Ultrasound

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Ultrasound

How can we look deep inside the human body without using harmful ionizing radiation or invasive surgery?


Ultrasound imaging (or sonography) uses high-frequency sound waves—far above the limit of human hearing, typically between 1 and 20 megahertz—to create real-time images of soft tissues, organs, and blood flow.

When an ultrasound wave travels through the body, it encounters boundaries between different tissues (such as muscle, fat, or bone). At each boundary, part of the sound wave is reflected back as an echo. The strength and timing of these returning echoes depend on a fundamental physical property known as acoustic impedance ($Z$). By measuring these echoes, an ultrasound machine calculates the depth and nature of internal structures to reconstruct a detailed visual image.

Watch the video below to explore the wave mechanics of ultrasound, how acoustic impedance dictates reflection versus transmission at tissue boundaries, and how sonograms are formed in medical diagnostics.


What is Acoustic Impedance?

To understand how an ultrasound machine constructs an image of internal organs and tissues, we first need to understand acoustic impedance ($Z$).


Acoustic impedance is a fundamental property of a medium that measures its resistance to the passage of sound waves. It depends on two physical factors: the density of the tissue ($\rho$) and the speed of sound ($v$) through that tissue, expressed by the formula:


Z = ρv


When an ultrasound wave travels through the body and encounters a boundary between two different tissues—such as muscle and fat, or tissue and bone—the difference in their acoustic impedances determines what happens to the wave:


  • Small Difference: Most of the sound wave passes straight through the boundary, while a small portion reflects back as an echo.


  • Large Difference: Almost all of the sound wave is reflected back at the boundary, preventing the sound from penetrating any deeper.


Watch the video to explore how acoustic impedance dictates reflection versus transmission at tissue boundaries, why acoustic gel is used during ultrasound scans, and how these returning echoes form a detailed sonogram image.



The Piezoelectric Effect: Generating Ultrasound


How can a medical device convert electrical pulses into mechanical sound waves—and then turn returning echoes back into electrical signals—at frequencies reaching several megahertz?


The key lies in the piezoelectric effect. Certain materials, such as specialised quartz crystals or synthetic ceramics like lead zirconate titanate (PZT), naturally change shape when an electric voltage is applied across them. By applying a rapidly alternating voltage, the crystal expands and contracts at high frequencies, creating the high-frequency acoustic pressure waves used in ultrasound imaging.


Conversely, when reflected sound echoes return and hit the crystal, they mechanically compress it, generating a proportional electrical voltage that the ultrasound machine reads to construct an image.

Watch the video to explore a physical model of the piezoelectric effect, how ultrasound transducers operate, and how these unique crystals bridge the gap between electricity and sound.

The Ultrasound A-Scan (Amplitude Scan)


How do we convert sound echoes into precise distance measurements inside the body?


The simplest mode of ultrasound imaging is the A-scan (Amplitude scan). In an A-scan, a stationary transducer sends a single beam of ultrasound pulses along a straight line into tissue. As the sound wave encounters boundaries between different organs or structures, echoes reflect back to the transducer.


Instead of creating a 2D picture, an A-scan plots these returning echoes on a one-dimensional graph:

  • Horizontal Axis (x-axis): Represents the time delay of the echo, which directly corresponds to the depth or distance of the tissue interface inside the body (d = v t⁄2).
  • Vertical Axis (y-axis): Represents the amplitude (strength) of the returning signal, determined by the difference in acoustic impedance at each boundary.

Watch the video to explore how A-scans are produced, how travel time translates into depth, and why this precise 1D measurement technique is essential in ophthalmology for measuring the dimensions of the eye.

The Ultrasound B-Scan (Brightness Scan)


While a one-dimensional A-scan measures depth along a single line, how do we transform those acoustic echoes into a dynamic, two-dimensional image of internal body structures?


The solution is the B-scan (Brightness scan). Instead of plotting echo height as a spike on a graph, a B-scan converts the amplitude of each returning signal into a dot of varying brightness:


  • Strong Echoes: Produced by boundaries with large acoustic impedance differences, appearing as bright white spots (e.g., bone or tissue interfaces).
  • Weak Echoes: Produced by subtle tissue transitions, appearing as dim grey spots.
  • No Echoes: Areas where sound passes through completely (like fluid-filled cysts), appearing black.

By sweeping an array of transducers across a region, the machine combines hundreds of individual scan lines in real time to render a detailed 2D cross-sectional image—the classic ultrasound image used in prenatal care and organ diagnostics.

Watch the video to explore how B-scans build upon A-scan physics to map complex anatomical structures in two dimensions.