Lecture 1: MRI suite zones, controlled access, contraindications, labelling terminology, patient care, screening questionnaires Flashcards
what are 2 reasons that MRI is considered safe
no harmful ionising radiation
no clearly demonstrated biological effects
what is the most common injury from MRI and which 2 injuries closely follows it in commonality
most common = burn
followed by projectiles and hearing damage
what are the 4 components of MRI hardware
magnet
radiofrequency coils
gradient coils
cryostat
what does the magnet do in terms of MRI hardware
it aligns protons in the body
what is the magnet in MRI hardware
large magnetic field that is homogenous over a large area
what does the radiofrequency coil do in terms of MRI hardware
transmit and receive RF energy into and from the body
what does the gradient coils do in terms of MRI hardware
induce linear change in magnetic field to enable spatial coding
what is the cryostat in terms of MRI hardware
large chamber of liquid helium in which the magnet coils are immersed
what do the H protons in the body do when there is no magnetic field vs when there is a mag field
H protons in body are spinning randomly but in MRI H protons align with magnetic field
what do the H protons do when RF energy is transmitted
RF energy transmitted, transmitted pulse in so protons moves away from alignment with magnet, away from parallel
what happens to the H protons when the RF pulse is turned on and off
how often does this happen in an examination
Turn pulse off and protons lose energy to realign with magnetic field
Happens very quickly and multiple times in an examination
what component of the MRI hardware is closest to the patient
RF coil
what does the RF coil transmit and receive
transmits RF pulse
receive signal when protons are realigning to mag field and release energy which is detected by RF receiver coils
where are the gradient coils located - what are they between
between the main magnetic coil and the RF coil
what does the gradient coil do
Gradient coil applies 3 very small mag field across patient in orthogonal planes so enables the localisation/coding of signals to determine where signal is coming from in the patient
what happens if you dont have a gradient coil
Without Gradient coils the signals would be coming back and wouldn’t know what signal was coming from where
what are the 3 electromagnetic fields in a MRI machine
main static magnetic field
RF field
gradient field
what is the gradient field in terms of time and why is that
Gradient field is time varying as they turn on and off v quickly in the scan
what temperature does the cryostat keep the MRI magnet to
-273 *C
what is the cryostat in terms of conductivity
a superconductor
what is the main magnet/magnetic field in a MRI machine
static magnetic field
what does the static magnetic field do
aligns Hydrogen protons to produce net magnetization
what does the radiofrequency field do
excites protons by transmitting radiofrequency pulses
RF coils also used to receive signals
what are the 4 safety considerations/effects that the static magnetic field can have on forces and patients
translational forces - projectiles
rotational forces - implants
medical device disruption
bio-effects
what are the 2 forces that result from the static mag field
translational and rotational forces
what does the translational force generated by the static mag field affect
projectiles
what does the rotation force generated by the static mag field affect
implants
what is the difference between passive and active implants
Passive implants hold things in place/together but active implants are things like neurostimulator/pacemaker
what are bioeffects generated by static mag field and what do they depend on
depend on scanner strength and include headaches/nausea/blinking lights etc
are bioeffects generated by static mag field serious
normally transient and not long term
what are the 2 safety considerations/effects that the radiofrequency field can have on patients and devices
tissue heating and burns
medical device heating
what does the gradient magnetic field do
localise the MR signals
what does cryogens do
maintains superconducting magnetic field
what are the 2 safety considerations/effects that the gradient magnetic field can have on patients
acoustic noise
peripheral nerve stimulation
what are the 2 safety considerations/effects that cryogens can have on patients
quench can lead to asphyxiation or bodily harm
what are the 2 safety considerations of GBCA
nephrogenic systemic fibrosis
Gd brain deposition
what are the 5 components of the MRI machine that should be safety considerations
RF field
static magnetic field
gradient magnetic field
cryogens
GBCAs
why can gradient mag field cause loud noises and peripheral nerve stimulation
Switches quickly generates loud noises and can cause peripheral nerve stimulation not dangerous but uncomfortable on certain pulses sequences that are very fast
what magnetic field is responsible for the most significant injuries and fatalities
static magnetic field
how strong is the magnetic field in the MRi
2 strengths
1.5T or 3T
