthermal Effects: Radiation Force:  When encountering a (partially) reflective
attempting to ‘push’ it along the direction of propagation. where Frad is the radiation force,
? is absorption coefficient and I is the acoustic intensity c is the medium’s
speed of sound. 2. Acoustic Streaming: When an acoustic wave is propagating in
a fluid, the acoustic radiation force creates a non-oscillatory, fluidic motion
which is called acoustic streaming. Acoustic Cavitation: The term acoustic
cavitation is used to define the interaction between an acoustic field and
microscopic bodies of gas in any medium or tissue.Ref
stable and inertial cavitation are two kinds of cavitation that discussed in
detail in Ref .   In
medicine, the applications of ultrasound are mainly divided into two
categories, Diagnostic Ultrasound and Therapeutic Ultrasound. For diagnostic
ultrasound, the ultrasonic signal level is low so that the propagation of
ultrasound in human tissue has no obvious physical, chemical or biological
effects. For therapeutic ultrasound, the ultrasonic signal level is
comparatively high depending on the different treatments.

For ultrasonic therapy, since the ultrasonic intensity
is high, some physical, mechanical, chemical and biological effects may be
produced because of the intense interaction between the intense ultrasound and
the human tissue. In other words, in ultrasonic therapy, the ultrasonic energy
is used to produce some permanent changes for the biological tissue structure,
status or function so that the treatment of certain human diseases can be
realized.

For diagnostic applications, exposure is chosen
primarily for their ability to give images with good spatial and temporal
resolution, using sufficient S/N ratio. The aim is obtaining required
diagnostic information significant cellular effect. In therapeutic ultrasound
the exposed target tissue undergoes reversible or irreversible change depending
on the goal of the treatment.

Therapeutic ultrasound divides into two classes,
applications that use ‘low’ intensity and those using ‘high’ intensities.

The intention of the lower intensity treatments is to
stimulate normal physiological responses to injury, or to accelerate some
processes such as the transport of drugs across the skin. Low intensity
applications include physiotherapy, bone healing, and drug uptake. The purpose
of the high intensity treatments is rather to selectively destroy tissue in a
controlled fashion. High Intensity applications mostly involved HIFU
applications.

Low intensity applications: Physiotherapy: Ultrasound is
an alternative method to hot pack, microwave and RF heating for soft tissue
injuries and bone and joint conditions. Selection of transducers is done by
physiotherapist. The sound is directly coupled in to the patient through a thin
layer of coupling medium. Ultrasonic enhancement of drug uptake: Sonophoresis:
Ultrasound may be used to improve the penetration of the pharmacologically
active drugs through the skin. Sonoporation and sonodynamic therapy:
Sonoporation is the term using for the phenomenon by which ultrasound may
transiently alter  the structure of the
cellular membrane and thus allow enhanced uptake of low and high molecular
weight molecules in to the cell. Gene therapy: Transferring of gens in to the
diseased tissue and organs is an interested subject. The ideal system would
increase the gene expression in to the target while having no effect in
non-target tissue. Ultrasound might be able to provide this localization.

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