Ultrasound is sound with a frequency above 20,000 Hz — too high for humans to hear. Medical ultrasound uses pulses of this high-frequency sound to build images of internal structures. Pulses are sent into the body, reflect at tissue boundaries, and the echo timing reveals the depth and shape of organs and tissues beneath the skin.
What is ultrasound?
Sound is a longitudinal mechanical wave — it requires a medium (solid, liquid, or gas) to travel through. The human ear can detect sound in the frequency range 20 Hz to 20,000 Hz (20 kHz). Any sound above 20,000 Hz is called ultrasound — it is the same type of wave as audible sound, just at a higher frequency.
Higher frequency means shorter wavelength (for the same speed). Because ultrasound wavelengths are very short (less than 1 mm in soft tissue), the waves can reflect off small features and produce detailed images. Lower-frequency sound has a longer wavelength and diffracts more around obstacles, making it unsuitable for detailed imaging.
Ultrasound is produced and detected by transducers containing piezoelectric crystals that convert between electrical signals and mechanical vibrations at ultrasonic frequencies.
How does the pulse-echo technique produce an image?
Ultrasound imaging uses the pulse-echo method:
- The transducer emits a very brief pulse of ultrasound into the body.
- The pulse travels through tissue until it reaches a boundary between different tissues (such as the boundary between muscle and an organ, or between tissue and fluid).
- At each boundary, some of the pulse is reflected (an echo) and some is transmitted onwards.
- The reflected echo travels back to the transducer, which detects it and converts it to an electrical signal.
- The time taken for the echo to return is measured precisely.
- The distance to the reflecting boundary is calculated from the time and the known speed of ultrasound in tissue.
- By sending many pulses in slightly different directions and combining all the echo data, a computer builds a 2D or 3D image.
How do you calculate distances using ultrasound?
The key equation uses the fact that the pulse travels to the boundary and back:
distance to boundary = (speed of ultrasound × time for echo to return) ÷ 2
d = v × t ÷ 2
The division by 2 is essential because the pulse travels the distance twice — once going out, and once coming back.
The speed of ultrasound in soft tissue is approximately 1,500 m/s.
Worked example
A transducer sends a pulse of ultrasound into a patient's body. The echo from an organ boundary returns after a time of 0.000200 s (200 microseconds).
- Speed of ultrasound in tissue, v = 1,500 m/s
- Time for echo, t = 0.000200 s
d = v × t ÷ 2 d = 1,500 × 0.000200 ÷ 2 d = 0.300 ÷ 2 d = 0.150 m (15.0 cm)
The organ boundary is 15.0 cm below the transducer.
What are the medical uses of ultrasound?
| Application | Detail |
|---|---|
| Prenatal scanning | Imaging the developing foetus; the standard NHS 20-week anatomy scan checks organ development and position. Safe for the foetus because ultrasound is non-ionising |
| Abdominal organ imaging | Liver, kidneys, gallbladder, spleen — checking for cysts, tumours, gallstones |
| Echocardiography | Imaging the heart's structure and valves in real time; measuring blood flow (Doppler ultrasound) |
| Musculoskeletal imaging | Tendons, ligaments, muscles — particularly useful because these show poorly on X-ray |
| Guided procedures | Needle placement for biopsies guided by real-time ultrasound to ensure accuracy |
| Kidney stones | Detecting and sometimes breaking up kidney stones using focused high-intensity ultrasound (lithotripsy) |
Why is ultrasound preferred to X-rays for prenatal scanning?
Ultrasound is preferred for imaging pregnancies and many soft-tissue structures because:
- No ionising radiation: ultrasound is a mechanical wave, not electromagnetic radiation. It does not carry enough energy per photon to ionise atoms or damage DNA. X-rays are ionising and pose a cancer risk with repeated exposure.
- Real-time imaging: ultrasound produces live moving images, so the beating heart of a foetus can be viewed directly.
- Soft tissue contrast: ultrasound distinguishes between organs and fluid-filled structures that appear very similar on X-rays.
- Portable and affordable: ultrasound machines are relatively compact and inexpensive compared with MRI scanners.
The main limitation of ultrasound is that it cannot penetrate air or bone well — gas and bone reflect nearly all the ultrasound, so gas-filled organs (lungs, bowel) and structures behind bone are poorly imaged.
How is the same principle used in sonar?
SONAR (Sound Navigation and Ranging) applies the same pulse-echo method in water:
- Ships and submarines emit pulses of ultrasound downward or outward
- Echoes reflect from the seabed, fish shoals, or other submarines
- The time taken for the echo to return, combined with the speed of sound in seawater (~1,500 m/s), gives the depth or distance
SONAR is used to map the ocean floor, locate fish for fishing fleets, detect submarines (military sonar), and navigate in poor visibility. Animals such as bats and dolphins use their own biological sonar (echolocation) with the same physics — emitting ultrasonic clicks and interpreting the returning echoes to build a precise map of their surroundings.
Frequently asked questions
Why must ultrasound frequency be high for detailed medical imaging?
Spatial resolution in imaging depends on the wavelength of the waves used — to resolve a feature, the wavelength must be smaller than the feature. Higher frequency means shorter wavelength (v = fλ, so λ = v ÷ f). At 1 MHz (1,000,000 Hz), the wavelength in soft tissue is about 1.5 mm; at 10 MHz it is about 0.15 mm. Medical scanners typically use 2–15 MHz to balance resolution (higher frequency = better detail) against penetration depth (higher frequency waves are absorbed more strongly by tissue and do not reach deep structures as well).
Why does ultrasound divide time by 2 in the distance calculation?
When a pulse is emitted, it travels from the transducer to the reflecting boundary and then back to the transducer. The total time measured is the round-trip time — the time for both journeys. The distance to the boundary is only one way, so you must halve the time before multiplying by speed: d = v × t ÷ 2. Forgetting to divide by 2 is the most common error in GCSE ultrasound calculations, and it doubles the calculated distance.
How is Doppler ultrasound different from standard ultrasound?
Standard pulse-echo ultrasound images static boundaries between tissues. Doppler ultrasound measures the speed and direction of movement — most commonly of blood flowing in vessels. When ultrasound reflects from moving red blood cells, the frequency of the reflected wave is slightly different from the emitted frequency (the Doppler effect). If blood is moving towards the transducer, the reflected frequency is higher; if moving away, it is lower. The size of the frequency shift reveals the blood's speed. Doppler ultrasound is used to assess blood flow through the heart, detect clots in veins, and monitor blood supply to the foetus during pregnancy.
What is the difference between ultrasound imaging and X-ray imaging?
Ultrasound uses high-frequency sound waves (mechanical waves) that reflect at tissue boundaries; X-rays use electromagnetic radiation that is absorbed differently by tissues of different densities. Ultrasound is better at distinguishing soft tissues (organs, fluid) and carries no ionising radiation risk. X-rays are better at imaging bone and detecting lung disease (because bone and lung air both produce very high contrast with X-rays). CT scans (a form of X-ray imaging) produce detailed 3D images but use much higher radiation doses than a single X-ray. MRI uses radio waves and strong magnetic fields and is excellent for soft tissue without radiation, but is far more expensive and takes longer than ultrasound.
For predict-first GCSE physics with Professor Newton — starting from the wave equation and echo timing before building toward full imaging systems — visit aitutors.me.