BackUltrasound Imaging
Ultrasound imaging is a modality that uses high-frequency sound waves to create images of the organs and structures inside the body
02/2026
BackUltrasound imaging is a modality that uses high-frequency sound waves to create images of the organs and structures inside the body
Ultrasound imaging, or echography, is a medical imaging technique that uses high-frequency sound waves to view inside the body. But more simply, imagine throwing a ball against a wall in the dark: by measuring how fast and how hard it bounces back, you could see where the wall is and what it's made of. Unlike X-rays, ultrasound does not use ionizing radiation, making it safe and widely used. By emitting sound waves and listening to the echoes that bounce back from tissues, an ultrasound machine can construct detailed images of organs, tissues, and blood flow. But how exactly does sound become an image?
Before we can create an image, we need to understand how sound waves travel through the body. Ultrasound waves are mechanical vibrations that propagate through a environment. When a sound wave hits a border between two tissues, it faces a choice: keep going (Transmission) or bounce back (Reflection).
The amount of reflection depends on the difference in Acoustic Impedance (Z) between the two tissues. If the difference is small (e.g., muscle to fat), most of the sound is transmitted. If the difference is huge (e.g., tissue to bone or air), almost all the sound is reflected, creating a bright white line and a dark shadow behind it.
Try clicking on "Air" in the interactive visualisation to see why ultrasound gel is essential during exams between the probe and the skin!
Now that we know how sound waves interact with tissues, how do we actually create and detect them? The answer lies in the Piezoelectric Effect.
Inside the ultrasound transducer (probe), there are special crystals (Piezoelectric Crystals). These crystals have special properties:
This dual capability allows the same crystal to act as both a speaker and a microphone, rapidly switching between sending pulses and listening for echoes.
The fundamental concept behind ultrasound is the pulse-echo principle. The ultrasound probe (transducer) sends out a short burst of high-frequency sound (the pulse) and then listens for the sound waves that bounce back (the echo).
By knowing the average speed of sound in human tissue (approximately 1540 m/s) and measuring the time it takes for the echo to return, the machine calculates the exact distance to the tissue boundary using the formula:
We divide by 2 because the sound has to travel to the target and back.
We've seen how a single pulse travels and reflects. Now, let's see how these echoes are converted into visual information.
When the echoes return to the probe, they are recorded as an electrical signal over time. This is called an A-Mode (Amplitude Mode) signal. The height of the spike corresponds to the strength of the echo.
To create an image, we convert this amplitude into brightness. A strong echo becomes a bright white dot, a weak echo becomes a gray dot, and no echo remains black. This single line of pixels is a B-Mode (Brightness Mode) line.
By sweeping the ultrasound beam across a plane (sending multiple pulses side-by-side), we build a 2D slice of the anatomy from many individual B-mode lines. Each line acts like a column of pixels, and together they form the complete image.
As ultrasound waves dive into the body, they face attenuation: sound energy is lost to friction and scattering as it travels deeper. To prevent this, the system must amplify the returning electrical signals to make them visible on the screen.
While our simulation uses a single Receiver Gain slider for simplicity, real-world medical devices use a vertical bank of multiple sliders called TGC pods. Each slider controls the amplification for a specific "depth slice" of the image. By sliding the last controls further to the right, we can manually compensate for the lost energy at greater depths. This ensures the final image has a uniform brightness from top to bottom, preventing deep structures from disappearing into the dark.
Beyond simply imaging tissues, ultrasound can also reveals fluids. By exploiting the Doppler effect, we can track the movement of blood and fluids through the body in real time.
The Doppler effect is the change in frequency of a wave in relation to an observer who is moving relative to the wave source. In ultrasound, we use this to measure the velocity of red blood cells. If blood is moving towards the probe, the returning echoes have a higher frequency. If moving away, a lower frequency. The difference of these frequencies is called the Doppler Shift (Δf).
Where :
The measured velocity depends heavily on the angle (θ) between the ultrasound beam and the blood flow. If the beam is perpendicular (90°) to the flow, cos(90°) = 0, and no flow is detected.
While this overview covers the main concepts of ultrasound, from how crystals work to the Doppler effect, real ultrasound machines are much more complex. The hard part is not just knowing the theory, but being able to use all these settings at the same time during an exam. When you combine this practical skill with a basic understanding of how sound travels through the body, you can find the tiny details you need. Ultimately, getting these settings right is what lets you turn simple sound waves into an interpretable picture of what's inside the body.
Azar, S. F., & Cascella, M. (2023)
StatPearls [Internet]. Treasure Island (FL): StatPearls Publishing.