Page 534 - Physics Coursebook 2015 (A level)

Cambridge International A Level Physics
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QUESTIONS
17 Two consecutive peaks in an ultrasound A-scan are separated by a time interval of 0.034 ms. Calculate the distance between the two reflecting surfaces. (Assume that the speed of sound in the tissue between the two surfaces is 1540 m s−1.)
18 Explain why an ultrasound B-scan, rather than X-rays, is used to examine a fetus.
Magnetic resonance imaging
Magnetic resonance imaging, or MRI, is a diagnostic technique used in medicine. It can provide images (including moving images) of the insides of a patient. It does not rely on exposing patients to ionising radiation such as X-rays; rather, it relies on the fact that some atomic nuclei behave like tiny magnets in an external magnetic field.
(MRI was originally known as nuclear magnetic resonance imaging, but the word ‘nuclear’ was dropped because it was associated in patients’ minds with bombs and power stations. To emphasise: MRI does not involve radioactive decay, fission or fusion.)
As in CT scanning, MRI scanning involves electromagnetic radiation, in this case radio frequency (RF) electromagnetic waves. The patient lies on a bed in a strong magnetic field (Figure 32.29), RF waves are sent into
Figure 32.29 A patient undergoing an MRI scan of the brain. This is a form of tomography; the display shows different ‘slices’ through the patient’s brain.
their body, and the RF waves that emerge are detected. From this, a picture of the patient’s insides can be built up by computer. As we will see, MRI gives rather different information from that obtained by the other non-invasive techniques such as X-rays or ultrasound.
Principles of nuclear magnetic resonance
The nuclei of certain atoms have a property called spin, and this causes them to behave as tiny magnets in a magnetic field. In MRI, it is usually the nuclei of hydrogen atoms that are studied, since hydrogen atoms are present in all tissues. A hydrogen nucleus is a proton, so we will consider protons from now on.
A proton has positive charge. Because it spins, it behaves like a tiny magnet with N and S poles. Figure 32.30a shows a number of protons aligned randomly.
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b
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SNSSS NSNNN
NSNS SNSN
SSSNS NNNSN
SSSS NNNN
Figure 32.30 How protons behave in a strong magnetic field. a Protons are randomly directed when there is no external magnetic field. b Because protons are magnetic, a strong external magnetic field causes most of them to align themselves with the field.
When a very strong external magnetic field is applied, the protons respond by lining up in the field (just as plotting compasses line up to show the direction of a magnetic field). Most line up with their N poles facing the S pole of the external field, a low energy state; a few line up the other way round, which is an unstable, higher energy state (Figure 32.30b).
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