Modos de Compensación de la Presbicia con Lentes de Contacto

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made up of water that can both reflect and transmit the sound waves produced from ultrasonography. The reflected sound generates the image at the boundary of the structure being scanned, while the transmitted sound penetrates into deeper levels within the body. In biological structures that have low water content such as bone, there is almost no transmission of the wave with all the sound being reflected back. This produces a bright echo that defines the outer boundaries of the bone but because there is no further sound penetration a shadow is formed and the ability of the sound waves to create images from a deeper level is not possible (Walker, et al 2004).

The velocity ( ) at which the sound wave travels is determined by the frequency (f) and the wave length ( ) and this relationship can be expressed by the equation

v = f

The speed (c) at which ultrasound propagates through tissue depends on the stiffness and density ( ) of the tissue which can be combined into a parameter known as its acoustic impedance (Z) with the stiffer tissue producing the higher speeds. This relationship is given by the equation

Z = c

The value given for the acoustic impedance of soft tissue has been reported as 1.64 x 106 kg.m3.s-1 while for bone it is 7.68 x 106 kg.m3.s-1 (Cameron, Skofronick, 1978). This shows that a sound wave pulse propagates faster through bone than through soft tissue. If the speed of the sound wave through the medium is known then the

reflection from the returning wave can be measured as a function of time and information can be obtained on the position of the tissue.

When a sound wave reaches the boundary between two mediums, the wave can change its behaviour and be reflected off the medium, diffracted or refracted around the medium and/or transmitted into the medium (Cameron, Skofronick, 1978).

The basis of ultrasonography as an imaging technique is the partial reflection of sound at the boundary of two structures that have different acoustic impedances to produce an echo (Cameron, Skofronick, 1978). The ratio of reflection of the sound wave (R) to the incident wave (A0) depends on the acoustic impedance at the boundary of two

media (Z1 and Z2) and is given by the equation

0 A R = 2 1 1 2 Z Z Z Z  

When sound waves encounter a medium or a boundary with varying acoustic

impedance it changes direction and bends around the medium or passes through small openings between two boundaries and spreads out in a process called diffraction. The amount the wave is diffracted depends on the wavelength and the difference in acoustic impendence of the median or the boundary. The sharpness of the diffraction increases with increasing wavelength and decreases with decreasing wavelength. When the wavelength is smaller than the medium or opening, no noticeable diffraction occurs.

Refraction produces a change in the direction of the sound wave as it passes from one medium to another and is accompanied by a change in its speed and wavelength. Refraction of sound waves is most evident when a wave passes through a medium

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depends on the angle of the incident wave and the speed of sound on either side of the boundary (Cameron, Skofronick, 1978).

Figure 2-37 shows the behavior of ultrasound at a boundary from a perpendicular and non-perpendicular incidence. In Figure 2-36 a) a proportion of the ultrasound wave is reflected back to the source while a proportion is transmitted and continues along the original path. In Figure 2-36 b) part of the wave is reflected but not returned to the transducer and part is refracted at a different angle.

a)

b)

Figure 2-36: Refection and refraction of an ultrasound wave. (Adapted from Evans, 2006).

transmitted. The ratio of the transmitted sound wave (T) to the incident wave (A0) is

given by the equation

2 1 2 0 2 Z Z Z A T  

As an ultrasound wave pulses through tissue the amplitude and intensity of the wave is reduced due to absorption or scattering in a process called attenuation. Absorption is the conversion of the mechanical energy of the sound wave into heat while

scattering results in the energy redirected out of the incident energy path. Structures with large differences in acoustic impedance at their boundary produce a scattering of the incident sound wave resulting in the wave being transmitted in all directions (Suetens, 2002).

Figure 2-37 illustrates the scattering of the sound wave. The pattern and the amount of scattering that occurs, depends on the size and the density of the structure within the propagation path of the incident energy. Structures smaller than the wavelength, produce a uniform scattering pattern while larger structures produce a more complex scattering pattern (Suetens, 2002).

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Figure 2-37: Scattering of ultrasound wave. (Adapted from Evans, 2006)

Once the transducer has sent the sound wave pulse and sufficient time has elapsed for all the echoes to return, another sound wave is produced from a slightly different direction and the sequence is repeated. The position of the structure that produced the echo is

calculated from the direction that the pulse was transmitted and the time delay between the transmission of the pulse and the receipt of the reflection (Suetens, 2002).