The principle of ultrasound: Difference between revisions

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.  

Let us talk about '''Impedance''' (Z).  This is an important concept and it is related to reflection of ultrasound energy.  It is calculated and is not measured directly.  The higher the difference of the acoustic impedance between two media, the more significant is the reflection of the ultrasound.  That is why we use coupling gel between the ultrasound transducer and the skin.  By using the gel, we decrease the impedance and allow the ultrasound to penetrate into the tissue.  Otherwise, the impedance between skin/transducer is so high that all the energy will be reflected and no image will be produced.  More of on reflection – it occurs only when the acoustic impedance of one media is different from acoustic impedance of the second media at the boundary.  If the ultrasound hits the reflector at 90 degrees (normal incidence), then depending on the impedances at the boundary the % reflection = ((Z2 - Z1) / (Z2 + Z1))^2.  Then transmission is 1 - % reflection.  Physics of oblique incidence is complex and reflection/transmission may or may not occur.  We do know that the incident intensity is equal to the sum of the transmitted and reflected intensities.