What is the unit of measurement for MRI?
MRI requires strong magnetic field between 0.2 and 3 Tesla that are generated by superconductors
1 Tesla = 10,000 Gauss = 1 weber/m2
(Earth’s magnetic field = 0.5- 1 Gauss)
Gauss - the gaussian unit if magnetic flux density (G)
T = 10,000 G
Describe the magnetic field
MRI requires strong magnetic field between 0.2 and 3.0 Tesla that are generated by superconductors.
To minimise the electrical resistance of the superconducting coils, they are immersed in liquid helium and cooled to below 4.2 Kelvin
What are indications for MRI?
Posterior Fossa and infratententorial pathology
Sinus and orbit pathology
Cerebral inflammatory disease
Brain abscess
Acute Ischaemic strokes
Spinal cord soft tissue pathology
Demyelinisation and myelopathies
Airway malformations
Vascular malformations
Liver vascular pathology
Joint soft tissue pathology
Describe physics of MRI
- Hydrogen atoms are abundant within the body within water molecules
- Hydrogen atoms possess a property called ‘spin’, allowing them to act like small magnets
- When surrounded by a strong magnet, spin aligns with magnetic field
- Spin can be turned out of alignment with magnetic field by applying pulses of electromagnetic radiation
- Energy emitted is detected by 3 receiving gradient coils and translated into a series of grey pixels in a screen to generate 3D image.
What is superconductivity?
A phenomenon where electrical resistance in a coil of wire decreases to zero below a critical temperature.
Passing of an electrical current through a coil of wire results in a strong magnetic field (electromagnet). Typical MRI uses a magnetic field strength of 1.5 Tesla
Achieved by cooling to 4.2K by surrounding coils in liquid helium (Coils in MR magnets need to be kept cold in order to maintain superconductivity)
This is usually surrounded by a jacket of liquid nitrogen, which has a boiling point of 77K to keep expensive liquid helium from boiling away.
What is quenching?
Quenching is a process involving the rapid boil-off of the coolant that causes immediate loss of superconductivity
If this happens the magnetic field will be lost and a large volume of helium gas will be produced. This is normally vented to the outside via a quench pipe.
In the event of a spontaneous or emergency field shutdown, (a quench), the liquid helium and nitrogen expands to a gas and must be vented very rapidly with the potential for a hypoxic environment in the MRI scan room
What are T1 and T2 weighted images
Different tissues within the body have different relaxation rates.
T refers to the relaxation time constant and images may be T1 weighted (generated a few milliseconds after the electromagnetic field is removed) or T2 weighted (generated later than T1).
Nuclei in hydrogen take a long time to decoy to their original position , so fluid will appear dark (min signal) in a T1 weighted image, but white in the later T2 image as the signal appears
What are contradictions to MRI?
- Cochlear implants
- Intraocular foreign bodies
- Aneurysm clips
- Pacemakers
- Implantable defibrillator
What are hazards and safety considerations for patients and staff?
- Presence of a strong magnetic field e.g causing local heating, interfere with monitoring, electrical currents causing symptoms such as n&v, vertigo
- Ferromagnetic objects and the projectile effect
- Implants and foreign bodies
- Equipment and monitoring issues
- Restricted access of environment
- High level acoustic noise
- Scavenging of Anaesthetic Gases
- Quenching of superconducting magnets
- Hazards of MRI during pregnancy
- Use of contrast agents
- Maintenance of body temperature
What hazards are associated with the strong magnetic field.
- Exert large forces on any ferromagnetic objects - projectiles, implants, foreign bodies etc
- Local heating may interfere with monitoring equipment
- Movement of blood around body results in generation of electrical potentials and current - cause symptoms such as n&v, vertigo or flashing lights due to theirneffects on retina.
State field contour which the ‘MR environment’ is defined
5 Gauss or 0.5 milliTesla (mT)
Within this area, there is a risk of projectile damage from ferromagnetic objects, risk of heating from radio-frequency coils and risk associated with implanted devices
What is meant by MRI safe? And MRI conditional?
MRI Safe: these devices pose no MR-related hazards to patients or staff when used
MRI conditional: this equipment poses no MR-related hazard in a specified MR environment under specific conditions of use e.g static field strength, rate of change of magnetic field
What precautions should be taken to prevent burns caused by monitoring equipment in MRI?
- Use only MRI safe monitoring equipment
- Check all equipment prior to use, that it’s intact and no breach in insulating surfaces
- Fibreoptic cable for ECG and pulse oximeter eliminate use of electrical current, which may result in induction currents and burns to underlying skin
- ECG meads should be high impedance, braided and short to minimise risk of induction currents
- Telemetric monitor to elimiante risk of induction current in connecting leads
- Do joy allow cables to coil or cross each other as induction of currents can result from capacitance coupling
- Ensure leads are positioned to exit the scanner down the centre rate than sides to keep away from RF coils
- Separate leads from patients skin with padding
What precautions would out take to minimise risks associated with MRI
- Equipment checks - MRI safe
- Checklist for patients and staff
- Ferromagnetic objects removed
- Padding over RF coils
- Ear protection
- Monitoring equipment amd breathing circuit
- Inaccessibility of airway
- Telemetric monitoring
- Remote site anaesthesia - ensure senior support available
- Awareness of quench evacuation procedures
- Risk of gadolinium based contrasts - avoid if eGFR < 30
- don’t repeat contrast within 7 days
- drugs to manage anaphylaxis
- risk if neohrogenic sclerosing fibrosis if patient is in ESRF
