Acoustics in Medicine: Where Wave Physics Becomes Clinical Technology
Medical acoustics takes the same wave mechanics that govern sonar and turns them into instruments that image, heal, and even operate inside the human body. In soft tissue, sound travels at roughly 1540 m/s and varies by only a few percent between organs — a narrow spread that makes diagnostic imaging possible while constraining resolution, penetration depth, and safety margins.
This quiz tests the physics behind the devices: pulse-echo timing, acoustic impedance ##Z = \rho c##, intensity reflection coefficients, the Doppler shift, and the therapeutic use of focused sound in lithotripsy and high-intensity focused ultrasound (HIFU).
What you will practise
- Converting echo delays into reflector depth at 1540 m/s
- Applying the reflection coefficient ###R = \left(\dfrac{Z_2 – Z_1}{Z_2 + Z_1}\right)^2###
- Reasoning about Doppler flow measurement, echocardiography, and obstetric scanning
- Comparing diagnostic and therapeutic ultrasound, and judging safety limits
Difficulty: Medium. Fifteen questions, each answerable from Lesson 26 alone.
Q1. Diagnostic ultrasound imaging fundamentally maps which property of tissue?
A pulse-echo scanner records echo delays and amplitudes; the resulting image is a map of acoustic impedance discontinuities, not of optical density.
Q2. In soft tissue the speed of sound is taken as approximately ##1540\ \text{m/s}##. A pulse returns after a round-trip delay of ##26\ \mu\text{s}##. The reflector depth is about:
Depth ##= \dfrac{c\,t}{2} = \dfrac{1540 \times 26\times10^{-6}}{2} \approx 0.020\ \text{m} = 2\ \text{cm}##, the standard worked example of the lesson.
Q3. Acoustic impedance of a medium is defined as:
Acoustic impedance is the product of density and sound speed, ##Z = \rho c##, and it governs reflection at an interface.
Q4. For normal incidence, the intensity reflection coefficient at an interface is given by:
The intensity reflection coefficient is the square of the impedance ratio, ###R = \left(\dfrac{Z_2 - Z_1}{Z_2 + Z_1}\right)^2###, and the transmitted fraction is ##1 - R##.
Q5. At a soft-tissue-to-bone interface with ##Z_1 \approx 1.6\times10^6## Rayl and ##Z_2 \approx 7.8\times10^6## Rayl, the fraction of incident intensity reflected is nearest to:
Substituting gives ##R = \left(\dfrac{6.2}{9.4}\right)^2 \approx 0.44##, so roughly 44 percent of the intensity returns — the reason bone casts acoustic shadows.
Q6. Why is a coupling gel applied between the transducer and the skin?
Air has very low acoustic impedance, so an air gap reflects almost all the incident intensity; the gel provides an impedance-matched path into tissue.
Q7. Liver and kidney appear similar on an ultrasound image mainly because:
Tissues with similar impedance produce weak echoes at their boundary and therefore look similar on screen.
Q8. Lung tissue blocks ultrasound almost entirely because:
Air has very low impedance, so the mismatch at a soft-tissue-to-air boundary is enormous and almost all the intensity is reflected.
Q9. Doppler ultrasound is used clinically chiefly to measure:
The Doppler shift in the frequency of echoes from moving blood gives the flow velocity, the basis of Doppler flow meters.
Q10. Echocardiography applies pulse-echo ultrasound to:
Echocardiography images the heart, and combined with Doppler techniques it assesses cardiac motion and blood flow.
Q11. Lithotripsy uses ultrasound to:
Lithotripsy is a therapeutic application in which focused acoustic pulses fragment kidney stones non-invasively.
Q12. High-intensity focused ultrasound ##\ce{(HIFU)}## is used surgically because focusing raises intensity enough to:
HIFU concentrates acoustic energy at a focus so that targeted tissue is heated and destroyed, while surrounding tissue is spared.
Q13. Which statement about the safety of diagnostic ultrasound is most consistent with the lesson?
Because ultrasound deposits energy, safety margins are set by limiting intensity and exposure time to control heating and cavitation effects.
Q14. Ultrasound contrast agents improve imaging mainly by:
Microbubbles present a very large impedance mismatch with blood and therefore scatter sound strongly, enhancing the echo signal.
Q15. Elastography and photoacoustic imaging differ from conventional pulse-echo ultrasound in that they:
Elastography maps tissue stiffness, while photoacoustic imaging detects acoustic waves generated by absorbed light pulses, extending beyond simple impedance mapping.