Lecture 2: Basic physical principles, MR equipment and safety issues

Question 1

at body temp when in a magnetic field what is the behaviour of the H atoms in our bodies

what does this produce

Answer 1

align with main magnetic field

gives net magnitization

Question 2

if we add in radiation at special frequency when the person has been placed in the MRI machine what does it do to the net magnitization

Answer 2

tips the net mag to become perp to the main mag field

Question 3

what is the energy we put into the system

Answer 3

radiofreq waves

Question 4

what does the imaging gradient do

Answer 4

increase mag field slightly at one end and decrease the mag field at another end of the body

Question 5

what is special resonant freq proportional to

Answer 5

the main mag field

Question 6

what is the magnetic susceptibility

Answer 6

degree of magnetization of a material in response to an applied mag field

Question 7

what are differences in mag susceptibility a source of in MRI

Answer 7

contrast

Question 8

what are the 3 types of mag susceptibility

Answer 8

diamag
paramag
ferromag

Question 9

what is diamagnetism’s mag susceptibility and what does it do in the applied field

Answer 9

negative mag susceptibility

opposed to applied field

Question 10

what are examples of diamagnets

Answer 10

water, copper, tissue

Question 11

what is paramagnetism’s mag susceptibility and what does it do in the applied field

Answer 11

small positive mag susceptibility

small attractive force

Question 12

what is ferromagnetism’s mag susceptibility and what does it do in the applied field

Answer 12

high positive magnetic susceptibility

strong attractive forces

Question 13

what are examples of paramagnets

Answer 13

aluminium

gadolinium

Question 14

what are examples of ferromagnets

Answer 14

iron
nickle
cobalt

Question 15

what is the precessionary motion aligned to

Answer 15

the main mag field

Question 16

what freq do H atoms precess at

Answer 16

at the larmor freq

Question 17

what are H atom spins

Answer 17

moving positive charge gives rise to magnetic moment

Question 18

what happens when H protons with mag moments are outside and inside the mag field

Answer 18

outside = randomly distributed

inside = align with mag field, some align parallel and some align anti parallel (all precess in direction of mag field but point in opp directions)

Question 19

which way do lower energy state H atoms point in the main mag field

Answer 19

in direction of main mag field

Question 20

what produces the net energy mag

Answer 20

when the H atoms that point opposite directions cancel out and its the excess protons that produce the net mag

Question 21

Radiofreq at special freq has to match what energy

Answer 21

energy difference between the 2 states

Question 22

describe excitation/RF transmit

Answer 22

Put energy in resulting in the effect of net mag being tipped from longitudinal to trans plane

Question 23

describe induction/RF recieve

Answer 23

As it’s a changing mag field, if we put the same RF coil next to the sample that changing magnetization will induce a current or voltage across the coil also oscillating at the lamor freq

Question 24

what is T2* decay

Answer 24

When we tip net mag into transverse plane the individ spins that make up the net mag were all precessing at the same freq as we used the same freq to excite them

They can start to precess at diff freq so not all are in phase and making up net magnitization so effect of that is that when the Vectors added together they don’t make the same large net mag anymore and the net mag goes down and the sig in the coil goes down in the exponential way

Question 25

what does T2* time characterize

Answer 25

time taken for signal to decrease when the protons dephase

Question 26

what is T1 recovery what happens

Answer 26

longitudinal mag starts to recover even though the signal decays away

Question 27

what plane does the T2 decay in

Answer 27

transverse plane

Question 28

what happens to T1 recovery rate for different tissues

Answer 28

tissues with different T1 recover at a different rate

Question 29

what doe the superconducting magnet provide

Answer 29

very strong and uniform magnetic field B0

Question 30

what does the RF coils tuned to resonant freq provide

Answer 30

a way to transmit and detect the emitted RF waves

Question 31

what type of magnetism does a permanent magnet produce

Answer 31

intrinsic

Question 32

what is a tesla

Answer 32

unit of mag field strength - mag field density

Question 33

what is a gauss

Answer 33

unit of magnetic induction - mag field density

Question 34

what are the 2 positives of permanent magnets

Answer 34

easily shielded less claustrophobic

Question 35

what are resistive mag fields

Answer 35

reg wound copper coil carrying direct current gen mag field along axis

Question 36

what are the 2 types of main mag fields

Answer 36

permanent and resistive

Question 37

what is the drawback of resistive mag field

Answer 37

high operating costs due to large power to gen current 50% of power dissipated as heat

