AS and A Level Physics Original material Cambridge University Press 2010 1

32 Worksheet (A2) Data needed to answer questions can be found in the Data, formulae and relationships sheet.

1 State the nature of X-ray radiation. [2]

2 The energy of an X-ray photon is 50 keV. a Calculate the energy of the photon in joules. [2] b Calculate the wavelength of the X-rays. [2]

3 One of the interaction mechanisms between X-rays and matter is the photoelectric effect. Name the two other interaction mechanisms. [2]

4 State one main difference between the images produced by a normal X-ray machine and by a CAT scan. [1]

5 Briefly explain what is meant by a non-invasive technique. [1]

6 State what is meant by ultrasound. [2]

7 The speed of ultrasound in soft tissue is 1.5 km s1. a Calculate the wavelength of ultrasound of frequency 1.8 MHz. [2] b Use your answer to part a to explain why high-frequency ultrasound is suitable for

medical scans. [1]

8 Define acoustic impedance. [1]

9 The table below shows useful data for biological materials.

Material Density / kg m3 Speed of ultrasound / m s1

Acoustic impedance Z / 106 kg m2 s1

soft tissue 1060 1540 1.63

muscle 1075 1590 1.71

bone ? 4000 6.40

blood 1060 1570 1.66

a Calculate the density of bone. [2] b Calculate the percentage of intensity of ultrasound reflected at the bloodsoft tissue

boundary. (Assume the waves are incident at right angles to the boundary.) [3] c Explain why it would be difficult to distinguish between blood and soft tissue in an

ultrasound scan. [2]

10 Name the five main components of an MRI scanner. [5]

11 Protons have a precession frequency of 40 MHz in a strong uniform magnetic field. a Describe what is meant by precession. [1] b State the frequency of the radio frequency (RF) radiation that will cause the protons to

resonate. [1] c Use your answer to b to determine the wavelength of the RF radiation. [2]

32 Worksheet (A2)

AS and A Level Physics Original material Cambridge University Press 2010 2

12 Briefly describe the production of X-rays and explain why an X-ray spectrum may consist of a continuous spectrum and a line spectrum. [7]

13 The intensity of a collimated X-ray beam is 250 W m2. a Define intensity. [1] b The diameter of the X-ray beam is 4.0 mm. Calculate the power transmitted by the beam. [2]

14 Describe what is meant by a contrast medium and state why it is used in X-ray scans. [2]

15 The potential difference between the cathode and the anode of an X-ray tube is 80 kV. Calculate the minimum wavelength of the X-rays emitted from this tube. [3]

16 The photoelectric effect is one of the attenuation mechanisms by which X-ray photons interact with the atoms in the body. Describe some of the characteristics of this mechanism. [3]

17 A collimated X-ray beam is incident on a metal block. The incident intensity of the beam is I0. a Draw a sketch graph to show the variation with thickness x of the intensity I of the beam. [3] b Write down an expression for the intensity I in terms of I0 and x.

Explain any other symbol you use. [2] c The linear absorption coefficient of a beam of 80 keV X-rays is 0.693 mm1 in copper. Calculate the thickness of copper necessary to reduce the intensity of the beam to 0.10 I0. [3]

18 a Describe the use of a CAT scanner. [5] b Compare the image formed in X-ray diagnosis with that produced by a CAT scanner. [3]

19 Outline how ultrasound may be used in medical diagnosis. [5]

20 Explain why, in medical diagnosis using ultrasound, a coupling medium is necessary between the ultrasound probe and the skin. [6]

21 a When an ultrasound pulse reflects from the front and back edges of a bone, it produces two peaks on an A-scan. The time interval between these two peaks is 13 s. The speed of the ultrasound in bone is 4000 m s1. Calculate the thickness of the bone. [3]

b Describe how a B-scan differs from an A-scan. [2]

22 a Outline the principles of magnetic resonance. [6] b Outline, with the aid of a sketch diagram, the use of MRI (magnetic resonance imaging)

to obtain diagnostic information about internal body structures. [10]

23 X-Rays, ultrasound and MRI are all used in medical diagnosis. State one situation in which each of these techniques is preferred and give reasons, one in each case, for the choice. [6]

Total: 104

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