You might be in hundreds of photos in your life, at birthday parties, graduations, or just snapping a selfie with friends. However, the very first photo of you was probably taken while you were still inside your mom, thanks to the power of ultrasound. Doctors can use an ultrasound transducer to make ultrasound waves that travel through your body, and reflect off different squishy bits, like organs, or a fetus. Then the transducer uses the reflected waves to create an image. That cold, sticky gel gets smeared on your body to help these waves make it to those inner tissues. However, how exactly does it work? Well, ultrasound waves are just another type of soundwave. Besides, sound waves travel by vibrating particles back and forth, like the ones that make up air.
The higher the frequency of a sound, the faster the vibrations. At really high frequencies, human ears cannot pick up around 20kiloHertz, the vibrations, so we call them ultrasound waves. Because ultrasound is just sound, it is safe for medical imaging. When sound waves travel from one medium to another, like from air to your skin, or skin to your liver, they can be partially reflected. Moreover, this high frequency, short-wavelength ultrasound waves are especially good at reflecting without spreading out too much. That is why they are good for making detailed images and for echolocation if you are a bat or dolphin. However, how do doctors make sure the waves travel into your body to reflect off organs, instead of just bouncing off your skin? Here is where ultrasound gel comes into play.
Something’s acoustic impedance how much it resists vibration due to sound—affects how much a sound wave will reflect when it travels between two materials. If there is a huge change in acoustic impedance, like from air to human skin, soundwaves will mostly bounce off. However, ultrasound gel is a mix of water and polymers like glycerin and propylene glycol and has an acoustic impedance that is similar to human skin. By using a transducer to make ultrasound waves in that gel, the waves have a much better chance of transferring into your body, with less reflection right off the bat. As the waves travel deeper, some will reflect every time there is a change in tissue type, like from the lining of a womb to tiny baby fingers.
The transducer keeps track of things like how long the reflected waves were travelling, and the frequency and amplitude of the wave. Then a computer uses all this information to generate images of what is inside you. All thanks to sounds that you cannot even hear. Thanks for asking, and thanks especially to all of our patrons on Patreon who keep these answers coming.