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Why is the speed of ultrasound average between two different boundires?

As I understand in ultrasound the speed of sound is an average speed of $1540ms^{-1}$ to calculate time intervals into distances.

My question why is this?

Think of a typical situation where we have a ultrasound pulse and it travels through soft tissue at a speed of $1600ms^{-1}$ at say $5cm$ deep then it comes into contact with a cyst full of fluid say $2cm$ and the speed of ultrasound is $1500ms^{-1}$ in the fluid.

Why cant we just work out the time for the ultrasound wave to hit the entrance point to the cyst, and then work out the ultrasound if it were to hit the exit point of the cyst and reflect back and combine the two times together.

If this more of a processing issue with the transducer and thus why we take an average or is there a physics concept that I am missing?

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  • $\begingroup$ I am having a hard time understanding your question. What do you mean? $\endgroup$ – MichaelK Aug 7 '18 at 10:21
  • $\begingroup$ I will edit my question and try and phrase it better, my apologies. $\endgroup$ – james2018 Aug 7 '18 at 11:39
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    $\begingroup$ I suspect in many cases the detailed composition of the object being imaged, and the speed of sound in all its various regions isn't known. So we just use an average speed and accept the small errors this causes. $\endgroup$ – John Rennie Aug 7 '18 at 11:56
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Conventional B-mode ultrasound is primarily attenuation based, so the contrast between different objects comes from how much of the reflected sound is detected by the transducer.

Why cant we just work out the time for the ultrasound wave to hit the entrance point to the cyst, and then work out the ultrasound if it were to hit the exit point of the cyst and reflect back and combine the two times together.

Your thinking assumes you (the software in the machine) knows the structure of the object being imaged. It also assumes that you (the software in the machine) knows the speed of sound for whatever materials are in the beam's path. The software in the ultrasound unit has no idea of what the speed of sound is for the material being imaged. To make constructing the ultrasound image possible, the machine assumes a typical speed of sound in soft tissue.

You know, all of the questions you've been asking could be fairly simply and quickly answered just by having conversations with your professor, or fellow students.

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