Introduction To MRI Flashcards
What is a magnetic field?
A region of space where magnetic forced are experienced
What is the strength of a magnetic field field measured in?
Teslas
How many Teslas are most scanners?
1.5 Teslas, but scanners can be different strengths
What is the direction of a magnetic field represented as?
The letter B
What does the MRI scanner work on?
The hydrogen atoms in the body
What is a magnetic moment?
A quantity that represents the direction and strength of a magnetic field
The stronger the magnetic field strength, the … the magnetic moment
Stronger
Why is the direction of magnetic moments important?
It shows us how the magnetic moment will align with the magnetic field
What are the features of an MRI magnet?
(4)
It requires a very strong magnetic field
This is achieved using a superconducting electromagnet (the scanner is a big electromagnet)
The electromagnet is configured using wire in the shape of a coil, and then current is passed through the wire, which induces a really high magnetic field
It’s deeper than a CT scanner and has a more narrow bore (hole)
Why does the MRI magnet use an electromagnet?
So that it can produce a really high magnetic field strength
What are the features of the superconducting magnet?
(4)
High resistance increases the temperature of the coil
So it’s cooled to a low temperature in order to operate
The MRI scanner is cooled with liquid helium via a pump
The magnet is always on
What is the sound that’s heard in the MRI room?
It’s the sound of the pump pumping the liquid helium into the MRI scanner
What do radiofrequency transmitter coils do?
They produce a radio frequency pulse that’s administered to the patient in order to produce an image (this causes the loud noise that patients can hear whilst in the MRI scanner)
Where are the radiofrequency coils located?
Within the main magnet
Label the components of an MRI scanner:
What do surface coils do?
They receive the MRI signal. They’re designed specifically for certain areas
Where do surface coils need to be placed?
(2)
Over the area to be imaged
As close to the patient as possible
Label the control room of MRI:
Why is a button needed in MRI?
It quenches the magnet. If there’s an emergency, the magnetic field can be reduced by pressing the button
Why is a phone needed in MRI?
In case a patient crashes
Why is a microphone needed in MRI?
So the radiographer can talk to the patient if need be, since they’re in the CT scanner for so long
Why is a screen needed in MRI?
It’s connected to a camera on the boar of the scanner so that we can watch the patient, as they’re in there for a while
What needs to have happened in order for a person to be allowed to enter the MR environment?
The person needs to have a safety questionnaire to ensure that they’re safe to be in there
What are the 2 types of areas in MRI?
Controlled access area
MRI environment
What rooms include the controlled access area?
(3)
Control room
Scanning room
MRI equipment room
(Access to these areas are restricted)
What rooms include the MRI environment?
Any area within the 5 gauss line
(Anyone entering needs to have been authorised and undertaken safety checks)
What is an atom made up of?
(3)
Protons
Electrons
Neutrons
(PEN)
What charge do protons have?
Positive charge
What charge do neutrons have?
No charge
What charge do electrons have?
Negative charge
How does an atom look like?
How is motion like in the atom?
(3)
Within the nucleus of the atom, the protons and neutrons spin in opposite directions
In atoms with the same number of protons and neutrons, the nucleus itself has no net spin
However if there’s an unequal amount of protons and neutrons, the nucleus itself will have a net spin
What is said to be MRI active Nuclei?
Atoms with an odd mass number
What is the mass number of a hydrogen atom?
1
What do hydrogen atoms contain?
A positively charged proton
Hydrogen atoms give a … magnetic moment
Large
Why are hydrogen atoms used in medical imaging?
Because it’s abundant in the human body
How does a hydrogen atom look like?
What happens to the magnetic moments if we don’t have a magnetic field?
The magnetic moments are randomly oriented
What happens to the magnetic moments when they’re placed in a strong magnetic field?
The magnetic moments align (line up) with the magnetic field
How can the magnetic moments align with the magnetic field?
(2)
Parallel (low energy nuclei)
Anti-parallel (high energy nuclei)
What do parallel nuclei create?
A net magnetic vector (NMV)
How does each hydrogen nucleus spin?
It spins on its axis (like a spinning top)
What is precession?
The spinning of the hydrogen atom’s nucleus
What is the precession frequency?
The speed that the hydrogen atom’s nucleus spins
When does resonance occur?
When an object is exposed to an external force that has a frequency close to its own
What happens during resonance?
(2)
An object is exposed to an external force that has a frequency close to its own
The hydrogen atom’s nucleus gains energy and resonates (quivers)
What is excitation?
When the radiofrequency pulse that causes resonance to occur is applied
What causes resonance to occur?
When a radiofrequency pulse is applied
What does Bo mean?
The magnetic field strength
How does the application of a radiofrequency pulse cause resonance to occur?
It causes the NMV to move out of alignment with Bo
What is the flip angle?
The angle between Bo and the NMV
What does resonance cause?
(2)
The NMV to move out of alignment with Bo
The magnetic moments of the hydrogen nuclei to move in phase with each other
Originally all the hydrogen atoms line up what happens when the radiofrequency pulse is added?
They move away from each other
When is the MRI signal produced?
When the in phase magnetism occurs across the receiver coil
What happens when the MRI signal is switched off?
(2)
The high energy nuclei lose energy
This results in the NMV realigning with Bo (relaxation)
What is relaxation?
When the NMV realigns with Bo
What are the 3 main hazards of MRI?
Static magnetic field (Bo)
Time varying magnetic field gradients
Radiofrequency magnetic fields
What static magnetic field do scanners used for clinical use have?
(2)
1.5T or 3T
What are the magnetic field strength hazards?
(3)
Sensory effects- dizziness, nausea, metallic taste normally above 2T
Projectile effects- any loose object containing ferromagnetic material will be attracted to the center of the scanner
Torque- metal implants and foreign bodies will try to align with Bo
What are the features of the projectile effects?
(3)
Any loose object containing ferromagnetic material will be attracted to the center of the scanner
It can reach speeds approaching 45mph entering the bore of the magnet (missile effect)
The attractive force increases as the distance from the isocenter is decreased
What is the missile effect?
When the speed of the attraction between the ferromagnetic material and the center of the scanner is very high- approaching 45mph
What is a quench?
(2)
If an incident occurs, the magnetic field strength can be demagnetised.
The liquid helium evaporates
How can we cause a quench?
Press the quench button
What are the consequences of pressing the quench button and causing a quench?
(2)
It usually results in irretrievable damage to the MRI machine
The cost of refilling the helium is £8000
When should the quench button only be pressed?
In exceptional circumstances- if there’s risk to life or limb
What happens when a spontaneous quench happens?
The helium gas should escape through the quench pipe. If this fails, it escapes into the room
What happens if the helium gas escapes into the room after a quench?
(4)
The oxygen in the room is forced downwards by the liquid helium
This creates an oxygen deficient to atmosphere
The oxygen is therefore monitored, and an alarm will go off if it fails below 21%
The door should open outwards or there should be a hatch to help with the pressure
