Page 537 - Physics Coursebook 2015 (A level)

Chapter 32: Medical imaging
The result is an image like the one shown in Figure 32.35. This image has been coloured to show up the different tissues, which are identified by their different relaxation times.
Advantages and disadvantages of MRI
MRI has several advantages compared to other scanning techniques:
■■ It does not use ionising radiation which causes a hazard to patients and staff.
■■ There are no moving mechanisms, just changing currents and magnetic fields.
■■ The patient feels nothing during a scan (although the gradient coils are noisy as they are switched), and there are no after-effects.
■■ MRI gives better soft-tissue contrast than a CT scan, although it does not show bone as clearly.
■■ Computer images can be generated showing any section through the volume scanned, or as a three-dimensional image.
One disadvantage of MRI is that any metallic objects in the patient, such as surgical pins, can become heated. Also, heart pacemakers can be affected, so patients with such items cannot undergo MRI scans. Loose steel objects must not be left in the room as these will be attracted to the magnet, and the room must be shielded from external radio fields.
Figure 32.36 shows how an MRI scan can be combined with a CT scan to show detail of both bone and soft tissue, allowing medical staff to see how the two are related. Compare this with Figure 32.35.
Figure 32.36 A combined CT scan and MRI scan, showing how the tissues revealed by MRI relate to the bone structure shown by X-rays.
QUESTION
21 An MRI scan might be considered a safer procedure than a CT scan.
a Explain why it might be considered to be safer.
b Why might a CT scan be chosen in preference
to an MRI scan?
c Explain why MRI is described as non-invasive.
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