A-LevelPhysicsMedical physics9702/42 · May/June 2017 · Q49 Marks

Explain the main principles behind the use of ultrasound to obtain diagnostic information about internal body structures.

(a)Explain the main principles behind the use of ultrasound to obtain diagnostic information about internal body structures.[6]
(b)A parallel beam of ultrasound has intensity I₀ as it enters a muscle of thickness 4.6 cm, as illustrated in Fig. 4.1. [Figure 4.1] The intensity of the beam just before it leaves the muscle is Iₜ. The ratio I₀/Iₜ is found to be 2.9. Calculate the linear attenuation (absorption) coefficient μ of the ultrasound in the layer of muscle. μ = ........................................................................ cm⁻¹ [3][3]

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Mark scheme

Marking points from the official mark scheme (May/June 2017). The mark or mark type is shown in brackets.

[B1] pulse (of ultrasound)
[B1] produced by quartz crystal/piezo-electric crystal
[B1] gel/coupling medium (on skin) used to reduce reflection at skin
[B1] reflected from boundaries (between media)
[B1] reflected pulse/wave detected by (ultrasound) transmitter
[B1] reflected wave processed and displayed
[B2] intensity of reflected pulse/wave gives information about boundary, time delay gives information about depth of boundary (max. 2 of additional detail points marked *)
[C1] I_T = I_o exp (–μx)
[C1] 2.9 = exp (4.6μ)
[A1] μ = 0.23 cm⁻¹

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