MRI fundamentals Flashcards
Advantages of MRI
High contrast sensitivity
Use of non-ionising radiation
Multiplanar capability
Multiple contrast mechanisms
Disadvantages of MRI
High equipment and siting costs; Relative long time
Significant artifacts
Incompatible with certain biomedical implants/ pacemakers
Potential patient claustrophobia
A material is considered MR active as
as long as its mass number is odd, as it has a net charge
Nuclei with a net magnetic moment align
align their axis of rotation to the applied
magnetic field (B0).
MRI has 2 magnetic fields
Bo and B1
What is Bo?
External main magnetic field - always activated
What is B1?
Radio-frequency field that is turned on and off
The frequency of the RF pulses has to match
match (resonate) with a particular group of
protons in the body.
What happens when the RF signal is switched off?
The protons
move back into alignment with the main
magnetic field, and they emit an RF signal, which is used to construct the MR image.
In the strong magnetic field the hydrogen nuclei can
align with (parallel) or against (antiparallel) to the magnetic field
Hydrogen nuclei possess
2 energy states
Low energy nuclei align parallel to ext field =
spin up nuclei
Spin up
low energy align to parallel to ext field
Spin down
High energy align their magnetic moments in the antiparallel direction
The protons continually
oscillate between the two states
At any given movement, there will be a slight
majority aligned with parallel than against the field (antiparallel)
The larger the Bo field,
the greater difference the enrgy levels and larer excess number aligned with field - increase in SNR
Net magnetisation vector
is the sym of all the protons pointing in direction of magnetic field
Increase B =
increase in NMV
Thermal energy is determined by
temoerature of patient
decrease temp = increase in NMV
What are the 2 ways to increase NMV
decrease in temp (not sig)& increase in B
Precession
is the additional/secondary spin from B0, causes circular path around B
Precessional freq
is the freq at which NMC wobbles around B0
Larmor Equation
w = y x B0
For hydrogen, y =
42.58 MHz/T
Gyro magnetic ratio is
constant for each nuclei
Resonance is the
absorption or emission of energy - only occurs at specific frequencies that match RF pulse
Excitation is the
application of RF pulse that causes reonsnace to occur - causes protons to move out of alignment
When the Rf is turned on,
1 = RF E is absorbed at res freq by protons
2 = Excited protons move from long to transverse plae - net magnetisation from B0
3 = excited protons spin in phase
What is the flip angle
the angle that NMV moves from long to trans
Flip angle magnitude is
dependent on amplitude and duration of RF pulse
NMV rotates into
the transverse plane at Larmor freq at 90 degrees
Once the RF is off
the absorbed RF is retransmitted; the excited spin return to original orientation with B0; in phase protons dephase
The signal at the highest level when
NMV is in the transverse plane and when there is phase coherance
T1 recovery is
when the amount of magnetisation in longitudinal plane increases
T2 decay is the
amount of magnetisation in the transverse plane decreases
T1 recovery is related to NMV
moving from transverse back to longitudinal
T2 decay is related to the
dephasing of the protons
T1 is the the time it takes
for 63% of longitudinal magnetisation to recover
T1 is short in
fats
T2 is the time it
takes for 63% of transverse magnetisation to be lost
In magnetic inhomogenities,
the loss of phase coherance occurs mre than from T2; transverse relat T2 is shortened to T2
T2* =
T2 decay and dephasing due to ext mag field inhomogenities
The signal is at the highest level
when the NMV is in the transverse plane and when there is phase coherance
Phase coherance happens
in transverse plane
