You are allowed ten minutes before the start of the examination to acquaint
yourself with the instructions below and to read the question paper.
Do not write anything until the invigilator informs you that you may start the
examination. You will be given five minutes at the end of the examination to
complete the front of any answer books used.
________________________________________________________________
June 2000
DE 4018
2 Answer Books
THE UNIVERSITY OF READING
Part III Examination in Science
Physics Unit 3/PH/L1
PHYSICS IN MEDICINE
1½ Hours
________________________________________________________________
Answer TWO Questions
•
Describe, using suitable diagrams, the operation of a typical x-ray tube.
Page 2
(4 marks)
Discuss the effect of the following on the spectrum emitted from an
•
•
•
•
x-ray tube
Increasing the tube voltage
Increasing the tube current
Increasing the atomic number of the target
Replacing the x-ray transparent window on the tube with an aluminium sheet.(8 marks)
In an x-ray image of a broken arm bone, the contrast between bone and soft tissue for
x-rays from a tube with a particular voltage is found to be 0.9. Using suitable
estimates and given that 3% of x-rays are transmitted through the soft tissue, calculate
the absorption coefficient of bone.
(6 marks)
Measurement of the dose received at the exit (ie, lower) surface of the skin in regions
where x-rays have only passed through soft tissue shows this to be 10-4 Gy. Estimate
the doses received at the entrance surface
of the skin and at the exit surface of the skin below the bone.
(2 marks)
2
(a)
A typical motor nerve axon is a myelinated fibre with short, unmyelinated
sections known as nodes of Ranvier.
Explain the functions of the myelin sheath and the nodes of Ranvier. (4 marks)
Estimate the speed of a nerve pulse in a typical motor nerve axon.
(6 marks)
(Some of the following data for a typical axon may be useful:
Resistivity = 2 m, Capacitance per unit area of nerve membrane = 5 x 10-5
Fm-2, Radius of axon core = 1 m, Distance between nodes of Ranvier = 1
mm)
2
(b)
An axon, is bounded by a membrane. The K+ concentration inside the axon is
165 moles / m3 and outside it is 8 moles / m3. Inside the axon the potential is 90 mV with respect to the outside. In which directions are the K+ ion flows
due to the electric field and due to diffusion? Which is the greater? State any
assumptions you make.
You may need the following: For a body at 37o C the Nernst equation, when
equilibrium is established, can be written in the form
VOUT - VIN = 60 log10 (XIN/ XOUT)
where X is the mole fraction of K+ ions, V the electric potential and the
subscripts IN and OUT refer respectively to inside and outside the membrane.(10 marks)
DE 4018
Page 3
3.
Explain the basic principles of the echo technique used in ultrasound imaging. What
gives rise to losses in the echo signal? Why is a jelly used between the patient and
piezoelectric transducer?
(12 marks)
Explain the A-mode, B-mode and M-mode ultrasound techniques.
(8 marks)
(End of Question Paper)
DE 4018