what is 1 tesla equivalent to in terms of gauss
10,000 Gauss
when is the magnet on in MRI
the magnet is always on
is the magnetic field confined to within the scanner
no
what is the fringe field
the magnetic field extending beyond the physical covers of the scanner
where does the fringe magnetic field get stronger
as you approach the magnet the fringe magnetic field quickly gets stronger
what does the 5 Gauss line indicate
the point in the fringe field beyond which it is considered unsafe for an unscreened person to pass
where is the magnetic field strongest in the MRI
strongest at the centre in the bore
is the 5Gauss line limited to the floor of the room
no it can extend outside the scanner room such as the control area and as its 3D it can also extend up into the roof
what are the 3 methods of site access restriction
4 zone concept
levels of MR personnel
screening procedures
what is the controlled access 4 zone prinicples
as you progress towards the magnet room you undergo increasing levels of screening and observation to prevent incidents that could threaten the safety of the patients and staff
what is considered zone 1
region includes all areas that are freely accessible to the general public
typically the first access to the MR dept
what is considered zone 2
interface between the publicly accessible uncontrolled zone 1 and the strictly controlled zone 3
patients are greeted in zone 2 and are not free to move throughout the region at will - usually patient screening occurs in this zone
what is considered zone 3
free access by unscreened non MRI personnel or ferromagnetic objects could result in serious injury or death
includes all areas with fringe field >5Gauss line
what is access like to zone 3
strictly physically restricted and all access must be controlled by and entirely under the supervision of MRI personnel
what is considered zone 4
the MR scanner room
how can zone 4 be accessed
only by zone 3
what are the requirements for non MR personnel access in zone 4 and 3
non MR personnel must be accompanied by or under the immediate supervision of a specifically identified level 2 MR person for the entirety of their duration within zone 3 or 4
what are the 2 levels of MR personnel
level 1 and 2
what are level 1 MR personnel
those who have passed minimal safety educational efforts to ensure their own safety as they work within zone 3
what are some examples of level 1 MR personnel
MRI dept office staff or patient aides
what are level 2 MR personnel
individuals who have been more extensively trained and educated in the broader aspects of MR safety issues
what are some examples of level 2 MR personnel
MRI technologist, radiologist, radiology dept nurses
what are the 3 classifications of implant and device labelling
MR safe
MR conditional
MR unsafe
what is a MR safe equipment label
item that poses no known hazards in all MRI environments
what is a MR conditional equipment label
item that has been demonstrated to pose no known hazards in a specified MRI environment with specific conditions of use
what is a MR unsafe equipment label
an item that is known to pose hazards in all MRI environments
what are some examples of MR safe equipment/materials
non conducting, non metallic, non magnetic objects
plastic petri dish
what are some examples of MR unsafe equipment/materials
pair of ferromagnetic scissors
what are 5 access control methods
warning signs
barriers
swipe access
appropriate supervision in restricted zones (zones 3 and 4)
ferromagnetic detectors
who has to complete the MRI screening questionnaire
everyone entering the MRI scanner room
how often do patients or their family have to complete the MRI screening questionnaire
every time
how often do staff have to complete the MRI screening questionnaire
first time and then verbal follow ups each time after
what are the 3 stages where safety screening should be administered for patients
on acceptance of booking - booking clerk
on arrival of the patient - written questionnaire checked by MRI personnel
immediately prior to entering zone 4 - verbal questioning by MRI personnel
why do we need know the patients height and weight in the MRI screening process
so scanner can accurately calculate limits for RF deposition
specific absorption rate measured watts per kilo and height and weight are entered to calculate how much RF is allowed to be put into that patient safely
what is patient preparation for MRI in terms of what it involves
change into plain cotton gown or scrubs - remove underwear if there is metal in it
what are 4 other considerations apart from getting the patient changed into a gown for patient preparation
patient mobility
anxiety/claustrophobia
looks for hairclips/watches/piercings
cultural considerations such as turbans, kirpans, hijab and burkas etc
how thick do patient positioning pads need to be
> 1cm thick
what are the 3 functions of patient positioning pads
burn prevention - RF field
immobilization
patient comfort
what are 3 things/equipment that should be considered when positioning the patient
leads/cables
earplugs and headphones for the acoustic noise
call bell
