Page 539 - Physics Coursebook 2015 (A level)

Chapter 32: Medical imaging
5 A transducer produces ultrasonic waves of frequency 800 kHz. The speed of sound in the crystal is 5200 m s−1.
Calculate the optimum thickness for the crystal. [2]
6 Explain what is meant by the Larmor frequency and state its units. [2]
7 State and explain two reasons why full-body CT scans are not offered for regular checking of healthy patients. [2]
8 a Explain with the aid of a simple, labelled diagram how X-rays are produced. [5] b Discuss the energy changes in the production of X-rays. [3]
9 Figure 32.37 shows the spectrum of X-rays produced from an X-ray source.
A
0
0 50 100 150
Energy of X-ray photon / keV
Figure 32.37 For End-of-chapter Question 9.
a Describe the process by which:
i the three sharp peaks of high-intensity X-rays are produced [2]
ii the broad band of X-rays is produced. [2]
b The X-rays in the shaded region, labelled A, are filtered out using an aluminium filter. Explain:
i why it is advantageous to filter these X-rays out [2]
ii why aluminium is a suitable material to filter them out. [2]
c Calculate the maximum frequency of X-rays produced by this tube. [3]
10 a
b A CT image is sometimes superimposed on an image from an MRI scan. Explain the advantages of using
11 a
this dual approach. [2]
An X-ray beam, containing X-rays with a variety of frequencies and which has an intensity of 4.0 × 105 W,
is incident on an aluminium plate of thickness 5.0 cm. The average linear attenuation coefficient is 250 m−1.
i Calculate the intensity of the transmitted beam. [3] ii ExplaintheadvantagesofpassingtheX-raysthroughthisaluminiumplatepriortotheirbeingincident
Describe the main principles of how a CT scan image is produced and compare this with the formation
of an X-ray shadow image. [7]
on a patient. [3] b Outline the use of an anti-scatter grid and explain its role in improving an X-ray shadow image. [5]
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Relative intensity