Magnetic Resonance Imaging Flashcards
MRI principles
- 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
MRI application
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)
Precession
= 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
example for precession
hyrogen atom in a magnetic field
- protons not perfectly aligned
- nuclei percess about the direction of the field
- precession frequency = Larmour Frequency
Spin states
- protons align with/against field
- aligned against field → anti-parallel → higher energy state
- transition (parallel → anti-parallel) induced via electromagnetic radiation excitement → RF pulse
Larmour frequency
frequency of percession of magnetic moments
- f = gamma * B0
net magnetization vector
- 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
radio frequency pulse
RF pulse at Larmour frequency will cause net magnetisation vector M to rotate about B1 in the rotating frame of reference
Flip angle alpha
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)
recovery of Mz
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
recovery of Mxy (after 90° FR pulse)
difference between T2 and T2* relaxation
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
T2 & T2* relaxation
= time at which magnetization has decayed to 37% of its initial value immediately after 90° RF pulse
T1 & T2 difference
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
spatial localization of MRI signal
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
steps for localization of MRI signal
- slice selection (z-gradient)
- frequency encoding (x-gradient)
- phase encoding (y-gradient)
slice selection
gradient applied in z-direction
- RF coil transmit singal f0 & associated bandwidth
- slice selected with determined thickness by magnetic field gradient strenght
frequency encoding
gradient in x-direction
- turning a gradient on for a different axis and leaving it on during the readout
phase encoding
gradient in y- direction
- rows experienced different phase shifting
k-space
- 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
Free Induction Decay
dependent on magnitude of net magnetization vector (Mz) immediately before the RF is applied
MR signal
MR signal is detected by RF coil
- solely produced by Mxy magnetization
- the higher the static field strenght, the larger the signal
properties of tissues mapped by MR
Proton density T1 recovery (spin-lattice relaxation) → fat is bright T2 recovery (spin-spin relaxation) → fat and water are bright
control of imaging
- 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
