MRI Flashcards
how does an MRI work?
H atoms have spin and charge and act like mini magnets
placed in a magnet and align with the magnetic field
Radiofrequency magnetic filed energy is applied 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 radiation
Scanner picks up the energy released and compares different rates of relaxation/ excitation to determine type of body tissue.
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
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
