Magnetic Resonance Imaging

Question 1

Why is MRI a better imaging technique than other methods? Give 6 reasons.

Answer 1

1. Images produced have higher contrast between different tissues.
2. Image is clearer and more detailed (higher resolution).
3. Can be used to look at obscured regions of the body eg. bone marrow.
4. No dangerous ionising radiation.
5. Can be uesed for extended investigations of sensative areas eg the brain diagnosing brain tumours.
6. Doctor has fine control over which part of body is investigated and the plane the image is formed in without moving patient.

Question 2

Why are larger diameter machines more expensive than smaller diameter ones?

Answer 2

It is harder to maintain the strong uniform magnetic field over a larger area.

Question 3

What is meant by a ‘superconductor’ ?

Answer 3

A material which has zero resistance below a certain threshold temperature.

Question 4

Give 2 reasons why superconducting electromagnets are used in MRI.

Answer 4

Superconducting, therefore no thermal energy is generated by I^2R heating, thus no wasted energy on cooling coils. Also, means large currents can be passed through coils to make B fields strong enough for use.

Question 5

What are the two types of field used in MRI?

Answer 5

Gradient B field and powerful uniform B field.

Question 6

Give the basic method through which an image is produced in MRI.

Answer 6

1. A powerful uniform magnetic field causes the nuclei of Hydrogen atoms to align themselves either with or against the field (in parallel to field).
2. This produces a net magnetisation in one direction.
3. A gradient B field is produced across the body, meaning net field strength at each point is different.
4. Protons undergo precession about the field lines at their Lamor frequency, which is proportional to B at each point.
5. rf waves are transmitted through the body. Where rf=Lamor frequency resonance occurs, changing protons to higher energy spin state.
6. When rf turned off protons relax, emitting energy. This is picked up in the detector coil.
7. Computer identifies origin of signal as it knows where Lamor frequency=rf and hence B at each point.
8. Gradient field changes direction so that protons in different slices resonate, allowing computer to process data and form final image, incorporating many slices.

Question 7

What are the two main sources of Hydrogen Nuclei in the body?

Answer 7

Organic molecules (such as fatty acids) and water.

Question 8

In how many planes and at what angle relative to each other are gradient B fields produced across the body?

Answer 8

Produced in three different planes at right angles (x, y, z).

Question 9

What does the intensity of the signal received by the detector coil depend on?

Answer 9

The density of Hydrogen nuclei that are radiating energy and the type of tissue.

Question 10

What are the 6 disadvantages of MRI?

Answer 10

1. Very expensive
2. Scans take a long time, leading to long patient waiting lists.
3. Very large (fat people) can’t fit in them.
4. Patients may feel claustrophobic inside them.
5. Patients may feel unnerved by the noise of the machine.
6. Patients with pacemakers or hip/joint replacements can’t use them-Pacemakers interfere with uniform B field and metallic replacements distort the field.