loadingUltrasound is ―reflected‖ if there is an orderly deflection of all or part of the beam. 超声波的―反射‖是指部分的或所有的波簇都发生有序的偏转。
If part of an ultrasound beam changes direction in a less orderly fashion, the event is usually described as ―scatter.‖
如果部分的超声波簇在改变方向时是无序的话则称为―散射‖。
As an ultrasound beam penetrates a medium, energy is removed from the beam by absorption, scattering, and reflection. These processes are summarized in Figure 19-2. As with x rays, the term attenuation refers to any mechanism that removes energy from the ultrasound beam. Ultrasound is ―absorbed‖ by the medium if part of the beam‘s energy is converted into other forms of energy, such as an increase in the random motion of molecules. Ultrasound is ―reflected‖ if there is an orderly deflection of all or part of the beam. If part of an ultrasound beam changes direction in a less orderly fashion, the event is usually described as ―scatter.‖
当一束超声波穿透介质时，波簇中的能量将随着吸收、散射和反射而减少。这些过程已经概括在图19-2中。和X射线一样，衰减指的是超声波的能量通过任何原理（机械）从超声波束中清除。由于部分的波簇的能量被转变成其他形式的能量，如增加随机运动的分子，所以超声波在介质中可以―被吸收‖。超声波的―反射‖是指部分的或所有的波簇都发生有序的偏转。如果部分的超声波簇在改变方向时是无序的话则称为―散射‖。
The behavior of a sound beam when it encounters an obstacle depends upon the size of the obstacle compared with the wavelength of the sound.
当声波簇遇到障碍物时，声波的反应取决于障碍物相对于声波波长的大小。 If the obstacle‘s size is large compared with the wavelength of sound (and if the obstacle is relatively smooth), then the beam retains its integrity as it changes direction.
如果障碍物的体积大于波长（且障碍物的表面光滑），那么波簇将改变方向且保持它的完整性。
Part of the sound beam may be reflected and the remainder transmitted through the obstacle as a beam of lower intensity.
部分的声波簇可能会被障碍物发射，剩余的部分将通过障碍物，从而造成声波簇的强度减弱。
The behavior of a sound beam when it encounters an obstacle depends upon the size of the obstacle compared with the wavelength of the sound. If the obstacle‘s size is large compared with the wavelength of sound (and if the obstacle is relatively smooth), then the beam retains its integrity as it changes direction. Part of the sound beam may be reflected and the remainder transmitted through the obstacle as a beam of lower intensity.
当声波簇遇到障碍物时，声波的反应取决于障碍物相对于声波波长的大小。如果障碍物的体积大于波长（且障碍物的表面光滑），那么波簇将改变方向且保持它的完整性。部分的声波簇可能会被障碍物发射，剩余的部分将通过障碍物，从而造成声波簇的强度减弱。
If the size of the obstacle is comparable to or smaller than the wavelength of the ultrasound, the obstacle will scatter energy in various directions.
如果障碍物的体积小于或等于超声波的波长，那么障碍物将会造成超声波的能量散射到各个方向上。
Some of the ultrasound energy may return to its original source after ―nonspecular‖ scatter, but probably not until many scatter events have occurred.
一些超声波的能量在经过散射―不是垂直反射‖后也许会回到发射源，但是这种可能性的前提是发生了大量的散射次数后。
If the size of the obstacle is comparable to or smaller than the wavelength of the ultrasound, the obstacle will scatter energy in various directions. Some of the ultrasound energy may return to its original source after ―nonspecular‖ scatter, but probably not until many scatter events have occurred.
如果障碍物的体积小于或等于超声波的波长，那么障碍物将会造成超声波的能量散射到各个方向上。一些超声波的能量在经过散射―不是垂直反射‖后也许会回到发射源，但是这种可能性的前提是发生了大量的散射次数后。
In ultrasound imaging, specular reflection permits visualization of the boundaries between organs, and nonspecular reflection permits visualization of tissue parenchyma (Figure 19-2).
在超声波图像诊断中，规则反射可以清楚地呈现组织器官间的边界，而不规则反射则可以清晰地显示软细胞组织。
Structures in tissue such as collagen fibers are smaller than the wavelength of ultrasound.
组织中的结构如骨胶原纤维是小于超声波的波长的。
Such small structures provide scatter that returns to the transducer through multiple pathways.
这么小的组织结构了超声波通过多重路径散射返回到传感器上。
The sound that returns to the transducer from such nonspecular reflectors is no longer a coherent beam.
返回到传感器上的超声波是来自不规则反射面的，不再是连贯的波簇。 It is instead the sum of a number of component waves that produces a complex pattern of constructive and destructive interference back at the source.
一些组成复杂的图像建立过程的波簇的综合和干涉波簇相消返回在超声波源。
This interference pattern, known as ―speckle,‖ provides the characteristic ultrasonic appearance of complex tissue such as liver.
这种干涉图样，被称为是―斑纹‖，提供了典型的超声波复合组织图案如肝脏的。
Reflection反射
In most diagnostic applications of ultrasound, use is made of ultrasound waves reflected
from interfaces between different tissues in the patient.
在大多数超声诊断应用中，利用的是超声波在病人不同组织界面的反射。 The fraction of the impinging energy reflected from an interface depends on the difference in acoustic impedance of the media on opposite sides of the interface.
从界面反射的少量冲击能取决于界面两侧介质的声阻抗。
The acoustic impedance Z of a medium is the product of the density ρ of the medium and the velocity of ultrasound in the medium: Z = ρc
介质的声阻抗Z是由介质的密度ρ和超声在介质中的速度：? =ρc
For an ultrasound wave incident perpendicularly upon an interface, the fraction αR of the incident energy that is reflected (i.e., the reflection coefficient αR) is 当超声波垂直入射到一个界面，入射能的一部分αR被反射回来（即反射系数αR）是
where Z1 and Z2 are the acoustic impedances of the two media. The fraction of the incident energy that is transmitted across an interface is described by the transmission coefficient αT , where
其中Z1和Z2是两个介质的声阻抗。少量的通过界面传播的入射能被称作传输系数αT，其中
Obviously αT + αR = 1. 很明显 αT + αR = 1。
With a large impedance mismatch at an interface, much of the energy of an ultrasound wave is reflected, and only a small amount is transmitted across the interface.
由于一个接口与一个大的阻抗不匹配，大部分超声波的能源被反射，只有一小部分是通过该界面传输。
For example, ultrasound beams are reflected strongly at air–tissue and air–water interfaces because the impedance of air is much less than that of tissue or water.
例如，因为空气的阻抗比组织或水小，所以超声束在空气-组织界面和空气-水中界面反射强。 Example 19-3
At a ―liver–air‖ interface, Z1 = 1.65 and Z2 = 0.0004 (both multiplied by 10?4 with units of kg-m?2-sec?1).
示例19-3
在一个―肝-空气‖的界面，Z1的= 1.65和Z2 = 0.0004（全部乘以10-4单位是M - 2秒，1个单位）。
Thus 99.95% of the ultrasound energy is reflected at the air–liver interface, and