Question 38

what is the positives of using resistive mag field

Answer 38

easily turned on/off

Question 39

what is a superconducting magnet similar to and how is it different

Answer 39

similar to resistive magnets but uses liquid helium to cool coil windings

Question 40

what happens to the electrical resistance in superconducting magnets and why

Answer 40

decreases and becomes super conducting (zero electrical resistance) when temperature falls below critical value ~263 degrees celsius

Question 41

larger currents in windings of a superconducting magnet leads to what effect on the magnetic field

Answer 41

stronger mag field

Question 42

what are the 2 positives of having a superconducting magnet

Answer 42

large homogenous magnetic field little power input

Question 43

what are the 2 cons of having a superconducting magnet

Answer 43

costly due to cooling system - liquid He always on

Question 44

what is the image quality of a 1.5T look like compared to a 3T

Answer 44

1.5T is a bit grainier than 3T

Question 45

what is SNR used to measure

Answer 45

evaluates image quality

Question 46

what is SNR proportional to

Answer 46

field strength

Question 47

what do RF coils generate and what are they tuned to

Answer 47

generates and detects the RF pulses tuned to the selected resonant freq

Question 48

what are the 2 types of RF coils

Answer 48

body and surface coils

Question 49

where does the body coil sit and what does it do

Answer 49

The body coil sits just on inside of magnet to do transmission that makes it nice and uniform - long cylinder that puts equal amount of radiofreq into the bore

Question 50

where do we want the surface coil to be located

Answer 50

as close as possible to the object

Question 51

what is done to correct the small signal of surface coils

Answer 51

amplifiers are used

Question 52

do body RF coils transmit or receive

Answer 52

transmit

Question 53

do surface RF coils transmit or receive

Answer 53

receive

Question 54

what is resonance freq proportional to

Answer 54

main mag field

Question 55

what happens to the spin precessions at either end of the body when you apply the gradient

Answer 55

spins precess faster at the higher and slower at the lower end where there is lower mag field

Question 56

what are the 3 main safety concerns of the static field

Answer 56

cryogen equipment malfunction displacement effects

Question 57

when does quench occur

Answer 57

cryogen is boiled off when need to shutdown mag field or if there is a mag fault

Question 58

what is the safety measure to combat cryogen risks

Answer 58

quench pipe regularly checked

Question 59

what are the 2 things that are considered in equipment malfunction

Answer 59

electrical equipment or active implants

Question 60

what are the 3 safety measure to combat equipment malfunction

Answer 60

5Gauss line is marked controlled areas for access procedures in place for scanning conditional implants

Question 61

what are the 2 things that are considered in displacement effects of static fields

Answer 61

projectile effect and rotation effect

Question 62

what 2 things does projectile effect due to attractive force depend on

Answer 62

field strength and spatial mag gradient

Question 63

what does the rotation due to torque depend on

Answer 63

B^2

Question 64

what are the 2 safety measure to combat displacement effects of the static field

Answer 64

effective shielding controlled access of people and objects to area

Question 65

what are the 2 types of shielding for the fringe field

Answer 65

passive and active

Question 66

what is passive shielding

Answer 66

added ferromag material that channels the field lines

Question 67

what is active shielding

Answer 67

secondary gradient coils are used which oppose the field created outside the magnet bore

Question 68

how do RF waves cause burns and heating

Answer 68

energy of RF wave is absorbed and converted to heat

Question 69

what are some tissue factors that influence the degree of heating effects from RF waves

Answer 69

some tissues lack heating control mechanisms depends on patient size/weight/age/health depends on ambient conditions

Question 70

what are the 2 safety measure to combat heating effects of RF waves

Answer 70

limit exposure of the patient with RF using SAR - specific absorption rate (W/kg) keep room temp finely controlled

Question 71

what are the 3 safety measure to combat burn effects of RF waves

Answer 71

good patient positioning (prevent skin-skin contact) effective screening (remove conductive materials) make sure any required materials/leads are well insulated

Question 72

what are the 2 main safety concerns of the gradient field

Answer 72

nerve stimulation acoustic noise

Question 73

what causes the gradient field's nerve stimulation

Answer 73

changing mag field with time will induce current in a conductor in the body currents can be induced along nerves which can stimulate things

Question 74

what are the 4 things that can be stimulated by the nerves by the gradient field and order them in order of seriousness

Answer 74

retina < peripheral nerve < peripheral muscle < cardiac

Question 75

what 2 things does stimulation thresholds of the gradient field rely on

Answer 75

shape of gradient pulse patient conductivity/implants/size

Question 76

what is a safety measure for gradient field nerve stimulation

Answer 76

limits on dB/dt - how fast the switching can go and limits set to never exceed peripheral nerve stimulation