MRI

• Magnetic resonance imaging (MRI)


  Signal originates from the nuclei of atoms resonating in a patient in the presence of a magnetic field
• Quantum mechanics


  Branch of physics that describes the behavior of very small objects, such as x-rays, protons, neutrons, and electrons
• Quantum mechanics deals with subatomic particles and photons
• Spin


  Quantity that every nucleus has, quantized into units of half-integer values and called the spin quantum number
• Allowed values of spin quantum number

  • 0
  • 1/2
  • 1
  • 3/2
  • and so on
• All hydrogen atoms of atomic mass 1 have a spin quantum number of 1/2
• Spin state


  Limited number of ways a nucleus can spin, either counter clockwise (+1/2) or clockwise (-1/2)
• A spinning, charged mass induces a magnetic field about itself, like the Earth
• Nuclear magnetic moment


  The magnetic field associated with the spinning, charged nucleus, related to its mass, charge, and rate of spin
• Neutrons with no charge have no associated magnetic field
• Proton


  Classifies what kind of atom something is, the number of protons is the atomic number
• Isotope


  Atoms with the same number of protons but different numbers of neutrons
• Hydrogen is the most abundant atom in the human body
• Equilibrium


  Before being energized or receiving RF, the nuclear magnetization is aligned longitudinally with the main magnetic field
• At equilibrium, the parallel aligned protons are in a low energy state
• Energy cannot be created or destroyed, it can only be transformed
• Antiparallel/Spin down


  High energy state of the protons
• In the presence of the main magnetic field, there are protons that are out of phase
• Net magnetization


  The overall magnetization of the protons, which is related to the external magnetic field strength
• The larger the net magnetization (M0), the stronger the MR signal
• Increasing the concentration of the nuclear species or the volume of tissue increases the MR signal due to higher proton density
• Hydrogen


  Has the highest gyromagnetic ratio of any nucleus other than tritium, and the second largest signal per number of nuclei
• Tritium would make an excellent MRI agent but is radioactive
• For 13C, the gyromagnetic ratio is about one fourth the value for 1H, resulting in a much weaker signal
• Higher magnetic field strength (B0) results in stronger net magnetization and a stronger MR signal
• Larmor frequency


  The precessional frequency of the nuclei, corresponding to electromagnetic radiation in the RF range
• The RF pulse must be tuned to the Larmor frequency to rotate the net magnetization vector
• Transverse magnetization (Mxy)


  The net magnetization in the transverse (XY) plane, which is the only magnetization that can be detected as an MR signal
• It is not possible to directly measure the longitudinal magnetization (Mz)
• Stationary frame of reference


  The view of someone standing next to the magnet, where all motions are compared to someone standing still
• Rotating frame of reference

  A frame of reference that matches the motion of the net magnetization, making it easier to visualize
• RF pulse


  The application of a designed time/intensity radiofrequency pulse that can rotate the net magnetization vector to any angle
• Hard pulse


  A strong, very short RF pulse
• Soft pulse


  A weaker but longer RF pulse
• Flip angle


  The angle through which the RF pulse rotates the net magnetization vector
• 90 degree RF pulse


  Rotates the net magnetization vector from equilibrium onto the transverse (XY) plane
• 180 degree RF pulse


  Rotates the net magnetization vector from equilibrium to the negative Z-axis
• Alpha (α) pulse


  A "partial flip" RF pulse with a rotation angle less than 90 degrees
• After a 90 degree RF pulse, the longitudinal magnetization (Mz) is zero and the transverse magnetization (Mxy) is equal to the equilibrium magnetization (M0)
• Dephasing of Mxy


  The gradual loss of phase coherence in the transverse magnetization due to interactions with local magnetic fields