Magnetic Resonance Imaging

Question 1

MRI principles

Answer 1

• use property of nuclear magnetic resonance NMR to image nuclei of atoms inside the body → magnetic nuclei in a magnetic field absorb & re-emit electromagnetic radiation

Question 2

MRI application

Answer 2

investigate human body, using non-ionizing radio frequency (RF) waves → waves are generated in the body, need a lot of water
- MRI examines magnetic properties of atomic nuclei
→ atomic nuclei with odd atomic weight possess a spin
→ nucleus with charge & spin produced small magnetic field (behave like a bar magnet)

Question 3

Precession

Answer 3

= slow movement of the axis of a spinning body around another axis due to a torque acting to change the direction of the first axis

Question 4

example for precession

Answer 4

hyrogen atom in a magnetic field

• protons not perfectly aligned
• nuclei percess about the direction of the field
• precession frequency = Larmour Frequency

Question 5

Spin states

Answer 5

• protons align with/against field
• aligned against field → anti-parallel → higher energy state
• transition (parallel → anti-parallel) induced via electromagnetic radiation excitement → RF pulse

Question 6

Larmour frequency

Answer 6

frequency of percession of magnetic moments

- f = gamma * B0

Question 7

net magnetization vector

Answer 7

• no RF pulse → parallel state → net magnetization parallel to magnetic field
• RF pulse → protins flip to anti-parallel state
• RF pusle stops → protons flip back to parallel state → M in z-plane

Question 8

radio frequency pulse

Answer 8

RF pulse at Larmour frequency will cause net magnetisation vector M to rotate about B1 in the rotating frame of reference

Question 9

Flip angle alpha

Answer 9

flip angle alpha of M determined by
- magnetic field induced by RF pulse (B1)
- duration of applied pulse tp
alpha = 90° → RF pulse rotates M into transverse plane (xy) → induces signal in receiver coil at Larmour frequency (same freqeuncy)
→ magnitude of signal depends on M(x-y)

Question 10

recovery of Mz

Answer 10

when is B1 released → M rebounds back to its original value
→ described by Free Iduction Decay FID
T1 = time constant; point where 63% of magnetization Mz has recovered from alignment with B0
→ measures how quickly protons realign with main magnetic field

Question 11

recovery of Mxy (after 90° FR pulse)

difference between T2 and T2* relaxation

Answer 11

T2- relaxation: defined by spin-spin interaction → dephasing of spin → loss of phase coherence
- RF pulse → protons rotate in-phase
- after RF pulse → protons de-phase → Mxy magnetisation decreases → signal decay
T2* relaxation: overall term of observed loss of phase coherence
- combines T2 relaxation & additional de-phasing caused by local variations
- more rapid decay of the signal

Question 12

T2 & T2* relaxation

Answer 12

= time at which magnetization has decayed to 37% of its initial value immediately after 90° RF pulse

Question 13

T1 & T2 difference

Answer 13

are tissue dependent

• T1 weighted: more homogenous → overall geometry observable; one tissue type is bright – FAT
• T2 weighted: different types of tissue visible; FAT and WATER are bright

Question 14

spatial localization of MRI signal

Answer 14

goal: received signal depends on position
solution: creating a gradient in exeternal magnetic field
- e.g. gradient in z-direction: causes resonant frequency to be different at each z location → RF pulse tuned to frequeny → only spins in slice corresponding to that frequency will flip

Question 15

steps for localization of MRI signal

Answer 15

• slice selection (z-gradient)
• frequency encoding (x-gradient)
• phase encoding (y-gradient)

Question 16

slice selection

Answer 16

gradient applied in z-direction

• RF coil transmit singal f0 & associated bandwidth
• slice selected with determined thickness by magnetic field gradient strenght

Question 17

frequency encoding

Answer 17

gradient in x-direction

- turning a gradient on for a different axis and leaving it on during the readout

Question 18

phase encoding

Answer 18

gradient in y- direction

- rows experienced different phase shifting

Question 19

k-space

Answer 19

• each value in the resulting image matrix corresponds to a grey scale intensity → indicative of MR characetirist of nuclei in voxel
• rows & columns are “frequency” or “phase” encoded

Question 20

Free Induction Decay

Answer 20

dependent on magnitude of net magnetization vector (Mz) immediately before the RF is applied

Question 21

MR signal

Answer 21

MR signal is detected by RF coil

• solely produced by Mxy magnetization
• the higher the static field strenght, the larger the signal

Question 22

properties of tissues mapped by MR

Answer 22

Proton density
T1 recovery (spin-lattice relaxation) → fat is bright
T2 recovery (spin-spin relaxation) → fat and water are bright

Question 23

control of imaging

Answer 23

• TE = time to echo
• TR = time to repeat (time btw RF pulses)
  → are selected to weight the contrast in the image
  T1: TE and TR (optimize contrast) are short
  T2: TE (for differentiation) and TR are longer