Sound is a mechanical disturbance set up in a medium,such that small parts of the medium, i.
e., particles, execute oscillatorymovements. This process does not involve any mass transfer, and originates froma local change in the stress or pressure field within the medium. Mechanicalenergy is ’embedded’ in the medium, in the form of elastic strains andvibrations of the molecules. However, due to the medium’s elasticity, when aparticle is displaced elastic forces that tend to restore it to its originalposition are developed. Through this oscillation and the interactionbetween different particles, acousticenergy can propagate across the medium in the form of a wave. In the case ofsound, these waves are called acoustic waves.
Therefore, sound requires amedium to propagate, be it a gas, liquid, or solid, but cannot propagate invacuum. Acoustic waves propagate as a series of compressionsand rarefactions of the medium. Acoustic waves of frequencies between 20 Hz and20 kHz are called audible while acoustic waves of higher frequencies arereferred to as ultrasound or ultrasonic waves. Depending on the direction ofthe particle motion, acoustic waves can be categorized as longitudinal ortransverse waves.
Acoustic Medium Properties: Density: The density ? (inkg/m3) of a medium is the ratio of the medium’s mass per unit volume. Density plays a major role in the behavior ofacoustic waves, as it influences a medium’s characteristic acoustic impedanceZ. Speed of Sound : A medium’s speed of sound c (in m/s), is the speed at whichan acoustic wave propagates through that medium. It is known from theoreticalacoustics, that a medium’s c is related to that medium’s compressibility ? anddensity ?.Compressibility is a measure of the relative volumechange of a medium as a response to a given pressure. Alternatively, c can becalculated through the medium’s elastic bulk modulus B (measured in Pa), whichis the reciprocal of ?, and acts as a measure of the medium’s tendency to bedeformed elastically, i.e.
, non-permanently, when a force is applied to it.Absorption Mechanisms & Coefficient: When anacoustic wave is propagating through a medium such as tissue, it experiences aloss of kinetic energy through conversion to thermal energy by a phenomenoncalled absorption. The absorption coefficient ? constitutes the sum of all theaforementioned losses and is a frequency dependent medium property, asdiscussed above. CharacteristicAcoustic Impedance: The characteristic acoustic impedance Z) is an inherentproperty of a medium.Acoustic Wave Properties: Acoustic Intensity: Adefinitive parameter associated with an acoustic wave is its intensity. Anultrasound wave carries kinetic energy as it propagates. Intensity, defined asthe energy propagating through unit area per unit time.
Reflection and Refraction areother properties of acoustic wave Ref. Diffraction:When an incident wave impinges upon a barrier with finite length, and thereforeedges, the wave tends to spread and/or bend around those edges. A similarphenomenon is observed when that barrier exhibits small openings. Thisphenomenon is referred to as diffraction, and causes the wave trajectories to bend and propagate in zonesthat would have been shadowed otherwise. Scattering: Scattering is a directconsequence of reflection and is the cornerstone of diagnostic ultrasound.Scattering occurs when an acoustic wave travels through an inhomogeneousmedium.
5. Attenuation: When an acousticwave is propagating through a medium, the amplitude of the acoustic pressure,and all related quantities such as intensity, are reduced exponentially as thewave progresses. This phenomenon is called attenuation.
Attenuation is a resultof absorption by the medium, as well as scattering, and determines the extentof penetration of an acoustic wave in a material or tissue. Physical Effects ofUltrasound: Acoustic waves interact with the medium in which they are propagating through the particle motion andpressure variations. This interaction yields a number of different physicaleffects, which can be classified into thermal effects and nonthermal effects. Thermaleffects are mostly related to the medium’s temperature increase, due to theconversion of acoustic energy into heat. The nonthermal effects are mechanicalin nature and include radiation force, acoustic streaming and the formation andcavitation of microbubbles.
Nonthermal Effects: Radiation Force: When encountering a (partially) reflectivesurface, radiation pressure will exert a radiation force on that interface,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’sspeed of sound. 2. Acoustic Streaming: When an acoustic wave is propagating ina fluid, the acoustic radiation force creates a non-oscillatory, fluidic motionwhich is called acoustic streaming. Acoustic Cavitation: The term acousticcavitation is used to define the interaction between an acoustic field andmicroscopic bodies of gas in any medium or tissue.Refstable and inertial cavitation are two kinds of cavitation that discussed indetail in Ref . Inmedicine, the applications of ultrasound are mainly divided into twocategories, Diagnostic Ultrasound and Therapeutic Ultrasound.
For diagnosticultrasound, the ultrasonic signal level is low so that the propagation ofultrasound in human tissue has no obvious physical, chemical or biologicaleffects. For therapeutic ultrasound, the ultrasonic signal level iscomparatively high depending on the different treatments. For ultrasonic therapy, since the ultrasonic intensityis high, some physical, mechanical, chemical and biological effects may beproduced because of the intense interaction between the intense ultrasound andthe human tissue. In other words, in ultrasonic therapy, the ultrasonic energyis used to produce some permanent changes for the biological tissue structure,status or function so that the treatment of certain human diseases can berealized.
For diagnostic applications, exposure is chosenprimarily for their ability to give images with good spatial and temporalresolution, using sufficient S/N ratio. The aim is obtaining requireddiagnostic information significant cellular effect. In therapeutic ultrasoundthe exposed target tissue undergoes reversible or irreversible change dependingon 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 tostimulate normal physiological responses to injury, or to accelerate someprocesses such as the transport of drugs across the skin. Low intensityapplications include physiotherapy, bone healing, and drug uptake.
The purposeof the high intensity treatments is rather to selectively destroy tissue in acontrolled fashion. High Intensity applications mostly involved HIFUapplications.Low intensity applications: Physiotherapy: Ultrasound isan alternative method to hot pack, microwave and RF heating for soft tissueinjuries and bone and joint conditions. Selection of transducers is done byphysiotherapist. The sound is directly coupled in to the patient through a thinlayer of coupling medium. Ultrasonic enhancement of drug uptake: Sonophoresis:Ultrasound may be used to improve the penetration of the pharmacologicallyactive drugs through the skin. Sonoporation and sonodynamic therapy:Sonoporation is the term using for the phenomenon by which ultrasound maytransiently alter the structure of thecellular membrane and thus allow enhanced uptake of low and high molecularweight molecules in to the cell. Gene therapy: Transferring of gens in to thediseased tissue and organs is an interested subject.
The ideal system wouldincrease the gene expression in to the target while having no effect innon-target tissue. Ultrasound might be able to provide this localization.