CAIE A-Level Physics A2 24.1 Production and Use of Ultrasound Questions

Practise analysing ultrasound generation, pulse-echo timing, acoustic impedance, reflection, transmission and attenuation for medical diagnosis.

Exam points

explain piezoelectric generation and detection of ultrasound using crystal deformation and induced e.m.f.
interpret reflected ultrasound timing and intensity to obtain internal-boundary depth and diagnostic information
calculate specific acoustic impedance from Z = ?c
calculate ultrasound intensity reflection or transmission at a boundary
calculate and interpret ultrasound attenuation using I = I?e??x

Covered syllabus points

24.1.1—A piezo-electric crystal changes shape when a p.d. is applied across it and 24.1.2—Ultrasound waves are generated and detected by a piezoelectric transducer 24.1.3—The reflection of pulses of ultrasound at boundaries between tissues can be 24.1.4—The specific acoustic impedance of a medium as Z = ρc 24.1.5—IR / I0 = (Z1 – Z2)2 / (Z1 + Z2)2 for the intensity reflection coefficient 24.1.6—I = I0e–µx for the attenuation of ultrasound in matter

[Maximum number: 7]

Piezo-electric transducers are used for the generation of ultrasonic waves.

(a)

State one other use, apart from in ultrasound, of piezo-electric transducers.

[ 1 ]

(b)

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

[ 6 ]

[Maximum number: 7]

Ultrasound is used to produce diagnostic information about internal body structures.

(a)

Explain how ultrasound waves are detected.

[ 3 ]

(b)

Table 9.1 contains information about air and soft tissue.

Table 9.1

[ 4 ]

(i)

Determine the unit for the specific acoustic impedance values shown in Table 9.1.

[ 1 ]

$\mathrm{kgm}^{-2} \mathrm{~s}^{-1}$

(ii)

Calculate the density of soft tissue.
density = $\mathrm{kg} \mathrm{m}^{-3}$

[ 1 ]

$\begin{aligned} \rho =Z / c=1.7 \times 10^{6} / 1600 =1100 \mathrm{~kg} \mathrm{~m}^{-3} \end{aligned}$

(iii)

Use data from Table 9.1 to explain why ultrasound cannot be used to produce an image inside an air-filled cavity such as the lungs.

[ 2 ]

intensity reflection coefficient $\approx 1$ or $Z_{1}$ and $Z_{2}$ are very different
almost no / no ultrasound transmitted (into air filled cavity)

[Maximum number: 5]

Outline the use of ultrasound to obtain diagnostic information about internal body structures.

[Maximum number: 1]

Piezo-electric transducers are used for the generation of ultrasonic waves.

State one other use, apart from in ultrasound, of piezo-electric transducers.

[Maximum number: 3]

Ultrasound is used to produce diagnostic information about internal body structures.

Explain how ultrasound waves are detected.

[Maximum number: 2]

Define specific acoustic impedance of a medium.

[Maximum number: 8]

(a)

Table 8.1 shows some data relating to the properties of air, gel and body tissue. The data are given to three significant figures.

Table 8.1

[ 2 ]

(i)

Complete Table 8.1 by calculating the missing values to three significant figures. Use the space below for any working that you need.

[ 2 ]

(b)

Use the information in (a) to calculate the intensity reflection coefficient for:

[ 4 ]

(i)

an air-tissue boundary

intensity reflection coefficient =

[ 2 ]

intensity reflection coefficient $=\left(Z_{1}-Z_{2}\right)^{2} /\left(Z_{1}+Z_{2}\right)^{2}$ $=(1680000-440)^{2} /(1680000+440)^{2}$
= 0.999

(ii)

a gel-tissue boundary.
intensity reflection coefficient =

[ 2 ]

intensity reflection coefficient $=\left(Z_{1}-Z_{2}\right)^{2} /\left(Z_{1}+Z_{2}\right)^{2}$ $\begin{aligned} =(1680000-1680000)^{2} /(1680000+1680000)^{2} =0 \end{aligned}$

A1

(c)

Use your answers in (b) to explain why gel is applied to the skin during ultrasound scanning.

[ 2 ]

[Maximum number: 4]

Ultrasound and X-rays are both types of wave that are used in medical diagnosis to form images of internal body structures.

(a)

For one type of wave passing through tissue, the wave has 72 % of its initial intensity after it has passed through 6.2 cm of the tissue.

Calculate the linear attenuation coefficient $\mu$ of the tissue for this wave.

\mu=\ldots \ldots \ldots \ldots \ldots \ldots \ldots \ldots \ldots \ldots \ldots \ldots \ldots \ldots \ldots \ldots . \ldots . \ldots . \ldots . \ldots . . . . . . . . . . . . . . . ~ \mathrm{~cm}^{-1}

[ 2 ]

$I=I_{0} \exp (-\mu x)$ $\begin{aligned} \ln (0.72)=-6.2 \mu \mu=0.053 \mathrm{~cm}^{-1} \end{aligned}$

(b)

Another wave of the same type as in (b)(i) passes through 9.3 cm of the same tissue.

Calculate the percentage of the initial intensity of the wave that is attenuated by the tissue.

[ 2 ]

$I / I_{0}=\exp (-9.3 \times 0.053)$
( = 0.61)
percentage attenuated $=100 \times(1.00-0.61)$ $=39 \%$

All question bank results loaded

Covered syllabus points

Question 1

Question (a)

Question (b)

Question 2

Question (a)

Question (b)

Question (i)

Question (ii)

Question (iii)

Question 3

Question 4

Question 5

Question 6

Question 7

Question (a)

Question (i)

Question (b)

Question (i)

Question (ii)

Question (c)

Question 8

Question (a)

Question (b)