Question

Can someone please explain (in 1-2 paragraphs) the physical basis of ultrasound imaging. How does it work, what is it used for, and how it's related to concepts of waves and impedance? Basically, how does ultrasound imaging accomplish its function via physics concepts?

Answer 1

The ultrasound beam originates from mechanical oscillations of crystals in a transducer, which is excited by electrical pulses (piezoelectric effect). The transducer converts one type of energy into another (electrical to mechanical/sound).

The ultrasound waves (pulses of sound) are sent from the transducer, propagate through different tissues, and then return to the transducer as reflected echoes. The returned echoes are converted back into electrical impulses by the transducer crystals and are further processed to form the ultrasound image presented on the screen.

The portion of the ultrasound beam that is reflected is determined by the difference in acoustic impedance between adjacent structures. Acoustic impedance is the product of a tissue's density and the velocity of the sound waves passing through it; therefore, the denser the tissue, the greater the acoustic impedance. The large differences in density and sound velocity between air, bone, and soft tissue create a correspondingly large difference in acoustic impedance, causing almost all of the sound waves to be reflected at soft tissue-bone and soft tissue-air interfaces.