MRI physics and principles Flashcards
general overview on how MRI works
- strong static magnetic field to align protons which create net magnetisation
- radio frequency magnetic filed to tip the magnetisation when both have the same frequency
- time-varying gradient magnetic filed to spatially localise the MR signals and reconstruct the image
what percentage of the human body is made of water
60-75%
what type of nuclei have a non-zero net spin
all nuclei with odd numbers of protons and or neutrons
what characteristic of an electron or proton creates a magnetic field
spinning charged particlel
Ines of magnetic fields ALWAYS form a closed loop
what do you call the magnet formed from electron or proton spin
magnetic dipole moment
explain the spin of a proton in and without B0
absence of b0= proton spins about its own axis
presence of b0= proton spins about its own axis AND precesses about axis of b0
what is larmor frequency
spins rotate (precess) at fixed frequencies
wroe 0 = 2 pi f = y B0
wroe 0 - larmor frequency
2 pi f - larmor frequency
y - gyromagnetic ratio
b0 - external magnetic field
what is the order of the electromagnetic spectrum from lowest to highest frequency
radio
micro
infrared
visible
ultraviolet
x-ray
gamma
when protons are randomly orientated, what net magnetisation do they hole
0, no net magnetisation
compare the energy of protons which have spins parallel/antiparallel to B0
- spins parallel to b0 (+z) have lower energy than those antiparallel (-z)
- more of spins are in parallel state than antiparallel
explain how the net magnetic moment (M) correlates with B0
- total parallel/antiparallel spins has M
- this behaves as a magnet pointing in the direction of B0
- M increases with B0
define flip angle
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.
what 2 things do flip angle depend on
- pulse energy
- pulse duration
how is the angle (flip angle) controlled
- by varying time or amplitude of pulse
- a 90 degree excitation pulse produces a flip angle of 90 degrees
why do we need to flip the net magnetisation
what is net magnetisation vector
The net magnetisation vector in MRI is the summation of all the magnetic moments of the individual hydrogen nuclei.
how is a signal detected in mri
- 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
define FID
free induction decay
- short-lived sinusoidal electromagnetic signal which appears immediately following the 90° pulse.
what is FID
- 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
what is the pathway needed to generate an MR signal/ conditions to satisfy
- 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
what 3 things happen when the RF transmitter turns off
- absorbed RF energy retransmitted (at resonance frequency)
- magnetisation returns exponentially to equilibrium
- excited protons begin to dephase
what type of recovery/decay and relaxation is T1
longitudinal recovery
- spin lattice relaxation
what type of recovery/decay and relaxation is T2
transverse decay
- spin spin relaxation
define spin-lattice relaxation
energy is given to surrounding lattice
define spin-spin relaxation
dephasing caused by interactions with other nuclei
why is the dephasing occurring during t2 relaxation
due to inhomogeneities within the magnetic field B0
what 2 things is dephasing of t2 dependant on
- scanner (intrinsic inhomogeneities of B0)
- local environment (susceptibility-induced field distortions produced by tissue or other materials placed within the field)
what is spin echo
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
what are the 3 classifications of materials based on their interactions with magnetic fields
- diamagnetic
- ferromagnetic
- paramagnetic
define diamagnetic
- substance not magnetic under normal conditions but when exposed to external field, reduced the strength of the field locally
- e.g wood, glass, water
define ferromagnetic
- always exhibits magnetic properties
-e.g iron, cobalt, nickel
define paramagnetic
- substance enot magnetic under normal conditions but when exposed to external field, increases the strength of field locally
- e.g gadolinium (contrast)
define magnetic susceptibility
degree to which various materials can be magnetised
compare diamagnetic, ferromagnetico and paramagnetic in terms of their magnetic susceptibility
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)
