Sound is a mechanical energy that is transmitted through a medium .The term MECHANICAL is defined as a physical change induced by a force, sound waves require elastic deformable medium for propagation .Ultrasound is defined as a high frequency mechanical wave that humans cannot hear with frequencies greater than 20K Hz. Waves are divided into two basic types :longitudinal and transverse, Ultrasound waves are longitudinal waves but only bone can cause production of transverse waves ,sometimes referred as shear waves or stress waves. Sound has a greater velocity in air (low density) than in bone (high density),Velocity is inversely proportional to the square root of the compressibility of the medium √B ,compressibility indicates the fractional decrease in volume when pressure is applied to the material, easier the medium to compress ,higher the compressibility, as bone is less compressible velocity of sound is more in bone .Reciprocal of compressibility is given by Bulk modulus, which means bulk modulus is directly proportional to velocity .As bulk modulus increases compressibility decreases in turn increases velocity of sound in the medium referring to this property as stiffness of medium The major interaction of interest for diagnostic ultrasound is that of reflection .if a sound beam is directed at right angles (normal incidence) to an interface (boundary between tissue type) larger than width of the sound beam, the beam is partially reflected back towards the sound source .the interface is called specular reflectors, and they are responsible for major organ outline seen in diagnostic ultrasound examination
The angle of reflection of the sound bean is equal to the angle of incidence of the sound beam. To obtain maximum reflected signal the transducer must be oriented so that the generated sound beam strikes the interface perpendicularly (normal incidence).to understand the concept of reflection of energy let us go back to basic physics of conservation of energy and conservation of momentum .conservation of energy would permit to transfer all its energy from one object to another, where as conservation of momentum prevents this due to differences in masses .Energy cannot be transmitted readily from large objects to small objects or from small objects to loarge subjects .If massive transfer of kinetic energy are required collision between objects of equal masses must occur .for sound waves which are a vibrating molecules ,they to behave in same manner ,as long as the molecule is transmitting energy to an identical sized molecule ,maximum transfer occurs .If there is a difference in the mass of the molecule ,less energy is transferred and the energy that is not transferred is reflected .In ultrasound ,the analogy to momentum is acoustic impedance .here ,one does not look at individual molecules ,but rather at their concerted action ,there by applying the concept of density.
Momentum (mechanics)=Mass × velocity Acoustic impedance( Ultrasound)= Density × Velocity
To simplify the term acoustic impedance I would say it is equal to resistance
High density materials give rise to high sound velocities and therefore high acoustic impedance, similarly low density materials such as gasses have low acoustic impedence .If acoustic impedence is the same in one medium as in another ,then the sound is readily transmitted from one medium to the other .a difference in acoustic impedance causes some portion of the sound to be reflected at the interface .this ist the main reason for ultrasound images .
The reflection coefficient for intensity is expressed as
α =reflection coefficient
z2 =impedance of medium 2
z1=impedance of medium1
The difference between two impedance subtracted any way and squared gives the same number, if acoustic impedance difference is small magnitude of the reflected wave is small. Bone is avoided during an ultrasound examination because acoustic impedance .As the difference between bone and tissue is very large most of the wave is reflected, little of the beam is transferred behind the bone and most of the incident beam returns to the detector, that is the reason why ultrasound is restricted to bony region .Thickness of the medium is never considered in calculation of reflection ,only impedance miss match is a point of concern
Another interaction that occurs between ultrasound and tissue is that of refraction .If the ultrasound beam strikes an interface between two media at an angle of 90 degree with respect to the surface ,a percentage of beam is reflected back to the first medium ,and the rest is transmitted into second medium without a change in direction ,if velocity of sound in both the medium will be equal then no refraction will occur although acoustic impedance may be different .For ultrasound image it is assumed that ultrasound beam always travel in straight lines through tissue .another important aspect is scattering and absorption ,all other modes of interaction except absorption decreases the ultrasonic beam intensity by redirecting the energy of the beam . Absorption is a process where in ultrasonic energy is transformed into another energy form, primarily heat which is used for physiotherapy .The absorption of the medium is related to frequency of beam ,viscosity of medium and relaxation time of the medium .If substances have a short relaxation time ,the molecules return to their original position before the next compression of wave arrives .however if the medium has a long relaxation time ,molecules may be moving back toward their original position as the wave crest strikes them ,more energy is required to stop and then reverse the direction of the molecule there by producing more heat