Introduction To MRI

Question 1

What is a magnetic field?

Answer 1

A region of space where magnetic forced are experienced

Question 2

What is the strength of a magnetic field field measured in?

Answer 2

Teslas

Question 3

How many Teslas are most scanners?

Answer 3

1.5 Teslas, but scanners can be different strengths

Question 4

What is the direction of a magnetic field represented as?

Answer 4

The letter B

Question 5

What does the MRI scanner work on?

Answer 5

The hydrogen atoms in the body

Question 6

What is a magnetic moment?

Answer 6

A quantity that represents the direction and strength of a magnetic field

Question 7

The stronger the magnetic field strength, the … the magnetic moment

Answer 7

Stronger

Question 8

Why is the direction of magnetic moments important?

Answer 8

It shows us how the magnetic moment will align with the magnetic field

Question 9

What are the features of an MRI magnet?
(4)

Answer 9

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)

Question 10

Why does the MRI magnet use an electromagnet?

Answer 10

So that it can produce a really high magnetic field strength

Question 11

What are the features of the superconducting magnet?
(4)

Answer 11

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

Question 12

What is the sound that’s heard in the MRI room?

Answer 12

It’s the sound of the pump pumping the liquid helium into the MRI scanner

Question 13

What do radiofrequency transmitter coils do?

Answer 13

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)

Question 14

Where are the radiofrequency coils located?

Answer 14

Within the main magnet

Question 15

Label the components of an MRI scanner:

Answer 15

Question 16

What do surface coils do?

Answer 16

They receive the MRI signal. They’re designed specifically for certain areas

Question 17

Where do surface coils need to be placed?
(2)

Answer 17

Over the area to be imaged

As close to the patient as possible

Question 18

Label the control room of MRI:

Answer 18

Question 19

Why is a button needed in MRI?

Answer 19

It quenches the magnet. If there’s an emergency, the magnetic field can be reduced by pressing the button

Question 20

Why is a phone needed in MRI?

Answer 20

In case a patient crashes

Question 21

Why is a microphone needed in MRI?

Answer 21

So the radiographer can talk to the patient if need be, since they’re in the CT scanner for so long

Question 22

Why is a screen needed in MRI?

Answer 22

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

Question 23

What needs to have happened in order for a person to be allowed to enter the MR environment?

Answer 23

The person needs to have a safety questionnaire to ensure that they’re safe to be in there

Question 24

What are the 2 types of areas in MRI?

Answer 24

Controlled access area

MRI environment

Question 25

What rooms include the controlled access area? (3)

Answer 25

Control room Scanning room MRI equipment room (Access to these areas are restricted)

Question 26

What rooms include the MRI environment?

Answer 26

Any area within the 5 gauss line (Anyone entering needs to have been authorised and undertaken safety checks)

Question 27

What is an atom made up of? (3)

Answer 27

Protons Electrons Neutrons (PEN)

Question 28

What charge do protons have?

Answer 28

Positive charge

Question 29

What charge do neutrons have?

Answer 29

No charge

Question 30

What charge do electrons have?

Answer 30

Negative charge

Question 31

How does an atom look like?

Answer 31

Question 32

How is motion like in the atom? (3)

Answer 32

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

Question 33

What is said to be MRI active Nuclei?

Answer 33

Atoms with an odd mass number

Question 34

What is the mass number of a hydrogen atom?

Answer 34

1

Question 35

What do hydrogen atoms contain?

Answer 35

A positively charged proton

Question 36

Hydrogen atoms give a … magnetic moment

Answer 36

Large

Question 37

Why are hydrogen atoms used in medical imaging?

Answer 37

Because it’s abundant in the human body

Question 38

How does a hydrogen atom look like?

Answer 38

Question 39

What happens to the magnetic moments if we don’t have a magnetic field?

Answer 39

The magnetic moments are randomly oriented

Question 40

What happens to the magnetic moments when they’re placed in a strong magnetic field?

Answer 40

The magnetic moments align (line up) with the magnetic field

Question 41

How can the magnetic moments align with the magnetic field? (2)

Answer 41

Parallel (low energy nuclei) Anti-parallel (high energy nuclei)

Question 42

What do parallel nuclei create?

Answer 42

A net magnetic vector (NMV)

Question 43

How does each hydrogen nucleus spin?

Answer 43

It spins on its axis (like a spinning top)

Question 44

What is precession?

Answer 44

The spinning of the hydrogen atom’s nucleus

Question 45

What is the precession frequency?

Answer 45

The speed that the hydrogen atom’s nucleus spins

Question 46

When does resonance occur?

Answer 46

When an object is exposed to an external force that has a frequency close to its own

Question 47

What happens during resonance? (2)

Answer 47

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)

Question 48

What is excitation?

Answer 48

When the radiofrequency pulse that causes resonance to occur is applied

Question 49

What causes resonance to occur?

Answer 49

When a radiofrequency pulse is applied

Question 50

What does Bo mean?

Answer 50

The magnetic field strength

Question 51

How does the application of a radiofrequency pulse cause resonance to occur?

Answer 51

It causes the NMV to move out of alignment with Bo

Question 52

What is the flip angle?

Answer 52

The angle between Bo and the NMV

Question 53

What does resonance cause? (2)

Answer 53

The NMV to move out of alignment with Bo The magnetic moments of the hydrogen nuclei to move in phase with each other

Question 54

Originally all the hydrogen atoms line up what happens when the radiofrequency pulse is added?

Answer 54

They move away from each other

Question 55

When is the MRI signal produced?

Answer 55

When the in phase magnetism occurs across the receiver coil

Question 56

What happens when the MRI signal is switched off? (2)

Answer 56

The high energy nuclei lose energy This results in the NMV realigning with Bo (relaxation)

Question 57

What is relaxation?

Answer 57

When the NMV realigns with Bo

Question 58

What are the 3 main hazards of MRI?

Answer 58

Static magnetic field (Bo) Time varying magnetic field gradients Radiofrequency magnetic fields

Question 59

What static magnetic field do scanners used for clinical use have? (2)

Answer 59

1.5T or 3T

Question 60

What are the magnetic field strength hazards? (3)

Answer 60

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

Question 61

What are the features of the projectile effects? (3)

Answer 61

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

Question 62

What is the missile effect?

Answer 62

When the speed of the attraction between the ferromagnetic material and the center of the scanner is very high- approaching 45mph

Question 63

What is a quench? (2)

Answer 63

If an incident occurs, the magnetic field strength can be demagnetised. The liquid helium evaporates

Question 64

How can we cause a quench?

Answer 64

Press the quench button

Question 65

What are the consequences of pressing the quench button and causing a quench? (2)

Answer 65

It usually results in irretrievable damage to the MRI machine The cost of refilling the helium is £8000

Question 66

When should the quench button only be pressed?

Answer 66

In exceptional circumstances- if there’s risk to life or limb

Question 67

What happens when a spontaneous quench happens?

Answer 67

The helium gas should escape through the quench pipe. If this fails, it escapes into the room

Question 68

What happens if the helium gas escapes into the room after a quench? (4)

Answer 68

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