DK2985_C000 1..28 - AlSharqia Echo Club

6 Transesophageal Echocardiography(A)(B)TI = 0.80 dBTI = 0.2-6 dB-(C)(D)LAAoLVRV-TI = 0.0-15 dBFigure 1.5 Transmitting power and thermal index. (A–D) The thermal index (TI) decreases when transmitting power is reduced[expressed in negative decibel (dB)]. A value of 0 dB means that the emitting source is at 100% of its power (Ao, aorta; LA, leftatrium; LV, left ventricle; RV, right ventricle).D. Specular Reflection1. Acoustic ImpedanceThe acoustic impedance (Z) is a characteristic of the interactionbetween an ultrasound wave and the medium. It canalso be defined as the product of r and c.Z ¼ r c (1:4)As both r and c are constants of a material, so is Z[Eq. (1.4)]. When ultrasound propagates from onemedium to another, the impedance generally changes atthe interface between the media. When the boundary isperpendicular to the direction of propagation, part of theincoming ultrasound energy is reflected back, whereasthe remainder penetrates the new medium and pursuesits course. The relative amount of reflected energydepends on the impedance difference, as described laterin this chapter. When the ultrasound l is much smallerthan the size of the interface, this phenomenon is termedspecular reflection. On the other hand, when the ultrasoundl is much larger than the size of the interface, it istermed nonspecular reflection or scattering.2. Angle DependenceWith any angle of incidence u with respect to the impedanceboundary, the reflected beam is oriented with thesame angle of incidence u. This more general case ofspecular reflection is illustrated in Fig. 1.6. Ultrasoundimaging is based on the detection of the reflected