Basic Physical Principles of MRI. James Voyvodic, Ph.D. Brain Imaging and Analysis Center. Synopsis of MRI. 1) Put subject in big magnetic field 2) Transmit radio waves into subject [2~10 ms] 3) Turn off radio wave transmitter 4) Receive radio waves re-transmitted by subject 0
James Voyvodic, Ph.D.
Brain Imaging and Analysis Center
1) Put subject in big magnetic field
2) Transmit radio waves into subject [2~10 ms]
3) Turn off radio wave transmitter
4) Receive radio waves re-transmitted by subject0
5) Convert measured RF data to image
There is electric charge
on the surface of the proton, thus creating a small current loop and generating magnetic moment m.
The proton also has mass which generates an
J when it is spinning.
Thus proton “magnet” differs from the magnetic bar in that it
also possesses angular momentum caused by spinning.
m= tmax / B
t = mB
= m B sinq
F = IBL
t = IBLW = IBA
J = mw=mvr
The magnetic moment and angular momentum are vectors lying along the spin axis
m = gJ
g is the gyromagnetic ratio
g is a constant for a given nucleus
the two alignment states depends on the nucleus
D E = hn
known as Larmor frequency
g/2p = 42.57 MHz / Tesla for proton
Note: Resonance at 1.5T = Larmor frequency X 1.5
The main field is static and (nearly) homogeneous
The Effect of Irradiation to the Spin System
Spin System After Irradiation
One can consider the quantum mechanical properties of individual nuclei, but to consider the bulk properties of a whole object it is more useful to use classical physics to consider net magnetization effects.
This is like a gyroscope
J = m/g
dm/dt = g (m× Bo)
m(t) = (mxocos gBot + myosin gBot) x + (myocos gBot - mxosin gBot) y + mzoz
This says that the precession frequency is the
SAME as the larmor frequency