Production and use of ultrasound
Cambridge A-Level Physics (9702) · Unit 24: Medical physics · 8 flashcards
Production and use of ultrasound is topic 24.1 in the Cambridge A-Level Physics (9702) syllabus , positioned in Unit 24 — Medical physics , alongside Production and use of X-rays and PET scanning. In one line: Specific acoustic impedance (Z) is a measure of a material's resistance to the propagation of sound. It is defined as Z = ρc, where ρ is the density of the medium and c is the speed of sound in the medium.
Marked as A2 Level: examined at A Level in Paper 4 (A Level Structured Questions) and Paper 5 (Planning, Analysis and Evaluation). It is not tested on the AS-only papers (Papers 1, 2 and 3).
The deck below contains 8 flashcards — 3 definitions and 5 key concepts — covering the precise wording mark schemes reward. Use the 3 definition cards to lock down command-word answers (define, state), then move on to the concept and calculation cards to handle explain, describe, calculate and compare questions.
Specific acoustic impedance (Z) and provide its formula
Specific acoustic impedance (Z) is a measure of a material's resistance to the propagation of sound. It is defined as Z = ρc, where ρ is the density of the medium and c is the speed of sound in the medium.
What the Cambridge 9702 syllabus says
Official 2025-2027 spec · A2 LevelThese are the exact learning outcomes Cambridge sets for this topic. The candidate is expected to be able to do each of these on the relevant paper.
- understand that a piezo-electric crystal changes shape when a p.d. is applied across it and that the crystal generates an e.m.f. when its shape changes
- understand how ultrasound waves are generated and detected by a piezoelectric transducer
- understand how the reflection of pulses of ultrasound at boundaries between tissues can be used to obtain diagnostic information about internal structures
- define the specific acoustic impedance of a medium as Z = ρc, where c is the speed of sound in the medium
- use IR / I0 = (Z1 – Z2)2 / (Z1 + Z2)2 for the intensity reflection coefficient of a boundary between two media
- recall and use I = I0e–μx for the attenuation of ultrasound in matter
Cambridge syllabus keywords to use in your answers
These are the official Cambridge 9702 terms tagged to this section. Mark schemes credit responses that use the exact term — weave them into your answers verbatim rather than paraphrasing.
Tips to avoid common mistakes in Production and use of ultrasound
- › Specify that the speed in the formula Z = ρc refers to the speed of ultrasound in the specific medium.
- › Consider all interfaces in a sample; a transmission percentage must be applied at every boundary where acoustic impedance changes.
- › Explain that pulsing is necessary to allow time for reflected pulses to be received between emissions.
Explain how a piezoelectric crystal is used to generate ultrasound.
A piezoelectric crystal changes shape when a p.d. is applied. An alternating voltage at the crystal's resonant frequency causes it to vibrate, producing ultrasound waves. The frequency of the applied voltage determines the frequency of the emitted ultrasound.
Explain how a piezoelectric crystal is used to detect ultrasound.
When an ultrasound wave strikes a piezoelectric crystal, it changes shape, generating an e.m.f. The magnitude of the e.m.f. is proportional to the intensity of the ultrasound wave, allowing for detection.
How is ultrasound used in medical imaging to obtain diagnostic information?
Pulses of ultrasound are emitted into the body. When these pulses encounter boundaries between tissues with different acoustic impedances, they are partially reflected. The reflected pulses are detected, and the time delay and intensity provide information about the depth and nature of the tissue boundaries.
Define specific acoustic impedance (Z) and provide its formula.
Specific acoustic impedance (Z) is a measure of a material's resistance to the propagation of sound. It is defined as Z = ρc, where ρ is the density of the medium and c is the speed of sound in the medium.
State the formula for the intensity reflection coefficient (IR / I0) at a boundary between two media and define the terms.
The intensity reflection coefficient is given by IR / I0 = (Z1 – Z2)² / (Z1 + Z2)², where IR is the intensity of the reflected wave, I0 is the intensity of the incident wave, and Z1 and Z2 are the specific acoustic impedances of the two media.
Describe what a high intensity reflection coefficient means at a tissue boundary in ultrasound imaging.
A high intensity reflection coefficient means that a large portion of the ultrasound pulse is reflected at the boundary between two tissues. This indicates a significant difference in the acoustic impedance of the tissues, which can help distinguish different anatomical structures.
State the formula for the attenuation of ultrasound intensity (I) as it travels through a medium.
The attenuation of ultrasound intensity is given by I = I0e–μx, where I is the intensity after traveling a distance x, I0 is the initial intensity, μ is the attenuation coefficient of the medium, and x is the distance traveled.
What does the attenuation coefficient (μ) represent in the context of ultrasound?
The attenuation coefficient (μ) represents the rate at which the intensity of an ultrasound wave decreases as it travels through a medium. A higher attenuation coefficient means that the ultrasound wave loses intensity more rapidly.
Review the material
Read full revision notes on Production and use of ultrasound — definitions, equations, common mistakes, and exam tips.
Read NotesMore topics in Unit 24 — Medical physics
Production and use of ultrasound sits alongside these A-Level Physics decks in the same syllabus unit. Each uses the same spaced-repetition system, so progress in one informs the next.
Key terms covered in this Production and use of ultrasound deck
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