MRI

• History of MRI


  • DEMOCRITUS - Greek philosopher, 400 B.C. theorized all matter is made of indivisible and invisible particles "atoms"
  • Magnesia - West Turkey, discovered "lodestones" used for navigation, religious and magical purposes
  • Hans Christian Oersted - 1819, discovered electricity produces magnetism
  • Michael Faraday - 1831, discovered electricity
  • JEAN-BAPTISTE-JOSEPH FOURIER - made the heart of MRI mathematics "the Fourier Transform"
  • Sir James Clerk Maxwell of Scotland - 1860, discovered magnetic lines of force could be mathematically expressed
  • Heinrich Hertz of Germany - 1868, discovered invisible electromagnetic waves exist with varying wave frequencies
  • Nikola Tesla - Discovered Rotating Magnetic Field
  • Isidor Isaac Rabi - first described and measured nuclear magnetic resonance in molecular beams
  • Felix Bloch and Edward Purcell - developed new ways and methods for nuclear magnetic precision measurements
  • Dr. Raymond Damadian - physician/physicist, performed 1st MRI whole body transaxial proton density weighted slice image in 1977
  • Dr. Paul Lauterbur - designed the gradient coils and developed a way to generate the first MRI images, in 2D and 3D, using gradients
  • Peter Mansfield from the University of Nottingham - developed a mathematical technique that would allow scans to take seconds rather than hours and produce clearer images

  
• Times of milestones in the development of an MRI scanner

  • 1974 - Selective excitation or sensitization of tomographic image slice invented by Sir Peter Mansfield's group
  • 1975 - Two dimensional Fourier Transformation invented by Richard Ernst's group
  • 1978 - First published image of human head produced by Clow and Young
  • Around 1984 - General Electric introduced high field 1.5 Tesla systems
  • 2003 - Paul Lauterbur and Sir Peter Mansfield awarded the Nobel Prize for Medicine or Physiology for their discoveries concerning magnetic resonance imaging

  
• Types of Magnets Used in MRI

  • Permanent Magnet
  • Electromagnets or Resistive Systems
  • Superconducting magnets
• Types of magnets in terms of field strengths

  • Ultrahigh field (4-7 Tesla) - used for research
  • High field (1.5-3 Tesla)
  • Midfield (0.5-1.4 Tesla)
  • Low field (0.2-0.4 Tesla)
  • Ultralow field (<0.2 Tesla)

  
• Magnetic Susceptibility


  • Diamagnetic
  • Paramagnetic
  • Ferromagnetic
• Atom

  Composed of a nucleus and revolving electrons
• Nucleus


  Comprises of protons and neutrons
• Atoms with an odd number of protons


  Exhibit the property of magnetic resonance
• Hydrogen


  Has a single proton and thereby a large magnetic moment, abundantly present in the body in the form of water and fat, produces the best magnetic resonance signals
• Magnetic field


  A vector quantity consisting of both a north and south pole
• Magnetic Dipole


  A magnetic field characterized by its own magnetic north and south poles separated by a finite distance

  
• Magnetic moment


  Refers to spinning motion of positive protons and the negative electrons that create a small magnetic field about the atom
• Magnetic intensity


  The amount of magnetic flux in a unit area perpendicular to the direction of magnetic flow
• Magnet


  A device that attracts iron and produces a magnetic field, the biggest and the most important part of the MRI system
• Spin


  Precesses or tumbles
• Precession


  The phenomenon of magnetic field spinning or gyrating around imaginary axis of its own creation
• Frequency precession


  The rate at which the nuclei complete a revolution about the precessional path (megahertz or millions of cycle per second)
• Gyromagnetic ratio


  The ratio between magnetic moment and angular momentum (disintegration constant in Nucmed)
• Angular momentum


  The angle formed between a precessing object and its imaginary axis
• Radiofrequency Pulse


  Refers to that portion of the electromagnetic spectrum in which electromagnetic waves can be generated by alternating current fed to an antenna
• The proton density weighted image is the most sensitive to all tissues, including fat.
• T1-weighted images are used to evaluate bone marrow, CSF, and brain tumors.
• Proton density (PD) weighted images are useful for evaluating soft tissue masses and fluid collections.
• T1-weighted images are more sensitive to soft tissue contrast than T2-weighted images.
• Protons have intrinsic spin and therefore behave like tiny magnets.
• Resonance


  Phenomenon resulting in the absorption and/or emission of electromagnetic energy by nuclei or electrons in a static magnetic field, after excitation by a suitable magnetic field

  
• T2-weighted images are more sensitive to fluid content within tissues.
• Protons have a positive charge and spin around their own axes like tiny tops.
• When radio frequency energy is applied, some of the aligned protons absorb this energy and change their alignment.
• Larmor Frequency


  Specific frequency of resonance, located based on the particular tissue and strength of the main magnetic field
• Fluid appears bright on T2-weighted images.
• Fat appears darker on T2-weighted images compared with other tissues.
• Relaxation time


  Time usually in fraction of a second in which the hydrogen nuclei switches from a magnetized state to a demagnetized state when magnetic pulse is turned off
• When radio frequency energy is introduced into the body, it causes some of the aligned protons to flip their orientation.
• T1 relaxation time/ Spin Lattice/Longitudinal Relaxation


  A biological parameter used in MRI to distinguish between tissue types, a measure of the time taken to realign with the external magnetic field
• T2 Relaxation time/ Spin-Spin Relaxation/ Transverse Relaxation


  Interaction between individual spins
• T2*/ T2-two-star


  Interaction between spins and Bo inhomogeneity
• TE/ Echo Delay Time/Time Echo


  Time between middle of exciting
• TR/ Repetition Time


  The period of time between the beginning of a pulse sequence and the beginning of the succeeding (essentially identical) pulse sequence
• Proton Density


  Tissues with the higher concentration or density of protons (hydrogen atoms) produce the strongest signals and appear the brightest on the image