Page 540 - Physics Coursebook 2015 (A level)

Cambridge International A Level Physics
12 a Explain what is meant by acoustic impedance and outline its role in the use of ultrasound scans. [3] b Brain tissue has a density of 1.04 × 103 kg m−3 and ultrasound travels at 1.58 × 103 m s−1 through it.
Calculate the acoustic impedance of brain tissue. [2] c Figure 32.38 shows the trace formed on the screen of an oscilloscope when ultrasound is reflected
from the front and rear surfaces of the head of a fetus. The time-base of the oscilloscope is set at 10 μs div−1.
Figure 32.38 For End-of-chapter Question 12.
i Explain why the second peak is lower than the first. [1]
ii Calculate the diameter of the head of the fetus. [3]
13 a With reference to nuclear magnetic resonance, explain what is meant by the gyromagnetic ratio. [1] b A doctor is conducting an MRI scan on a patient. There is a steady magnetic field of 1.8 T and superimposed
on this is a much weaker field which oscillates at an RF frequency of 48 MHz.
i Assuming the RF frequency causes resonance of the protons, calculate their gyromagnetic ratio. [3]
ii Explain what resonance means in this application. [2]
iii DescribewhathappenswhentheRFfieldisremovedandhowthisleadstoinformationaboutthe
internal organs of the patient. [3]
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