You may be in hundreds of photos in your life, at birthdays, graduations, and so on. But the very first photo of you was probably taken while you was still inside your mom’s belly, thanks to the power of ultrasound.
What is ultrasound?
It’s just a sound with a frequency higher than 20 kilohertz that cannot be heard by humans.
Doctors can use an ultrasound transducer to make ultrasound waves that travel through the body and reflect off organs, bones, or a fetus in order to create an image. The cold squeaky gel smeared on the body helps the waves make it to the inner tissues, opposed to just making the sound bounce off the skin.
When one sound travels from a medium to another, they can be partially reflected. These high-frequency waves, short wavelength ultrasounds are especially good at reflecting without spreading too much.
But how do doctors make sure the waves travel into your body to reflect off organs, instead of just bouncing off your skin?
Here where ultrasound gel comes into play. Something called acoustic impedance means how much obstruction or opposition does a flow encounter (in our case, the flow is the sound wave)
If there’s a huge change in acoustic impedance, like from air to human skin, sound waves will mostly bounce off. But ultrasound gel is a mix of water and polymers like glycerin and propylene glycol which has an acoustic impedance really similar to human skin. By using a transducer to direct ultrasound waves directly into the gel, the waves have a much better chance of transferring into your body, with less reflection right off the bat. Because the two mediums, the gel and skin, ar not so different, the acoustic impedance is low. As the waves travel deeper, some will reflect every time there’s a change in tissue type, like from the lining of a womb to tiny baby finger. The transducer keeps track of things like how long the reflected waves were traveling, the frequency, and amplitude of the wave. Then, a computer generates images based on this information.