MRI Flashcards
how does an MRI work?
H atoms have spin and charge and act like mini magnets - 1 proton in nucleus + charge and one neutron - hence imbalance of charge. at rest, no allignment
placed in a magnet and align with the magnetic field - spin on axis - known as precision
Radiofrequency magnetic filed energy is applied (at same frequency of larmour frequency i.e. resonance) causing the H atoms to flip 90 degrees to the original field = higher energy state
When RF turned off they relax back to original state and release energy
Scanner picks up the energy released and compares different rates of relaxation/ excitation to determine type of body tissue. different bonds e.g. OH and CH will give off different frequencies and at different rates
various gradient magnets are used and switched on and of (the noise) repetitively to help create a 3D image
what is the strength of the magnet produced by MRI?
1- 3 Tesla
static magnet
how much noise does MRI produce ?
100 Db
what is the larmor frequency?
the H atoms spin on an axis = precision
within a magnetic field they will spin at a natural frequency = larmor frequency
this frequency depends on the strength of the field and the atom in question
w= λ x Bo
(Bo = magnetic field strength)
λ = a constant
what is the difference between T1 and T2 image?
T1 = water is black, fat white.
T2 = water is white and fat white
what does T1 relate to in terms of relaxation states?
T1 = spin lattice relaxation = time constant longitudinal relaxation. This is the time taken for the net magnetism to realign with the main magnetic field.
what does T2 relate to in terms of relaxation?
T2 = spin spin relaxation = time constant for transverse relaxation. this is the time taken for protons to relax back to resting phase (in light with magnet)
what contrast medium can be used with MRI?
Gadolinium
used with T1 weighted MRI
alters relaxation of H making the image more visible.
when in contrast MRI’s used?
angiography
GIT
brain tumours that have crossed the BBB
what are the benefits of an MRI?
non ionising
better for soft tissues
better for making 3D images.
how is the high strength magnet created?
cryogenic superconducting magnets
the metal is cooled to close to absolute zero (0 K) by immersing in liquid helium
this causes the metals resistance to become close to absolute zero meaning its current is v high allowing is to create a very strong magnetic field.
what is quench?
liquid helium heated rapidly
vented from scanner
no liquid helium to create a superconducting magnet
no magnetic field
what are the disadvantages/ safety concerns for an MRI ?
Practicalities for trust
* Large
* Expensive
For patient:
* Claustrophobia – trauma
* Noisy – can cause damage to ears
* Contraindicated for those with metal implants
For anaesthetist:
* Cant access patient
* A lot of equipment incompatible
* Far away from clinical area e.g. ITU
* Availability of drugs and equipment is limited.
* Interference with monitoring from magnet induced currents.
Safety:
* Risk of metal objects being projected and injuring patient
* Radiofrequency energy causes induced current and heating of metals = may cause burns.
what are the indications for a GA MRI?
Unstable patient – intubated and ventilated
Uncooperative patient – child, learning difficulties , Claustrophobia
Intra op MRI – do surgery, check with MRI, do more surgery if needed
how do CTs work?
CTs take a series of X-ray images around a central axis
What equipment is unsafe in MRI
Any ferrous equipment
E.g. laryngoscopes, stethoscopes, gas cylinders, drip stands, defibrillators
Pulmonary artery catheter small bit of metal at tip – can get induced currents and cause microshock if put in a magnet.
How is safety of MRI improved?
Faraday cage - Entire room is coated with copper/ aluminium
For monitoring – special MRI compatible equipment
*e.g. ecg electrodes- carbon electrodes
* e.g. graphite ecg leads.
*e.g. nylon BP cuffs
*e.g. special pulse oximeters – fibre optic cabling to prevent induced currents and burns
wires should run down centre of patient - furthest from the walls of the doughnut.
what is the difference between T1 and T2 MRI
T1 is about how fast tissues “relax” after the magnetic pulse, with fat being bright and water dark. spin lattice relaxation. i.e. time constant for longitudanal relaxation - allign back to original field
T2 is about how fast protons lose sync with each other, with water being bright and fat darker.
spin spin relaxation i.e. time constant for tranverse relaxation i.e. line to tranverse field.
what are the problems with quenching the MRI
can cause damage to scanner
a lot of money to replace He
risk of hypoxic area - designed to vent He externally however risk it gets into the MRI room, diluting O2
what do these 3 symbols mean?
top = MRI safe - no known hazards
middle - MRI conditional - Hazard in specific MRI environment e.g. at certain field strength
bottom - MRI unsafe
which part of LMA can affect an MRI image?
pilot balloon - small magnetic spring, can cause interference
what is meant by diamagnetism?
weakly magnetic materials
no unpaired electrons in outershell
or imbalance of P + N in nucleus
e.g. copper
how does the MRI magnetic field compare to earths?
MRI = 1-3Telsa
Earth 0.0005T
what are the two units for magnetic strength?
tesla
Guass - old
why are H atoms used in MRI?
due to their abundance in soft tissue - good for soft tissue imaging.
other atoms are also paramagnetic e.g. phosphorus in ATP
why does MRI not pick up signals from other atoms?
the larmor frequency is specific to each atom at that given magnetic field. i.e. w = yBo
the radiofrequency energy is of that frequency energy such that only H atoms are exited to higher energy state
only these will release energy when they relax.
Hazards of anaesthesia in MRI
Magnetic field…
* dangerous projectiles e.g. gas cylinders
* ferromagnetic foreign bodies become dislodged e.g. metals in eye
* pacemaker inactivated / reprogrammed
dynamic magnetic field
* induce currents - interference but also biological tissues
patient
* noise, claustrophobia
radiofrequency heating
* burns from any conducting material on patient hence ecg leads carbon fibre and pulse ox is fibre optic
helium escape
* hypoxic
anaesthetist
* unfamiliar environment with unfamiliar equiptment
* away from patient
do you know any critical incidence related to MRI?
ferromagnetic object - loose coins/ keys in pockets accelarating and causing injury. more serious would be O2 cylinders, wheelchairs. may result in serious patient/ staff injury
Cardiac arrest in MRI scanner- need to remove patient, cant take arrest trolley and defib into the room
what are the components of an MRI machine..
MRI machine
* main magnet
* smaller gradient magnets - create gradient fields to improve image
* radiofrequency coils - produce RF energy
* RF reciever - picks up emitted energy
* computer - processes to create image
patient table
liquid helium cooling device
cold insulation - to protect surrounding from supercool magnet
faraday cage - whole room kept in faraday cage
when are magnets used in practice?
MRI
deactivating ICD
paramagnetic O2 analyser
