Ultrasound imaging uses high frequency sound waves to look inside the body without using ionizing radiation. A handheld probe sends pulses of sound into tissue and listens for echoes that return from boundaries between different materials. The timing and strength of those echoes are converted into a grayscale image on a monitor.
This technology matters because it helps doctors view organs, blood flow, muscles, and developing babies in real time.
Understanding Medical Technology: The Physics of Ultrasound
The probe contains tiny piezoelectric crystals. These crystals change shape when an electric voltage is applied. Their movement creates a brief pressure wave in the body.
The same crystals can work in reverse. Returning pressure waves squeeze the crystals and produce tiny electrical signals. A computer measures these signals many millions of times each second.
Gel is important because air between the probe and skin reflects nearly all the sound. The gel removes this air gap, so much more sound enters the body. Firm contact helps too, though too much pressure can change the shape of soft tissue.
Sound does not reflect equally at every boundary. Reflection depends on acoustic impedance, which is linked to a material's density and the speed of sound within it. A large difference in impedance sends back a strong echo.
This is why bone surfaces and air-filled regions can appear very bright. It is also why the area behind them may be hard to see. Much of the sound is reflected or scattered before it reaches deeper structures.
Fluid has fairly uniform properties, so it usually produces few echoes and often appears dark. A full bladder can therefore provide a useful window for viewing nearby pelvic organs.
An ultrasound image is built from many narrow scan lines rather than from one broad sound pulse. Modern probes have rows of small crystal elements. The machine activates selected elements with carefully chosen time delays.
Their waves combine to focus the beam at a chosen depth or steer it in a chosen direction. On return, the machine compares signals from many elements to decide where an echo came from. This process is called beamforming.
Focusing improves detail because echoes from neighbouring small structures are less likely to blend together. The clearest part of an image is often near the selected focus, not everywhere on the screen.
Some scans show motion rather than just structure. In Doppler ultrasound, moving blood cells slightly change the frequency of the reflected sound. The size and direction of this change can be used to estimate blood speed along the beam.
Colour flow images place this information over a normal image. The colours do not mean arteries are always one colour or veins another. They show motion toward or away from the probe according to the chosen display setting.
A blood vessel viewed at a steep angle gives a less reliable speed estimate. Operators try to keep the beam as parallel to blood flow as practical.
Students should learn that images can contain artifacts. Shadowing occurs behind bone or gas because little sound continues beyond the boundary. Enhancement occurs behind fluid because sound loses less energy there than in surrounding tissue.
Multiple reflections can create repeated false structures. Incorrect settings for depth, gain, focus, or probe angle can hide real features or make normal tissue look unusual.
Ultrasound is generally used with low power and short scanning times, but energy is still deposited in tissue. Skilled operators use the lowest output that gives a useful image and avoid unnecessary exposure, especially during pregnancy.
Key Facts
- Ultrasound frequency is typically 2 MHz to 15 MHz, far above the human hearing range of about 20 Hz to 20,000 Hz.
- Wave speed in soft tissue is approximated as v = 1540 m/s.
- Echo depth is found from d = vt/2 because the sound travels to the boundary and back.
- Wave relationship: v = fλ, where v is wave speed, f is frequency, and λ is wavelength.
- Higher frequency gives better resolution but less penetration because more sound is absorbed and scattered.
- Brighter pixels usually represent stronger returning echoes from larger changes in acoustic impedance.
Vocabulary
- Ultrasound
- Ultrasound is sound with a frequency above the range of human hearing, used in medicine to create images from echoes.
- Transducer
- A transducer is the probe component that converts electrical signals into sound waves and returning sound waves into electrical signals.
- Echo
- An echo is a reflected sound wave that returns to the probe after hitting a boundary between tissues.
- Acoustic impedance
- Acoustic impedance is a measure of how strongly a material resists sound wave motion and affects how much sound is reflected at a boundary.
- Resolution
- Resolution is the ability of an imaging system to distinguish two nearby structures as separate objects.
Common Mistakes to Avoid
- Using d = vt instead of d = vt/2 for echo depth is wrong because the measured time includes both the outgoing and returning trip.
- Thinking ultrasound images are photographs is wrong because the image is calculated from echo timing and strength, not visible light.
- Assuming the highest frequency is always best is wrong because high frequency improves detail but reduces penetration into deeper tissue.
- Ignoring tissue boundaries is wrong because most useful echoes come from changes in acoustic impedance between materials, not from uniform tissue.
Practice Questions
- 1 An ultrasound echo returns to the probe 52 microseconds after a pulse is sent. Using v = 1540 m/s, how deep is the reflecting boundary?
- 2 A probe emits ultrasound at 5.0 MHz in soft tissue where v = 1540 m/s. What is the wavelength of the sound wave in millimeters?
- 3 A doctor switches from a 3 MHz probe to a 12 MHz probe for a shallow scan. Explain how this change affects image detail and penetration depth.