Overview Mri Hardware

Question 1

Mr system components

Answer 1

• Gradient coils-spatially encode MR system-source behind the noise
• Rf coils -transmit RF pulses and receive MR signal -can cause burns if not adequately assessed

Question 2

Types of magnets

Answer 2

• Permanent magnets
• Resistive electromagnets
• Superconductive magnets

Question 3

Permanent magnets

Answer 3

• Use permanent magnetised ferromagnetic materials such as ion
• Not adequate for complex Mri examinations
• No costs
• Drawback-weight
• Used in open bore mri units -advantage -reduce claustrophobia and good in interventional use
• Poor magnetic field homogeneity

Question 4

Resistive

Answer 4

• • Utilise principles of electromagnetism to generate the magnetic field-made by running an electric current through coils of wire
• Need large currents and cooling-constant power supply to maintain a homogeneous field
• Magnetic field strength limited by power supply
• Now superseded by Superconductive magnets
• The resistive magnet does not require cryogens, but needs a constant power supply to maintain a homogenous magnetic field, and can be quite expensive to maintain

Question 5

Superconductive magnets

Answer 5

o Concept- A current traveling through a loop/coil of wire creates a magnetic field along the axis of the loop in line with Faraday’s law.
o Superconductivity is achieved by reducing the resistivity of the coil of wire-ideally there is no resistivity in the coil of wire
o This reduced resistivity is achieved by cooling the coil of wire to temperatures where the coil has lowest resistance/energy
o This slow resistivity is obtained by circulating a cryogen(liquid helium) (cryogen in the case of MRI)
o Superconductive materials exhibit 0 electrical resistance when cooling below a certain critical temperature-allows them to carry large currents without power loss -crucial for generating the strong magnetic fields in MRI
o Coil material used in nearly all modern superconductive magents is Naiobium titanium- becomes superconductive below 9degrees calvin

Question 6

Advantages and disadvantages

Answer 6

Advantages
o Higher field strengths
o Better performance
o Highest temporal and spatial homogeneity of the magnetic field
o High signal to noise ratio (SNR)
o Shorter scan times
o High patient throughput
o Closed bore

Disadvantages
• Potential claustrophobia -higher field strength =magnet is excerting a higher force on ferromagnetic materials

Question 7

Fringe fields

Answer 7

• Originates from Mr scanner and can extend to vicinty outside the magent – and so its attraction to ferromagnetic materials -
• The Strength of fringe field =strength of magnet
• Can interfere with nearby electronic devices
• Can turn ferromagnetic materials into projectiles
• Fringe field can be contained by magentic shielding – imp to protect personnel and equipment

Question 8

Active shielding

Answer 8

• Use of additional superconductive coils that generate opposing magnetic fields to generate the fringe field.
• Coils are placed strategically to produce a magentic field opposing and reducing the extent of the fringe field- therefore cancelling it out in areas outside the desired region-minimising the area affected by the magnetic field allowing mr systems to be installed in small places

Question 9

Passive shielding

Answer 9

• Use of physical ferromagnetic materials placed around the mri magnet such as iron
• The materials absorb and redirect the magnetic flux-preventing it from extending the boundaries
• Does not require the control areas of active shielding-simple
• Effectiveness does lower field strength