The principle of ultrasound: Difference between revisions

Back to propertied of pulsed ultrasound, we need to discuss '''spatial pulse''' length.  Up to now we introduced properties that were related to timing.  Spatial Pulse Length is the distance that the pulse occupies in space, from the beginning of one pulse till the end of that same pulse.  It is measured in units of distance with typical values from 0.1 to 1 mm.  SPL (mm) = # cycles x wavelength (mm).  Axial or longitudinal resolution (image quality) is related to SPL.  Axial resolution = SPL/2 = (# cycles x wavelength)/2.   

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.