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Topic 15: A peep inside

Siemens Education

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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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CT Scan

Ultrasound Scan

MRI Scan

X-ray image

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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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Typical diagnostic application: slipped disc

Nerves in this area are compromised from the

pressure of the prolapsed disc

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Typical diagnostic application: sprained wrist

(if compressed gives pain, swelling and

restriction of movement)

(Tendons of the wrist) (Nerve and artery of the ulna bone run through here to the hand)

(One of the bones in the

wrist)

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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)