Siemens Education
Topic 15:
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A peep inside
Course requirements for GCSE Physics
Students should:
• Develop knowledge and understanding of the relationship between the
properties of electromagnetic waves and their uses
• Use scientific theories, models and evidence to develop hypotheses,
arguments and explanations
• Develop and use models to explain systems, processes and abstract ideas
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Learning objective
We are learning to:
• Explain how MRI scanners produce images
• Apply their understanding of waves and particles to this application
• Describe typical uses of MRI images
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How can a doctor make the
best possible diagnosis?
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The importance of images
• There are various situations in which medical diagnosis will be assisted by
detailed imaging, such as broken bones, tumours, etc and the pros and cons
of various imaging technologies, such as ultrasound, CT scans, X-rays and
MRI scans, need to be considered
• X-rays are often used for the imaging of hard material such as bones
• Ultrasound is quick and easy but gives lower resolution images
• CT scans are high quality but only work in one plane, whereas MRI scans can
work in any plane, producing high quality images of the inside of the body
from the desired perspective
• CT and X-ray imaging uses ionising radiation
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MRI Scan
X-ray image
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CT Scan
Ultrasound
Scan
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A carbon atom
A hydrogen atom
Atomic structure
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What does an MRI scanner
consist of?
There are four key components:
1) A powerful permanent magnet
2) Gradient coils, which produce a
variable magnetic field
3) Radio frequency (RF) coils,
which produce radio waves
4) Scanner, which detects energy
emitted from the body
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How does MRI work? (1)
Although there are many different
elements in the body, hydrogen is very
common. It is present in water and in
fats. Hydrogen nuclei are the key to
the production of MRI scans.
Normally, these nuclei spin in random
directions around their individual
magnetic fields.
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How does MRI work? (2)
The MRI scanner has a powerful
magnet, which produces a magnetic
field, many times stronger than that of
the Earth. In this magnetic field the
nuclei line up either north or south.
About half go each way, but there are a
few unmatched ones.
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How does MRI work? (3)
When radio waves are produced by the
RF coils, the unmatched nuclei spin the
other way. The gradient magnets alter
the magnetic field so that images can
be produced from any direction.
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How does MRI work? (4)
When the radio frequency coils are
switched off and the pulse of RF
waves stops, the extra nuclei return to
their normal position, emitting energy.
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How does MRI work? (5)
The energy released by these nuclei
is detected by the scanner, which
sends a signal to the computer, which
converts it to an image.
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What advantages does MRI offer?
• It can work in any plane: think of the different directions one could slice an
apple in. Each slice is a two dimensional view; this is what the images are
like. Being able to select the plane offers medical staff real advantages
• The images have a very high resolution: the detail is good and it can
discriminate between matter that is similar
• There are no known biological hazards with MRI: it doesn’t use ionising
radiation
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What are the drawbacks?
• Noise: continuous hammering around
• Patient has to be still for anything up to 90 minutes: movement causes
blurring and requires a repeat scan
• Expensive: complex equipment which needs skilled operation and takes time
to scan
• Claustrophobic: some patients find it unpleasant
• Dangerous: magnetic materials can be caused to fly across the room
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Typical diagnostic application:
slipped disc
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Nerves in this area are
compromised from the
pressure of the prolapsed
disc
Typical diagnostic application:
slipped disc
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Typical diagnostic application:
sprained wrist
(One of the
bones in the
wrist)
(if compressed gives
pain, swelling and
restriction of movement)
(Tendons of the wrist)
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(Nerve and artery of the ulna bone
run through here to the hand)
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Sample assessment task
• This diagram shows a simplified
cutaway view of an MRI scanner
• Explain how it produces images
for clinical diagnosis (6)
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