MRI

Question 1

how does an MRI work?

Answer 1

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

Question 2

what is the strength of the magnet produced by MRI?

Answer 2

1- 3 Tesla
static magnet

Question 3

how much noise does MRI produce ?

Answer 3

100 Db

Question 4

what is the larmor frequency?

Answer 4

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

Question 5

what is the difference between T1 and T2 image?

Answer 5

T1 = water is black, fat white.
T2 = water is white and fat white

Question 6

what does T1 relate to in terms of relaxation states?

Answer 6

T1 = spin lattice relaxation = time constant longitudinal relaxation. This is the time taken for the net magnetism to realign with the main magnetic field.

Question 7

what does T2 relate to in terms of relaxation?

Answer 7

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)

Question 8

what contrast medium can be used with MRI?

Answer 8

Gadolinium
used with T1 weighted MRI

alters relaxation of H making the image more visible.

Question 9

when in contrast MRI’s used?

Answer 9

angiography
GIT
brain tumours that have crossed the BBB

Question 10

what are the benefits of an MRI?

Answer 10

non ionising
better for soft tissues
better for making 3D images.

Question 11

how is the high strength magnet created?

Answer 11

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.

Question 12

what is quench?

Answer 12

liquid helium heated rapidly
vented from scanner
no liquid helium to create a superconducting magnet
no magnetic field

Question 13

what are the disadvantages/ safety concerns for an MRI ?

Answer 13

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.

Question 14

what are the indications for a GA MRI?

Answer 14

Unstable patient – intubated and ventilated

Uncooperative patient – child, learning difficulties , Claustrophobia

Intra op MRI – do surgery, check with MRI, do more surgery if needed

Question 15

how do CTs work?

Answer 15

CTs take a series of X-ray images around a central axis

Question 16

What equipment is unsafe in MRI

Answer 16

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.

Question 17

How is safety of MRI improved?

Answer 17

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.

Question 18

what is the difference between T1 and T2 MRI

Answer 18

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.

Question 19

what are the problems with quenching the MRI

Answer 19

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

Question 20

what do these 3 symbols mean?

Answer 20

top = MRI safe - no known hazards
middle - MRI conditional - Hazard in specific MRI environment e.g. at certain field strength
bottom - MRI unsafe

Question 21

which part of LMA can affect an MRI image?

Answer 21

pilot balloon - small magnetic spring, can cause interference

Question 22

what is meant by diamagnetism?

Answer 22

weakly magnetic materials
no unpaired electrons in outershell
or imbalance of P + N in nucleus

e.g. copper

Question 23

how does the MRI magnetic field compare to earths?

Answer 23

MRI = 1-3Telsa
Earth 0.0005T

Question 24

what are the two units for magnetic strength?

Answer 24

tesla
Guass - old

Question 25

why are H atoms used in MRI?

Answer 25

due to their abundance in soft tissue - good for soft tissue imaging.

other atoms are also paramagnetic e.g. phosphorus in ATP

Question 26

why does MRI not pick up signals from other atoms?

Answer 26

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.

Question 27

Hazards of anaesthesia in MRI

Answer 27

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

Question 28

do you know any critical incidence related to MRI?

Answer 28

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

Question 29

what are the components of an MRI machine..

Answer 29

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

Question 30

when are magnets used in practice?

Answer 30

MRI
deactivating ICD
paramagnetic O2 analyser