How an MRI Works
Basic Principles

A moving electric charge produces a magnetic field. 

  • The faster the spin, larger the magnetic field
  • Water is the biggest source of protons in the body, followed by fat
  • The direction of the protons point is random
Magnetic Field Applied
Description
A Slight Excess Number of Protons Align with the Magnetic Field.
Some protons align ith the field, and some align against the field
  • A lot of these cancel each other out
  • There are a few extra protons that align with the magnetic field, so the net result is an alignment WITH the field. 
The Number of Protons Aligned with the Field in Excess is Proportional to the Strength of the Field. 
The larger the external field, the greater the difference in energy levels and the larger the excess number aligned with the field. 
Why doe the protons align both with and against the external magnetic field?
The alignment with the field is a lower energy state
  • protons are continually oscillating between the two states, but there will be a slight majority aligned with the field at any given moment.
The Number of Excess Protons is Large
We can ignore quantum mechanics and focus on just classical mechanics. 
  • The total magnetic field of excess protons is called Mo
Spinning Protons Precess
Protons wobble about its axis and the resonance frequency is proportional to the strength of the magnetic field. 
Larmor Equation
Wo=yBo
Wo=resonance frequency
y=gyromagnetic ratio - 42.56 MHz/Tesla for Protons
Bo=strength of magnetic field (in Teslas)
Protons Absorb energy from RF
At optimum quantum level, a single proton jumps to a higher energy state
Note
Description
Apply a RadioFrequency (RF) Pulse on Resonance
Description
Laboratory Frame
An observer from the lab will see the magnetization vector (Mo)  spin downwards the XY plane
Rotating Frame
Observer is riding the vector, and sees the external world rotating about him. Mo then seems to tip a(alpha) angle toward the Y'
Tip Angle a
Alpha is a function of the strength and duration of the RF pulse. 
The RF Transmitter is turned off
The following three things happen simultaneously.
RF Energy is Retransmitted
  • This is the "NMR" signal
  • At the resonance frequency
  • Signal proportional to Proton Density
Mz Begins to Recover
  • Exponential recovery of Mz
  • Time constant is called T1
  • Longitudinal or Spin-Lattice Relaxation
Spins (Mxy) Begin to Dephase
  • Exponential decay of signal
  • Time Constant is called T2 or T2*
  • Transverse or Spin-Spin Relaxation
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