MRI fundamentals

Question 1

Advantages of MRI

Answer 1

 High contrast sensitivity
 Use of non-ionising radiation
 Multiplanar capability
 Multiple contrast mechanisms

Question 2

Disadvantages of MRI

Answer 2

High equipment and siting costs; Relative long time
Significant artifacts
Incompatible with certain biomedical implants/ pacemakers
Potential patient claustrophobia

Question 3

A material is considered MR active as

Answer 3

as long as its mass number is odd, as it has a net charge

Question 4

Nuclei with a net magnetic moment align

Answer 4

align their axis of rotation to the applied
magnetic field (B0).

Question 5

MRI has 2 magnetic fields

Answer 5

Bo and B1

Question 6

What is Bo?

Answer 6

External main magnetic field - always activated

Question 7

What is B1?

Answer 7

Radio-frequency field that is turned on and off

Question 8

The frequency of the RF pulses has to match

Answer 8

match (resonate) with a particular group of
protons in the body.

Question 9

What happens when the RF signal is switched off?

Answer 9

The protons
move back into alignment with the main
magnetic field, and they emit an RF signal, which is used to construct the MR image.

Question 10

In the strong magnetic field the hydrogen nuclei can

Answer 10

align with (parallel) or against (antiparallel) to the magnetic field

Question 11

Hydrogen nuclei possess

Answer 11

2 energy states

Question 12

Low energy nuclei align parallel to ext field =

Answer 12

spin up nuclei

Question 13

Spin up

Answer 13

low energy align to parallel to ext field

Question 14

Spin down

Answer 14

High energy align their magnetic moments in the antiparallel direction

Question 15

The protons continually

Answer 15

oscillate between the two states

Question 16

At any given movement, there will be a slight

Answer 16

majority aligned with parallel than against the field (antiparallel)

Question 17

The larger the Bo field,

Answer 17

the greater difference the enrgy levels and larer excess number aligned with field - increase in SNR

Question 18

Net magnetisation vector

Answer 18

is the sym of all the protons pointing in direction of magnetic field

Question 19

Increase B =

Answer 19

increase in NMV

Question 20

Thermal energy is determined by

Answer 20

temoerature of patient
decrease temp = increase in NMV

Question 21

What are the 2 ways to increase NMV

Answer 21

decrease in temp (not sig)& increase in B

Question 22

Precession

Answer 22

is the additional/secondary spin from B0, causes circular path around B

Question 23

Precessional freq

Answer 23

is the freq at which NMC wobbles around B0

Question 24

Larmor Equation

Answer 24

w = y x B0

Question 25

For hydrogen, y =

Answer 25

42.58 MHz/T

Question 26

Gyro magnetic ratio is

Answer 26

constant for each nuclei

Question 27

Resonance is the

Answer 27

absorption or emission of energy - only occurs at specific frequencies that match RF pulse

Question 28

Excitation is the

Answer 28

application of RF pulse that causes reonsnace to occur - causes protons to move out of alignment

Question 29

When the Rf is turned on,

Answer 29

1 = RF E is absorbed at res freq by protons 2 = Excited protons move from long to transverse plae - net magnetisation from B0 3 = excited protons spin in phase

Question 30

What is the flip angle

Answer 30

the angle that NMV moves from long to trans

Question 31

Flip angle magnitude is

Answer 31

dependent on amplitude and duration of RF pulse

Question 32

NMV rotates into

Answer 32

the transverse plane at Larmor freq at 90 degrees

Question 33

Once the RF is off

Answer 33

the absorbed RF is retransmitted; the excited spin return to original orientation with B0; in phase protons dephase

Question 34

The signal at the highest level when

Answer 34

NMV is in the transverse plane and when there is phase coherance

Question 35

T1 recovery is

Answer 35

when the amount of magnetisation in longitudinal plane increases

Question 36

T2 decay is the

Answer 36

amount of magnetisation in the transverse plane decreases

Question 37

T1 recovery is related to NMV

Answer 37

moving from transverse back to longitudinal

Question 38

T2 decay is related to the

Answer 38

dephasing of the protons

Question 39

T1 is the the time it takes

Answer 39

for 63% of longitudinal magnetisation to recover

Question 40

T1 is short in

Answer 40

fats

Question 41

T2 is the time it

Answer 41

takes for 63% of transverse magnetisation to be lost

Question 42

In magnetic inhomogenities,

Answer 42

the loss of phase coherance occurs mre than from T2; transverse relat T2 is shortened to T2

Question 43

T2* =

Answer 43

T2 decay and dephasing due to ext mag field inhomogenities

Question 44

The signal is at the highest level

Answer 44

when the NMV is in the transverse plane and when there is phase coherance

Question 45

Phase coherance happens

Answer 45

in transverse plane