MR Equipment and Safety Flashcards
What do we see in MRI images?
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%)
What is the MR signal from protons due to?
spin
What is the heart of MR system?
The magnet
What is a measure of field strength?
Gauss (G)
e.g. 1 tesla equals to 10,000 gauss
What is the Earth’s magnetic field?
Approximately 0.05T
What are the principal types of magnet used in MRI?
- Superconducting magnet [1.5 or 3T]
- Permanent magnet [0.3T]
- Electromagnet [0.6T]
Where does the main field usually point?
Horizontally along the bore
What is the magnetic field always present for?
- Superconducting magnet
2. Permanent magnet
What does superconducting magnets require?
Liquid helium as cryogenic cooling fluid
What does a sudden loss of super-conductivity result in?
A magnet quench where the windings heat up, the field collapses in less than one minute and large amounts of helium boil ass gas
What is Radiofrequency coil generated by?
Transmitter coil which surrounds whole or part of body
What is detected using a receiver coil?
MR signals produced in the body
What are MR signal?
Very weak and are sensitive to electrical interference
What is the localization of MR signal in body achieved by?
Generating short-term variations in magnetic field across patients
Where are the gradient coils for each direction x,y,z built into?
Bore of magnet
How are the gradients applied?
Carefully controlled pulse sequences
What are the safety concerns?
- 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
What do we need to do an MRI?
- Main magnet
- Uniform static field
- B0 - RF coil -B1
- Field gradient coils
How is MRI done?
- Put subject in magnet
- Disturb equilibrium with magnetic field (RF frequencies)
- Receive RF signal from subject
- Process data to get images
What are the main B0 field characteristics?
- Strength
- Homogeneity
- Stabillity in time
What are the features of French 11.75 T magnet?
- Inner diameter: 90cm
- > 100 tonnes of equipment including 60 just for the superconductor
- ~136km of multifilament carrying a current of 1,493A
- Stored energy (338 MJ)
What can be achieved at higher field?
- SNR
- Improve image quality or spectral resolution
- Improve spatial resolution
- Reduce examination time/patient discomfort
What can increased chemical shift improve?
Spectral fat suppression and spectroscopy
What may be traded for increased spatial resolution or decreased imaging time?
The improvement in SNR with increasing field strength
What are features of Electromagnet?
- Low field (0.1 to 0.5T)
- Expensive in electricity to run (40-100kW)
- Tend to be unstable in field strength
What are the features of permanent magnet?
- Low field (0.1 to 0.5T)
- Heavy and potentially thermally unstable
- Cannot turn off
What are the features of superconducting magnet?
- Approx 50km of superconducting wire (200Amps)
- Wire cooled with 4K liquid helium and vaccum jackets
- A refrigerator turns he gas into liquid again
- Very stable, B0 field up to 9.4T
- No need for power supply
What does magnet reconstruction include?
A large volume of liquid helium to keep wires at their superconducting temperature
What does highest field strength offer?
Best homogeneity and stability over time
What does Magnetic field gradient allow one to do?
- Gather spatial information in 1,2 or 3 dimensions
2. Select a specific area of interest in sample
What happens within RF coil?
- ‘Excite’ water protons with an RF signal (transmit, Tx)
* Receive RF signal from subject’s excited protons (Rx)
What does RF coil generate?
Time-varying RF field B1 perpendicular to B0
-Coil tuned at larmor frequency set at B0
What are examples of RF coils?
- Solenoid
- Saddle
- Birdcage
- Surface
What decreases with distance from the coil?
Surface coil
What are the RF coil characteristics?
- Homogeneity (B1) over volume of interest
- Efficiency/sensitivity (reciprocity principle)
- Rule of thumb: the closer the better
What can Array coils be designed to do?
- Boost SNR [increase image quality]
- Speed-up image acquisition
- Motion reduction/detection/correction
What do shim coil do?
- 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
What is RF shield?
A ‘faraday cage’ designed to prevent any RF with frequencies close to the scanner operating frequency from interfering with MR equipment
What is magnetic shield?
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
Is MRI safe?
- 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
What are the main potential bio-effects?
- B0
- B1 (RF)
- Switching Gradients
