The principle of ultrasound: Difference between revisions

We will now talk about '''interaction of ultrasound''' with tissue.  As we discussed in the section of amplitude, the energy of ultrasound decreases (attenuation) as it travels through tissue.  The stronger the initial intensity or amplitude of the beam, the faster it attenuates.  Standard instrument output is ~ 65 dB.  So for a 10 MHz transducer, the maximum penetration would be as follows:  1 dB/cm/MHz x 10 MHz x (2 x max depth) = 65 dB.  Max depth = 65/20 = 3.25 cm.  If we use a 3.5 MHz transducer and apply the same formula for max depth, will get Max depth = 65/7 = 9.3 cm.  Attenuation of ultrasound in soft tissue depends on the initial frequency of the ultrasound and the distance it has to travel.  As we saw in the example above, in soft tissue the greater the frequency the higher is the attenuation.  So we can image deeper with lower frequency transducer.  The further into the tissue the ultrasound travels, the higher the attenuation is, so it is ultimately the limiting factor as to how deep we can image clinically relevant structures.   

There are 3 components of interaction of ultrasound with the tissue medium: absorption, scattering, and reflection.  Absorption of ultrasound by tissue implies loss of energy that is converted to heat.  The highest attenuation (loss of energy) is seen in air, the lowest is seen in water. Reflection is the process were propagating ultrasound energy strikes a boundary between two media (i.e., the RV free wall in the parasternal long axis) and part of this energy returns to the transducer.  If the reflector is very smooth and the ultrasound strikes it at 90 degree angle (perpendicular), then the reflection is strong and called specular.  If the incidence is not 90 degree, then specular reflectors are not well seen.  Another instance when specular reflection is produced is when the wavelength is much smaller than the irregularities of the media/media boundary.  Diffuse or Backscatter reflections are produced when the ultrasound returning toward the transducer is disorganized.  This occurs when the ultrasound wavelength is similar size to the irregularities of the media/media boundary. When the ultrasound wavelength is larger than the irregularities of the boundary, the ultrasound is chaotically redirected in all directions or scatters.  If the reflector is much smaller than the wavelength of the ultrasound, the ultrasound is uniformly scattered in all directions and this is called Rayleigh scattering.  Red blood cell would be an example of Rayleigh scatterer.  Rayleigh scattering is related to wavelength to 4th power.  Backscatter is what produces the relevant medical imaging.