Perpendicular Incidence occurs when soundwave hits a surface or tissue boundary that is perpendicular to the media. The variables involved in perpendicular incidence consists of the incident pulse, reflected pulse, and transmitted pulse. The incident pulse is the sound sent out from the transducer and hits the first media at a 90 degree angle at 100%. Once the incident pulse hits the surface, and depending upon the impedance of the first medium, an echo is "reflected" back to the sound source and this is called the reflected pulse. If, however, the impedance on the first medium is less, then a pulse echo is transmitted through the first medium into the second thereby inducing a transmitted pulse. The reflected pulse is the only sound that changes direction opposite to the incident pulse, whereas the transmitted pulse is unchanged and remains in the same direction as that of the incident pulse.
Impedance is the resistance of the medium, and is equal to the density(p) multiplied by the propagation speed. Impedance units are in rayls, and the greater the acoustic impedance, the more dense the material (i.e. bone, air). Therefore, if the medium is more resistant (more incompressible), then density and propagation speed increases. This will cause more echoes to reflect back to the sound source thus an image will be produced (mostly shadowing or attenuation). On the other hand, if resistance or impedance decreases, then more transmitted pulses are generated and no image will be produced (i.e. CW).
Impedance can be broken down into two types: Intensity Reflection Coefficient (IRC) and Intensity Transmission Coefficient (ITC). IRC is the fraction or percentage of the reflected intensity going back through the first medium, and is determined by dividing the relected echo and incident pulse. ITC, on the other hand, is the fraction or percentage of transmitted intensity going through the first medium and into the second, and is determined by dividing the transmitted echo and incident pulse. Both IRC and ITC depend upon the impedances of media one and media two (Z1 & Z2). If there is a large difference between the two media, a stronger echo is produced and IRC increases. If the two media are almost alike, a weaker echo is produced and ITC increases. If however, both mediums are equal, then no echo is produced thus no reflection and ITC is equal to the incident intensity.
If sound hits the media at an oblique angle, then Oblique Incidence occurs. In Oblique Incidence, echoes may reflect back to the sound source or may transmit or "refract" through the medium. Oblique Incidence is defined as the change or shift in the direction of sound due to differences of propagation speeds. As previously mentioned, Perpendicular Incidence depend upon the resistance or impedance of the medium, whereas in Oblique Incidene, the variables invovled depend upon the propagation speeds on either side of the boundary. Oblique Incidence consists of the incident angle, reflected angle, and transmitted angle. The incident angle is "always" proportional to the reflected angle. The transmitted angle plays a role in refraction and depends upon the difference between the propagation speed of medium one and medium two.
Refraction is the change of direction in sound when crossing a boundary. It is caused by Oblique Incidence and a difference in propagation speeds on either side of the boundary. That is, if the propagation speed in medium two is greater, then the transmitted angle increases thus making it greater than the incident angle (and vice-versa). If, however, both of the propagation speeds are equal, then the transmitted angle will be proportional to the incident angle. The relationship between the incident and transmitted angle can be expressed in Snell's Law
No comments:
Post a Comment