MRI physics and principles

Question 1

general overview on how MRI works

Answer 1

1. strong static magnetic field to align protons which create net magnetisation
2. radio frequency magnetic filed to tip the magnetisation when both have the same frequency
3. time-varying gradient magnetic filed to spatially localise the MR signals and reconstruct the image

Question 2

what percentage of the human body is made of water

Answer 2

60-75%

Question 3

what type of nuclei have a non-zero net spin

Answer 3

all nuclei with odd numbers of protons and or neutrons

Question 4

what characteristic of an electron or proton creates a magnetic field

Answer 4

spinning charged particlel

Question 5

Ines of magnetic fields ALWAYS form a closed loop

Question 6

what do you call the magnet formed from electron or proton spin

Answer 6

magnetic dipole moment

Question 7

explain the spin of a proton in and without B0

Answer 7

absence of b0= proton spins about its own axis

presence of b0= proton spins about its own axis AND precesses about axis of b0

Question 8

what is larmor frequency

Answer 8

spins rotate (precess) at fixed frequencies

Question 9

wroe 0 = 2 pi f = y B0
wroe 0 - larmor frequency
2 pi f - larmor frequency
y - gyromagnetic ratio
b0 - external magnetic field

Question 10

what is the order of the electromagnetic spectrum from lowest to highest frequency

Answer 10

radio
micro
infrared
visible
ultraviolet
x-ray
gamma

Question 11

when protons are randomly orientated, what net magnetisation do they hole

Answer 11

0, no net magnetisation

Question 12

compare the energy of protons which have spins parallel/antiparallel to B0

Answer 12

• spins parallel to b0 (+z) have lower energy than those antiparallel (-z)
• more of spins are in parallel state than antiparallel

Question 13

explain how the net magnetic moment (M) correlates with B0

Answer 13

• total parallel/antiparallel spins has M
• this behaves as a magnet pointing in the direction of B0
• M increases with B0

Question 14

define flip angle

Answer 14

MRI phenomenon by which the axis of the hydrogen proton shifts from its longitudinal plane (static magnetic field B0) Z axis to its transverse plane XY axis by excitation with the help of radiofrequency (RF) pulses.

• the flip angle determines the amount of RF energy that is absorbed by the body during each pulse
• the amount of rotation the net magnetization (M) experiences during application of a radiofrequency (RF) pulse.

Question 15

what 2 things do flip angle depend on

Answer 15

• pulse energy
• pulse duration

Question 16

how is the angle (flip angle) controlled

Answer 16

• by varying time or amplitude of pulse
• a 90 degree excitation pulse produces a flip angle of 90 degrees

Question 17

why do we need to flip the net magnetisation

Question 18

what is net magnetisation vector

Answer 18

The net magnetisation vector in MRI is the summation of all the magnetic moments of the individual hydrogen nuclei.

Question 19

how is a signal detected in mri

Answer 19

• current induced in conductor is exposed to changing magnetic field
• in transverse (xy) plane theres Mxy, the rotating component of M
• a coil whose axis is in the xy plane will detect Mxy

Question 20

define FID

Answer 20

free induction decay

• short-lived sinusoidal electromagnetic signal which appears immediately following the 90° pulse.

Question 21

what is FID

Answer 21

• after RF pulse, short duration decay
• RF pulse removed, M returns to original equilibrium position M0, pointing along Bo
• spins precess out of phase = Mxy decays
• spins release energy = Mz recovers to Mo

Free= spins precess freely after RF pulse switched off
Induction = spins induce current in RF receive coil
Decay= signal decays with time due to longitudinal recovery and dephasing

Question 22

what is the pathway needed to generate an MR signal/ conditions to satisfy

Answer 22

• Mxy precessing in TRANSVERSE plane
• Net magnetisation Mz
• Strong magnet (Bo) to make Mz
• RF pulse to flip Mz to xy plane and get Mxy
• RF transmit coil to generate pulse
• Magnetic moments spin IN PHASE
• Re phase the spin back after dephasing
• Receive coil in path of precessing Mxy

Question 23

what 3 things happen when the RF transmitter turns off

Answer 23

1. absorbed RF energy retransmitted (at resonance frequency)
2. magnetisation returns exponentially to equilibrium
3. excited protons begin to dephase

Question 24

what type of recovery/decay and relaxation is T1

Answer 24

longitudinal recovery
- spin lattice relaxation

Question 25

what type of recovery/decay and relaxation is T2

Answer 25

transverse decay - spin spin relaxation

Question 26

define spin-lattice relaxation

Answer 26

energy is given to surrounding lattice

Question 27

define spin-spin relaxation

Answer 27

dephasing caused by interactions with other nuclei

Question 28

why is the dephasing occurring during t2 relaxation

Answer 28

due to inhomogeneities within the magnetic field B0

Question 29

what 2 things is dephasing of t2 dependant on

Answer 29

- scanner (intrinsic inhomogeneities of B0) - local environment (susceptibility-induced field distortions produced by tissue or other materials placed within the field)

Question 30

what is spin echo

Answer 30

the refocusing of spin magnetisation by a pulse of resonant electromagnetic radiation - A spin echo sequence aims to remove the effects of the static field (T2*) but leave the tissue characteristic T2 effect. - A single RF pulse generates a free induction decay (FID), but two successive RF pulses produce a spin echo

Question 31

what are the 3 classifications of materials based on their interactions with magnetic fields

Answer 31

1. diamagnetic 2. ferromagnetic 3. paramagnetic

Question 32

define diamagnetic

Answer 32

- substance not magnetic under normal conditions but when exposed to external field, reduced the strength of the field locally - e.g wood, glass, water

Question 33

define ferromagnetic

Answer 33

- always exhibits magnetic properties -e.g iron, cobalt, nickel

Question 34

define paramagnetic

Answer 34

- substance enot magnetic under normal conditions but when exposed to external field, increases the strength of field locally - e.g gadolinium (contrast)

Question 35

define magnetic susceptibility

Answer 35

degree to which various materials can be magnetised

Question 36

compare diamagnetic, ferromagnetico and paramagnetic in terms of their magnetic susceptibility

Answer 36

diamagnetic= very low magnetic susceptibility (doesnt increase field strength) paramagnetic= high magnetic susceptability (increases field strength) ferramagnetic= very high magnetic susceptibility (greatly increases field strength)