introduction to MRI

Question 1

basic hardware of MRI machine

Answer 1

• primary magnet
• gradient coils
• radiofrequency RF coils

Question 2

what are the 3 magnet types

Answer 2

• permanent
• resistive
• superconductive

Question 3

list features of a permanent magnet

Answer 3

• limited field strength
• 20-30 tonnes
• iron, cobalt, nickel used
• low power consumption
• they generate a magnetic field without an external power source

Question 4

list features of a resistive magnet

Answer 4

• require stable supply to create B0
• water cooling required
• field strength = 0.3T
• uses electric current flowing through coil of wire to generate magnetic field

Question 5

what does current produce in resistive magnet

Answer 5

• produces heat through resistive turns

Question 6

can strength of resistive magnet be managed

Answer 6

yes, can be adjusted by changing the current

Question 7

list features of superconductive magnets

Answer 7

• use a coil that when cooled to low temperatures, loses all electrical resistance
• allows persistent strong magnetic field to be maintained with minimal energy loss
• can generate strong and stable magnetic fields = ideal for high field applications
• always ON

Question 8

materials of superconducting magnet

Answer 8

niobium/ titanium alloy

Question 9

what are gradient coils used for

Answer 9

used to spatially encode MR signals produced by photons in the body, allowing creation of 2D and 3D images

Question 10

gradient coils role

Answer 10

• used to introduce small, controlled magnetic field gradients in addition to main magnetic field
• provide slops within field, 3 orthogonal X Y Z
• noisy

Question 11

what are radiofrequency RF coils

Answer 11

responsible for sending RF pulses into patient’s body during MRI scans
- these pulses are used to excite protons in body causing them to emit MR signals

Question 12

what is passive shielding

Answer 12

involves the use of materials and physical barriers to block/ absorb electromagnetic interference without requiring external power source

Question 13

what is active shielding

Answer 13

employs active electronic components such as amplifiers and feedback control systems to actively counteract and cancel out unwanted electromagnetic interference

Question 14

what are faraday cages designed to do

Answer 14

designed to block electromagnetic fields
- conducted from electrically conductive materials such as copper, aluminium/ steel

Question 15

basic principles of MRI

Answer 15

• hydrogen nuclei protons are MR active
• behave like random mini bar magnets (N + S pole)
• protons aligned in same direction as magnetic field
• creates net magnetisation in direction of magnetic field

Question 16

what is spin?

Answer 16

refers to the intrinsic property of certain atomic nuclei, particularly hydrogen nuclei, that makes them suitable for detection in MRI

Question 17

what is the principle element that detects signals

Answer 17

protons
- tissues with more water will give higher signal

Question 18

two possible orientations of spin

Answer 18

• parallel
• anti-parallel

Question 19

what is the energy difference between parallel and anti-parallel

Answer 19

directly proportional to the strength of B0 (magnetic susceptibility)

Question 20

how can an MRI scanner be made - superconducting?

Answer 20

• selection of superconducting material
• formation of superconducting wire coils
• cooling system
• integration into magnet structure

Question 21

3 advantages of the superconducting MRI scanner

Answer 21

• higher magnetic field strength
• stability and persistent current mode
• energy efficiency

Question 22

3 disadvantages of superconducting MRI scanners

Answer 22

• high initial cost
• high maintenance
• long ramp-up time to reach optional magnetic field strength

Question 23

what are RF pulses used to do

Answer 23

manipulate the magnetic properties of (hydrogen) nuclei within body

Question 24

what is B0

Answer 24

the main magnetic field

Question 25

what happens before RF pulse applied

Answer 25

protons within the body are randomly orientated with respect to main magnetic field (B0) - this is the resting state

Question 26

RF pulse application

Answer 26

- RF pulse is short burst of radiofrequency energy applied perpendicular to B0 - frequency of RF pulse matches resonant frequency of protons

Question 27

what is resonance and tipping

Answer 27

when RF pulse matches frequency of protons it causes them to absorb energy and enter higher energy state (known as resonance) - RF pulse tips magnetic moments of protons away from direction of main magnetic field

Question 28

flip angle

Answer 28

the angle at which the magnetic field is tipped is called the flip angle - the flip angle is a critical parameter that determines amount of magnetization tipped into transverse plane

Question 29

what happens when the RF pulse is turned off

Answer 29

the tipped magnetic moments begin to rotate back towards B0 - while they are precessing they emit radiofrequency signals known as free induction decay (FID)

Question 30

what does free induction decay contain

Answer 30

these signals contain information about local tissue characteristics

Question 31

signal detection

Answer 31

these emitted signals are detected by the coils in MRI system and are used to create detailed images of internal structure

Question 32

what are relaxation times

Answer 32

it describes how long the magnetisation takes to get back to equilibrium after an RF pulse

Question 33

what are the two types of relaxations

Answer 33

- T1 recovery - T2 relation

Question 34

what is T1 recovery

Answer 34

- refers to recovery process in MRI that is associated in T1W images - process where energy absorbed by the excited protons/ spins are released back into surrounding lattice re-establishing thermal equilibrium

Question 35

what type of magnetisation does T1 recovery refer to

Answer 35

- longitudinal magnetisation is returned to its equilibrium

Question 36

what is T2 relaxation also known as

Answer 36

spin-spin relaxation

Question 37

what is T2 relaxation

Answer 37

- whenever spin comes close to each other, an interaction takes place which affects their individual magnetic moment and relative phase angle (phase scrambling) - with time phase scrambling increases until it reaches a point where spins run out of phase thus reaching ZERO magnetisation

Question 38

what does 'phase' refer to

Answer 38

represents position/ location of signal in space

Question 39

what does 'magnitude' refer to

Answer 39

represents the amplitude/ strength of signal

Question 40

what is T2 star

Answer 40

when T2 relaxation time is influenced by both magnetic field inhomogeneities and other effects (like susceptibility effects) the resulting decay is referred to as T2 star

Question 41

what is T2 star sensitive to

Answer 41

- certain substances such as deoxy blood, iron, air - substances that can cause magnetic field variations