How does ultrasound imaging work?

[anorred]

From what I understand, ultrasound beams are generated from hundreds of piezo crystals that constructively interfere with one another. When the beam is reflected, it returns to the crystals and the crystal directly above the reflection (the crystal that obtains mechanical vibrations) records the data and assumes the reflection point is right underneath the crystal. Data is then interpreted based on the time it takes for the wave to travel back and forth to the crystal. Please help me understand this phenomenon better. I want to know how these images are created.

 

[Simon Bridge]

The time is interpreted as a density - and you also know what sorts of material densities are likely.
The rest is up to the electronic processing - usually done by a computer.

The topic is usually part of a course in "inverse problems" and the exact process is not trivial.

 

[Bobbywhy]

To form an image of internal structures, a transducer is placed against the skin. Transducers can consist of only one piezoelectric element or several, but never "hundreds", depending on the requirements of the system. The transducer sends out high frequency sound waves that reflect off of body structures. The returning sound waves, or echoes, are displayed as an image on a monitor. The image is based on the frequency and strength (amplitude) of the sound signal and the time it takes to return from the patient to the transducer.
There are several technical websites available for learning the principles of operation using the search terms “ultrasonic imaging physics”.

Here are two general descriptions:
https://en.wikipedia.org/wiki/Medical_ultrasonography
http://www.criticalecho.com/content/tutorial-1-basic-physics-ultrasound-and-doppler-phenomenon