BackUltrasound Imaging
Ultrasound 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?
The Physics of Sound Propagation
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).
- Reflection: A portion of the wave bounces back towards the probe. This is the "echo" that creates our image.
- Transmission: The rest of the wave continues traveling deeper into the body.
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!
Interactive wave propagation according to tissue type
The Piezoelectric Effect: Transmitting and Receiving
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:
- Transmitting: When an electrical voltage is applied to the crystal, it changes shape rapidly, vibrating and producing a sound wave.
- Receiving: Conversely, when a returning sound wave (echo) hits the crystal, the mechanical pressure deforms it, generating an electrical signal that the machine can read.
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.
Interactive Piezoelectric Crystal
The Pulse-Echo Principle
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.
Interactive A-Mode (Amplitude)
From Echo to Image: The B-Mode Line
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.
A-Mode to B-Mode Conversion
B-Mode: Building a 2D Image
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.
Sweeping Scan: Reconstructing the Image
The Frequency Trade-off:
- Higher frequencies (e.g., 10 MHz) provide excellent resolution, allowing us to see tiny details, but they suffer from high attenuation (they lose energy quickly and cannot penetrate deep). Great for a thyroid or a tendon.
- Lower frequencies (e.g.,2.5 MHz) penetrate deeper but produce blurrier images. Essential for looking at a liver or a deep-seated fetus.
TGC (Time Gain Compensation): Balancing the Deep
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.
Resolution vs Penetration
Doppler Ultrasound: Measuring Flow
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).
Doppler Equation
Where :
- v : Blood velocity (the final result).
- Δf : Measured frequency shift.
- c : Speed of sound in tissues (fixed at 1540 m/s).
- f₀ : Emission frequency of your probe.
- cos(θ) : Angle correction factor.
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.
Conclusion
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.
References
Azar, S. F., & Cascella, M. (2023)
StatPearls [Internet]. Treasure Island (FL): StatPearls Publishing.