A-LevelPhysicsMedical physics9702/42 · Feb/Mar 2018 · Q510 Marks

(a) Explain the main principles behind the use of ultrasound to obtain diagnostic information about internal body structures. (b) (i) Define specific acoustic impedance. (ii) Two media have specific acoustic impedances of Z₁ and Z₂. The magnitudes of the acoustic impedances may be almost equal or very different. State how these differences affect the intensity reflection coefficient at the boundary between the two media. Z₁ ≈ Z₂ Z₁ » Z₂ or Z₁ « Z₂

(a)Explain the main principles behind the use of ultrasound to obtain diagnostic information about internal body structures.[6]
(b)(i)Define specific acoustic impedance.[2]
(b)(ii)Two media have specific acoustic impedances of Z₁ and Z₂. The magnitudes of the acoustic impedances may be almost equal or very different. State how these differences affect the intensity reflection coefficient at the boundary between the two media. Z₁ ≈ Z₂ Z₁ » Z₂ or Z₁ « Z₂[2]

Mark scheme

Marking points from the official mark scheme (Feb/Mar 2018). The mark or mark type is shown in brackets.

[B1] pulses of ultrasound
[B1] reflected at boundaries (between media)
[B1] reflected pulses detected by (ultrasound) generator
[B3] Any three from:
[(B1)] (reflected signal) processed and displayed
[(B1)] time delay (between transmission and receipt) gives information about depth (of boundary)
[(B1)] intensity of reflected pulse gives information about (nature of) boundary
[(B1)] gel used to minimise reflection at skin / maximise transmission into skin
[(B1)] degree of reflection depends upon impedances of two media (at boundary)
[M1] product of density and speed
[A1] of sound in the medium
[B1] (Z₁ about equal to Z₂, ) coefficient very small / nearly 0
[B1] (Z₁ very different to Z₂, ) coefficient nearly 1

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