Magnetic Resonance Imaging

Front 1

NMR

Back 1

Nuclear Magnetic Resonance which was later changed to a nicer sounding MRI

Front 2

Bp (MRI)

Back 2

the small magnetic field produced from the interaction between the proton's charge and it's spin

Front 3

Why must MRI induce a magnetic field?

Back 3

The protons in the atom will either allign paralle or anti-parallel with the induced field

Front 4

Distinguish between spin up and spin down for protons in an applied magnetic field.

Back 4

Spin up indicates low energy protons (parallel with the applied magnetic field) whereas spin down indicates high energy (induced magnetic field in an anti-parallel direction)

Front 5

Bz (MRI)

Back 5

the large external magnetic field applied in MRI

Front 6

Why does MRI work only for elements with odd numbers of protons?

Back 6

For elements with an even number of protons, the spin up and spin down cancel out creating no resulting magnetic field that can be detected.

Front 7

Resonant Absorption

Back 7

The precise frequency and energy of a photon required to excite a spin up proton to a spin down position.

Front 8

Resonant Emission

Back 8

The precise frequency and energy of a photon released when an excited spin down proton returns to a spin up position

Front 9

Consider proton precession in regards to its spin.

Back 9

Proton spin is not exactly parallel (or anti-parallel) to the applied magnetic field (Bz) but at a slight constant angle depending on quantum mechanics

Front 10

Larmour Frequency (FL)

Back 10

the number of times per second that BN precesses around Bz calculated using the Larmor Equation

Front 11

BN (MRI)

Back 11

the angled magnetic field of a proton which rotates about the applied magnetic field Bz

Front 12

What does gamma stand for in the Larmor equation?

Back 12

Gamma is the gyromagnetic ratio which varies with material. For protons, gamma = 42.5 MHz/T

Front 13

What kind of photon is applied in MRI to flip proton spins

Back 13

Radio waves (RF)

Front 14

What sort of patter is seen in resonant emission as protons return to spin-up positions?

Back 14

The protons don't all return at the same time but at slower and slower rates - an exponential decay

Front 15

BL (MRI)

Back 15

Longitudinal Magnetic Field - the resultant magnetic field of the individual protons in precession (BN) which is exactly parallel or anti-parallel to the applied magnetic field Bz
the left and right components cancel out

Front 16

BT (MRI)

Back 16

the transverse magnetic field of protons in precesseion; When the protons are in random phase, BT=0
BT can be observed only when the proton precessions are in phase (due to radio frequency)

Front 17

How can you eliminate BL?

Back 17

If the right number of radio photons (pulse) is selected, the number of of spin up and spin down protons can cancel each other out giving a BL of 0.

Front 18

Why is BL so difficult to measure? What is done to overcome this?

Back 18

BL is very small and also parallel to BZ. the radio waves put the proton precession in phase which creates BT (transverse field)

Front 19

How is BT (transverse field) measured?

Back 19

By placing metal coils near the tissue, BT will induce a detectable RF electrical current in the coils

Front 20

State the effect of radio pulses on a nuclear magnetic field.

Back 20

Reduction of BL by changing spin of protons.
Productino of BT by putting the protons in phase.

Front 21

Relate between the Larmor frequency and BT.

Back 21

BT precesses about the z-axis (BZ) at the Larmor frequency.

Front 22

What happens when the radio waves are switched off? [2]

Back 22

BL recovers as excited protons return to spin up and BT decays as precession falls out of phase. The two fields are independent of each other.

Front 23

Spin-Lattice Relaxation

Back 23

The recovery of BL characterised by the time constant T1

Front 24

T1 (MRI)

Back 24

the time required for BL to recover to 63% of its pre-pulse value. Applies to tissues rather than liquids. [varies from tissue to tissue from 0.3 to 2.0 seconds]

Front 25

T0 (MRI)

Back 25

the time which corresponds to when BL = 0; equal number of parallel and anti-parallel fields or
the time which corresponds to when BT is maximised (same phase) and the radio pulse is turned off

Front 26

Why do protons go out of phase after radio waves are removed?

Back 26

Extrinsic effects including extrenal magnetic fields.
Intrinsic effects including the magnetic fields of surrounding atoms

Front 27

At what rate does BT decay when the RF pulse is switched off?

Back 27

BT decays exponentially with the same half-life as the induced RF current decay.

Front 28

Spin-Spin Relaxation

Back 28

name for the decay of BT which is characterised by T2

Front 29

T2 (MRI)

Back 29

the time required for BT to decay to 37% of it's maximum value; associated with liquids