Medical Physics

Describe how the piezoelectric crystal in a transducer generates ultrasound waves for use in medical diagnosis.

State what is meant by specific acoustic impedance.

Describe how $\alpha$ depends on the relative values of $Z_1$ and $Z_2$.

A parallel ultrasound beam of intensity $I_0$ enters a region of soft tissue. After passing a distance of $2.1\ \text{cm}$ through this tissue, the intensity of the ultrasound is $0.62I_0$.

Calculate the linear attenuation coefficient $\mu$ of ultrasound in the soft tissue. Give a unit with your answer.

$\mu$ = ______ unit ______

Define specific acoustic impedance.

Explain how ultrasound waves are detected by a piezoelectric crystal.

Calculate the intensity reflection coefficient for ultrasound incident on a water–steel boundary.

intensity reflection coefficient = ______

Explain, without calculation, what is likely to happen when ultrasound is incident on a body tissue–water boundary.

State what is meant by contrast in an X-ray image.

Show that the intensity of the X-rays emerging in region B is $0.13I_{0}$.

Determine the linear attenuation coefficient $\mu$ of material Q.

$\mu$ = ______ $\text{cm}^{-1}$

Use the information in (b)(i) to suggest why the X-rays emerging from the structure form an image that has poor contrast.

Explain how X-rays are used in computed tomography (CT) scanning to produce a three-dimensional image of an internal structure.

Define specific acoustic impedance.

Explain how ultrasound waves are detected by a piezoelectric crystal.

Calculate the intensity reflection coefficient for ultrasound incident on a water–steel boundary.

intensity reflection coefficient = ______

Explain, without calculation, what is likely to happen when ultrasound is incident on a body tissue–water boundary.

Calculate the minimum wavelength of the X-rays that are produced.

wavelength = ______ $\text{m}$

The melting points of two metals are given in Table 9.1.

Table 9.1

Suggest why the metal target is made from tungsten rather than copper.

An X-ray beam is incident normally on a sample of soft tissue and bone as shown in Fig. 9.1.

Data for the two materials are given in Table 9.2.

Table 9.2

The total thickness of soft tissue is $x$. The total thickness of bone is also $x$.

The incident intensity of the X-ray beam is $I_0$. The transmitted intensity of the X-ray beam is $13\%$ of the incident intensity.

Determine $x$, in cm.

$x$ = ______ $\text{cm}$

Define the specific acoustic impedance of a medium.

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.

percentage transmitted = ______ $\%$

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 ______

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

Define specific acoustic impedance of a medium.

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.

intensity reflection coefficient = ______

Show that the specific acoustic impedance of gel is $1.68 \times 10^6\ \text{kg m}^{-2} \text{s}^{-1}$.

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

an air–tissue boundary

intensity reflection coefficient = ______

a gel–tissue boundary.

intensity reflection coefficient = ______

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

State the name of the charged particles that are accelerated by the applied p.d.

Explain how X-rays are produced at the metal target.

Calculate the minimum wavelength of X-rays produced when the applied p.d. is $5.80\ \text{kV}$.

wavelength = ______ $\text{m}$

X-rays pass through a medium that has an attenuation coefficient of $1.4\ \text{cm}^{-1}$.

Calculate the percentage of the X-ray energy that is absorbed by a $2.8\ \text{cm}$ thickness of this medium.

percentage absorbed = ______ $\%$

Show that the specific acoustic impedance of gel is $1.68 \times 10^6\ \text{kg m}^{-2} \text{s}^{-1}$.

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

an air–tissue boundary

intensity reflection coefficient = ______

a gel–tissue boundary.

intensity reflection coefficient = ______

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

Complete Table 10.1 to state, for each type of wave:

• the method of production of the wave
• whether the wave that is detected and used to form the image is the wave that has been absorbed, reflected or transmitted by the internal body structure.

Table 10.1

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

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

$\mu$ = ______ $\text{cm}^{-1}$

Another wave of the same type as in (b)(i) passes through $9.3\ \text{cm}$ of the same tissue.

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

percentage attenuated = ______ %