(a)

The alternating voltage in (a) is now applied to a piezoelectric crystal in air.

[ 4 ]

(i)

Explain what happens to the air surrounding the crystal.

[ 3 ]

(ii)

A second piezoelectric crystal is placed in the air near to the first crystal.

Explain the effect of the surrounding air in (b)(i) on the second crystal.

[ 1 ]

(a)

Suggest electrical sensing devices, one in each case, that may be used to monitor changes in

[ 1 ]

(i)

the intensity of an ultrasound beam.

[ 1 ]

(a)

State what is meant by the specific acoustic impedance of a medium.

[ 2 ]

(b)

The specific acoustic impedances Z of some media are given in Fig. 10.1.

Fig. 10.1

[ 7 ]

(i)

The density of a sample of bone is $1.7 \times 10^{3} \mathrm{~kg} \mathrm{~m}^{-3}$ .

Determine the wavelength, in mm , of ultrasound of frequency $9.0 \times 10^{5} \mathrm{~Hz}$ in the bone.
wavelength = mm

[ 3 ]

(ii)

Ultrasound of intensity I is incident normally on the boundary between two media of specific acoustic impedances $Z_{1}$ and $Z_{2}$ , as shown in Fig. 10.2.

Fig. 10.2

The intensity of the ultrasound reflected from the boundary is $I_{\mathrm{R}}$ .
The ratio $\frac{I_{\mathrm{R}}}{I}$ is given by the expression

\frac{I_{\mathrm{R}}}{I}=\frac{\left(Z_{1}-Z_{2}\right)^{2}}{\left(Z_{1}+Z_{2}\right)^{2}} .

By making reference to the data for air, gel and soft tissue, explain quantitatively why, during medical diagnosis using ultrasound, a gel is usually put on the skin.

[ 4 ]

(a)

By reference to ultrasound waves, state what is meant by acoustic impedance.

[ 2 ]

(b)

An ultrasound wave is incident on the boundary between two media. The acoustic impedances of the two media are $Z_{1}$ and $Z_{2}$ , as illustrated in Fig. 10.1.

Fig. 10.1

Explain the importance of the difference between $Z_{1}$ and $Z_{2}$ for the transmission of ultrasound across the boundary.

[ 3 ]

(c)

Ultrasound frequencies as high as 10 MHz are used in medical diagnosis. State and explain one advantage of the use of high-frequency ultrasound compared with lower-frequency ultrasound.

[ 2 ]

[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 ]

(ii)

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

[ 1 ]

(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 ]

(a)

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

[ 6 ]

(b)

State and explain one advantage of the use of high frequency ultrasound as compared with low frequency ultrasound for medical diagnosis.

[ 2 ]

(c)

The absorption (attenuation) coefficient for ultrasound in muscle is $23 \mathrm{~m}^{-1}$ . A parallel beam of ultrasound is passed through a muscle of thickness 6.4 cm .

[ 5 ]

(i)

Calculate the ratio
$\frac{\text { intensity of transmitted beam }}{\text { intensity of incident beam }}$ . intensity of incident beam
ratio =

[ 3 ]

(ii)

An ultrasound transmitter emits a pulse. Suggest why, when the signal from the pulse is processed, any signal received later at the detector is usually amplified more than that received at an earlier time.

[ 2 ]

(a)

By reference to ultrasound waves, state what is meant by the specific acoustic impedance of a medium.

[ 2 ]

(b)

A parallel beam of ultrasound of intensity I is incident normally on a muscle of thickness 3.4 cm , as shown in Fig. 11.1.

Fig. 11.1

The ultrasound wave is reflected at a muscle-bone boundary. The intensity of the ultrasound received back at the transducer is $I_{\mathrm{R}}$ .

Some data for bone and muscle are given in Fig. 11.2.

Fig. 11.2

[ 2 ]

(i)

The intensity reflection coefficient $\alpha$ for two media having specific acoustic impedances $Z_{1}$ and $Z_{2}$ is given by

\alpha=\frac{\left(Z_{1}-Z_{2}\right)^{2}}{\left(Z_{1}+Z_{2}\right)^{2}}

Calculate the fraction of the ultrasound intensity that is reflected at the muscle-bone boundary.
fraction =

[ 2 ]

(ii)

Calculate the fraction of the ultrasound intensity that is transmitted through a thickness of 3.4 cm of muscle.
fraction =

(iii)

Use your answers in (i) and (ii) to determine the ratio $\frac{I_{\mathrm{R}}}{I}$ .
ratio =

(a)

(i)

Define the specific acoustic impedance of a medium.

[ 2 ]

(ii)

Use data from Table 9.2 to calculate the percentage of the intensity of ultrasound that is transmitted at a boundary between soft tissue and bone. \%

[ 2 ]

(iii)

The ultrasound is now incident on the sample of soft tissue and bone shown in Fig. 9.1.

Suggest two reasons why the transmitted intensity through the sample is less than the answer in (c)(ii).

1

2

[ 2 ]

(a)

(i)

Explain how ultrasound pulses are used to obtain diagnostic information about internal body structures in medical diagnosis.

[ 3 ]

(ii)

Define specific acoustic impedance.

[ 2 ]

(iii)

Two media have specific acoustic impedances $Z_{1}$ and $Z_{2}$ .

State the approximate value of the intensity reflection coefficient at the boundary between the two media when:
- $\quad Z_{1}$ is almost equal to $Z_{2}$

intensity reflection coefficient =

- $\quad Z_{1}$ is very different from $Z_{2}$ .
intensity reflection coefficient =

[ 2 ]

(a)

Describe how reflected ultrasound pulses may be used to obtain diagnostic information about internal structures.

[ 2 ]

(b)

(i)

Define specific acoustic impedance of a medium.

[ 2 ]

(ii)

Table 10.1 shows some data for water and for glass.

Table 10.1

Determine the intensity reflection coefficient for ultrasound that is incident on a water-glass boundary.
Determine the intensity reflection coefficient for ultrasound that is incident on a water-glass boundary.
intensity reflection coefficient =

[ 3 ]