Medicine
Ultrasound Imaging
Quick fact
Ultrasound uses frequencies between 2 and 18 megahertz, far above human hearing, and can capture moving images in real time, like a beating heart or a fetus kicking.
Why this is interesting
You've probably seen a grainy black-and-white image of a baby before birth—but how does ultrasound see inside the body without any X-rays or radiation?
Read the full explanation
Understanding Ultrasound Imaging
Think of ultrasound as echolocation, similar to how bats navigate in the dark. A handheld device called a transducer sends out short bursts of high-frequency sound waves into the body. These waves travel through different tissues and bounce back when they hit boundaries between organs or fluids. The transducer then listens for the returning echoes. By measuring the time it takes for each echo to return and its strength, a computer constructs a two-dimensional image—a sonogram. Denser tissues (like bone) reflect more sound and appear bright (white), while fluids (like amniotic fluid) transmit most sound and appear dark (black). Soft tissues show varying shades of gray. This process repeats many times per second, creating a live video of internal structures.
A deeper explanation
At the heart of an ultrasound transducer are piezoelectric crystals—materials that change shape when an electric voltage is applied. When a voltage pulse is applied, the crystal rapidly vibrates, producing sound waves at the desired frequency. These waves propagate through the body and encounter interfaces where the acoustic impedance (resistance to sound) changes. At each interface, a portion of the wave is reflected. The stronger the impedance mismatch, the stronger the echo. The same crystal acts as a receiver: returning sound waves deform the crystal, generating a tiny electrical signal. The system precisely times these signals—the round-trip travel time gives the depth of the reflector. Modern ultrasound also uses the Doppler effect to measure blood flow: sound reflected from moving red blood cells shifts in frequency, revealing speed and direction. This non-invasive technique is invaluable for guiding biopsies, assessing heart function, and monitoring pregnancy, all without harmful radiation.