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P2 PHYSICS > MR Equipment and Safety > Flashcards

MR Equipment and Safety Flashcards

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1
Q

What do we see in MRI images?

A

A proton is the nucleus of hydrogen atom (1H)
Each water molecule has 2 hydrogens and 1 oxygen
Hydrogen is the most common element in the body (~60%)

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2
Q

What is the MR signal from protons due to?

A

spin

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3
Q

What is the heart of MR system?

A

The magnet

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4
Q

What is a measure of field strength?

A

Gauss (G)

e.g. 1 tesla equals to 10,000 gauss

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5
Q

What is the Earth’s magnetic field?

A

Approximately 0.05T

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6
Q

What are the principal types of magnet used in MRI?

A
  1. Superconducting magnet [1.5 or 3T]
  2. Permanent magnet [0.3T]
  3. Electromagnet [0.6T]
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7
Q

Where does the main field usually point?

A

Horizontally along the bore

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8
Q

What is the magnetic field always present for?

A
  1. Superconducting magnet

2. Permanent magnet

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9
Q

What does superconducting magnets require?

A

Liquid helium as cryogenic cooling fluid

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10
Q

What does a sudden loss of super-conductivity result in?

A

A magnet quench where the windings heat up, the field collapses in less than one minute and large amounts of helium boil ass gas

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11
Q

What is Radiofrequency coil generated by?

A

Transmitter coil which surrounds whole or part of body

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12
Q

What is detected using a receiver coil?

A

MR signals produced in the body

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13
Q

What are MR signal?

A

Very weak and are sensitive to electrical interference

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14
Q

What is the localization of MR signal in body achieved by?

A

Generating short-term variations in magnetic field across patients

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15
Q

Where are the gradient coils for each direction x,y,z built into?

A

Bore of magnet

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16
Q

How are the gradients applied?

A

Carefully controlled pulse sequences

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17
Q

What are the safety concerns?

A
  • The MR scanner is very noisy during operation – patient/person in room given earplugs or ear defenders to reduce their exposure to acoustic noise
  • Potential hazards of working with cryogens include asphyxiation in oxygen-deficient atmosphere, cold burns, frostbite and hypothermia
  • Possibility of inducing asthma in susceptible person if cold gas is inhaled
  • Resist the temptation to touch feed pipes just after a helium fill to see how cold they get
  • Contact with cryogens should be restricted to fully trained engineering staff
  • ‘crash trolley’ in the MR suite with appropriate resuscitation equipment and drugs, ideally made of non-magnetic material so that it can be taken into the magnet room in an emergency
  • Trained physicians should be nearby when GD-based agents are being used
18
Q

What do we need to do an MRI?

A
  1. Main magnet
    - Uniform static field
    - B0
  2. RF coil -B1
  3. Field gradient coils
19
Q

How is MRI done?

A
  1. Put subject in magnet
  2. Disturb equilibrium with magnetic field (RF frequencies)
  3. Receive RF signal from subject
  4. Process data to get images
20
Q

What are the main B0 field characteristics?

A
  1. Strength
  2. Homogeneity
  3. Stabillity in time
21
Q

What are the features of French 11.75 T magnet?

A
  1. Inner diameter: 90cm
  2. > 100 tonnes of equipment including 60 just for the superconductor
  3. ~136km of multifilament carrying a current of 1,493A
  4. Stored energy (338 MJ)
22
Q

What can be achieved at higher field?

A
  1. SNR
  2. Improve image quality or spectral resolution
  3. Improve spatial resolution
  4. Reduce examination time/patient discomfort
23
Q

What can increased chemical shift improve?

A

Spectral fat suppression and spectroscopy

24
Q

What may be traded for increased spatial resolution or decreased imaging time?

A

The improvement in SNR with increasing field strength

25
Q

What are features of Electromagnet?

A
  1. Low field (0.1 to 0.5T)
  2. Expensive in electricity to run (40-100kW)
  3. Tend to be unstable in field strength
26
Q

What are the features of permanent magnet?

A
  1. Low field (0.1 to 0.5T)
  2. Heavy and potentially thermally unstable
  3. Cannot turn off
27
Q

What are the features of superconducting magnet?

A
  1. Approx 50km of superconducting wire (200Amps)
  2. Wire cooled with 4K liquid helium and vaccum jackets
  3. A refrigerator turns he gas into liquid again
  4. Very stable, B0 field up to 9.4T
  5. No need for power supply
28
Q

What does magnet reconstruction include?

A

A large volume of liquid helium to keep wires at their superconducting temperature

29
Q

What does highest field strength offer?

A

Best homogeneity and stability over time

30
Q

What does Magnetic field gradient allow one to do?

A
  1. Gather spatial information in 1,2 or 3 dimensions

2. Select a specific area of interest in sample

31
Q

What happens within RF coil?

A
  • ‘Excite’ water protons with an RF signal (transmit, Tx)

* Receive RF signal from subject’s excited protons (Rx)

32
Q

What does RF coil generate?

A

Time-varying RF field B1 perpendicular to B0

-Coil tuned at larmor frequency set at B0

33
Q

What are examples of RF coils?

A
  1. Solenoid
  2. Saddle
  3. Birdcage
  4. Surface
34
Q

What decreases with distance from the coil?

A

Surface coil

35
Q

What are the RF coil characteristics?

A
  • Homogeneity (B1) over volume of interest
  • Efficiency/sensitivity (reciprocity principle)
  • Rule of thumb: the closer the better
36
Q

What can Array coils be designed to do?

A
  1. Boost SNR [increase image quality]
  2. Speed-up image acquisition
  3. Motion reduction/detection/correction
37
Q

What do shim coil do?

A
  1. Help make the B0 field as homogenous as possible

2. Can be optimised to flatten B0 over a large area or a small volume of interest

38
Q

What is RF shield?

A

A ‘faraday cage’ designed to prevent any RF with frequencies close to the scanner operating frequency from interfering with MR equipment

39
Q

What is magnetic shield?

A

Can be used to contain the fringe field of the magnet and avoid it causing malfunctioning in electronic devices outside the scanner room e.g. pacemakers, pc monitors

40
Q

Is MRI safe?

A
  • Yes!
  • MR signal comes from tissue directly
  • MR uses non-ionising radiation – it is completely non-invasive
  • There are no known long-term effects – longitudinal studies can be performed safely
41
Q

What are the main potential bio-effects?

A
  1. B0
  2. B1 (RF)
  3. Switching Gradients

P2 PHYSICS (16 decks)

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