MEDRADSC 3DE3 Lecture Notes - Lecture 4: Free Induction Decay, Larmor Precession, Gyromagnetic Ratio
18 Jun 2018
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Review of:
oLarmor equation (we need to calculate the precise radiofrequency to have a transfer of
energy, causing excitation/resonance), calculated by multiplying the gyro magnetic ratio
(constant, depends on the wobble of the h spin and the main mag field) and Bnot
oNMV
Use the gradients to temporarily cause a change in field strength, the change
causes change in precessional frequency outside of the ROI
Resonance
oExcitation
oMR signal
Image Contrast
Review of Larmor frequency
Precessional frequency= Larmor frequency because it is determined by the Larmor equation (B0
x gyro ratio)
All MR active nuclei have their own gyromagnetic constant so that when they are exposed to the
same field strength- they precess at different frequencies (hydrogen is different then fluorine or
carbon) … this allows us to preferentially image hydrogen
Gyromagnetic ratio is a constant of proportionality—B0 is proportional to the Larmor equation…
therefore if B0 increases
Resonance
A phenomenon that occurs when an object is exposed to an oscillating disturbance that has a
frequency that is close or matches its own natural frequency
Resonance of H—RF pulse of exact Larmor frequency must be applied
Other active nuclei aligned with B0 will not resonate
We need to expose h spins to the same precessional frequency of H and causes resonance,
Resonance causes all the spins to start to be in line with each other, they are along the same
precessional path at the same time
causes the net mag vector to be flipped out of the longitudinal plane- they can all oppose the
main magnetic field
Excitation- application of RF pulse that creates or results in resonance (also causes the spin to
be in phase with eachother)
As Bo increases energy difference between high and low increases so RF pulse must increase
to create any resonance
Result- NMV flipped to opposing Bnot(one result of resonance)
olow energy nuclei have more energy via (the RF pulse) resonance- move up to high
energy population
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NMV now lies at an angle to Bo
Angle called flip angle
oFlip angle depends on amplitude and duration of RF pulse
oUsually 90 degrees – however this varies
Longitudinal and transverse planes
Transfer of energy (resonance) results in:
1. NMV flips
2. Spins come into phase with each other (precessional paths are exactly the same)
(without application of RF pulse, they are out of phase)
Phase: second result of resonance
describes the position of each magnetic moment on precessional path
around Bo at any given moment
Out of phase (incoherent) vs. In phase (coherent)
What do you think this means??
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NMV
NMV is a vector quantity—created by two components 90 degrees to each other (mag in the
long plane vs transverse plane)
oThe vector we are transferring energy to (expose the H atom to the same
radiofrequency as its precessional frequency)
After the application of an RF pulse- the NMV is flipped into the transverse plane
oWhen the net mag vector is passed 90 degrees , causes the spins to go to spin-down, as
it passes a coil that is placed at 90 degrees, it induces a charge (motion and magnetism
induces a charge) when the net mag vector passes the coil that is at 90 degrees to Bnot
oLongitudinal plane= same as B0
oTransverse plane is where the receiver coil is
oAs the H spins pass the transverse plane, charge induced in receiver coil
Once the RF pulse is removed the NMV recovers (long grows, transverse shrinks)
MR Signal
Induction of MR signal: NMV flipped, as it goes down and passes transverse receiver coil we
have the induction of MRI signal
Magnitude of signal depends on the size of the vector in the transverse plane
Resonance - NMV flipped
Faraday’s Law of Induction- if any conductive loop is place in area of moving magnetic field, a
voltage is induced in this receiver coil
Voltage - MR signal
oLarmor frequency, magnitude of signal depends on amount of magnetization present in
transverse plane
find more resources at oneclass.com
find more resources at oneclass.com
Document Summary
Use the gradients to temporarily cause a change in field strength, the change causes change in precessional frequency outside of the roi. Precessional frequency= larmor frequency because it is determined by the larmor equation (b0 x gyro ratio) Gyromagnetic ratio is a constant of proportionality b0 is proportional to the larmor equation therefore if b0 increases. A phenomenon that occurs when an object is exposed to an oscillating disturbance that has a frequency that is close or matches its own natural frequency. Resonance of h rf pulse of exact larmor frequency must be applied. Other active nuclei aligned with b0 will not resonate. We need to expose h spins to the same precessional frequency of h and causes resonance, Excitation- application of rf pulse that creates or results in resonance (also causes the spin to be in phase with eachother) As bo increases energy difference between high and low increases so rf pulse must increase to create any resonance.
