MRI SIR NIÑO

• MRI


  Magnetic Resonance Imaging
• EDWARD PURCELL


  • Discovered Nuclear Magnetic Resonance (NMR) in 1946
  • Won the Nobel Prize of 1952 with Felix Bloch
  • Proposed that atomic nuclei behave like magnets
• EDWARD PURCELL


  • Charged particles have magnetic fields
  • Led to the development of Nuclear Magnetic Resonance (NMR) spectrometers
  • Spectrometry – analysis of molecular configuration based on NMR spectrum
• FELIX BLOCH


  • Nobel Prize winner of 1952, along with Edward Purcell
  • Proposed the Bloch Equations
  • Relationship between nuclear magnetization and relaxation times
• RAYMOND DAMADIAN


  • Malignant tissue have different NMR parameters compared with normal tissue
  • First NMR image of a rat tumor in 1974
  • Built The Indomitable with his team in 1977
  • First scan took five hours to complete
• PAUL LAUTERBUR


  • Expanded Herman Carr's idea of using magnetic gradients
  • Managed to obtain pictures that demonstrate the difference between regular water and heavy water (deuterium oxide)
• PETER MANSFIELD


  • Improved upon Paul Lauterbur's technique by using Larmor Frequency and Fourier Transformation
  • Larmor Frequency - rate of spin wobble when placed in a magnetic field, directly proportional to magnetic field strength
  • Fourier Transformation - Mathematical technique to decompose an MRI signal, Interpolation that speeds up image formation
  • Credited with "slice selection" in MRI

  
• The original name for MRI was Nuclear Magnetic Resonance Imaging, but the "nuclear" was removed because of the various negative connotations of the word.
• BASICS OF MAGNETISM


  • Any charged particle in motion generates a magnetic field, hence "Electromagnetism"
  • The magnetic field exists in a 90-degree angle to the motion of the particle, hence "Fleming's Hand Rule"

  
• DEFINING A MAGNET


  • Device that attracts iron and produces a magnetic field
  • SI unit of measurement – Tesla (T) or Gauss (G), 10,000 G = 1 T
  • Nikola Tesla – proponent of alternating current (AC)
  • Carl Friedrich Gauss – first to measure the magnetism of Earth using a magnetometer, Magnetometer – instrument that measures changes in the Earth's magnetic field
• Magnetic dipole


  Some elements form magnetic dipoles based on the spin of their nucleus and electrons, Dipole – means a magnet that has a north and south pole, The alignment of many magnetic dipoles result in a magnetic domain
• Types of magnetism


  • Ferromagnetism
  • Paramagnetism
  • Diamagnetism
• Ferromagnetism


  Strong attraction, Can be permanently magnetized, "Alnico" alloy
• Paramagnetism


  Slight attraction to a magnet, Loosely influenced by magnetic fields, Contrast agents for MRI
• Diamagnetism


  Weakly repelled by magnetic fields, Pure water, plastic, Used in MRI shielding
• Non-magnetic


  Unaffected by magnetic fields, Wood, glass
• Aspects of magnetism


  • Magnetic Intensity
  • Magnetic Permeability
  • Magnetic Susceptibility
• Magnetic Intensity


  Amount of magnetic flux in an area perpendicular to the direction of magnetic flow, Measurement of field "Strength"
• Magnetic Permeability


  Ability of a material to attract lines of magnetic field intensity, Characteristic of a material to "strengthen" or "weaken" a magnetic field
• Magnetic Susceptibility


  A measure of magnetic properties of a material, Determines whether a material can be affected by an external magnetic field
• Types of magnets


  • Permanent
  • Resistive Magnet / Electromagnet
  • Superconducting Magnet
• Permanent Magnet


  Magnetized material that do not lose magnetism, Low field strength (0.064T – 0.3T), Usually has an open design (less claustrophobia), Low magnetic strength means longer scan time and less image quality
• Resistive Magnet / Electromagnet


  Magnetic field is generated by a current that passes through coiled wire, Either Air-Core or Iron-Core, Up to 0.3T, Generates a lot of heat, requires water-cooling, Open design, Can be switched off and on
• Superconducting Magnets


  Most common among MRI machines, Similar to a resistive magnet inside, except the metal is cooled to near absolute zero, making it a superconductor, Superconductivity – a property that some materials gain when reaching extremely low temperatures wherein their electric resistance becomes zero, Requires multiple vacuum shields to prevent heat from reaching the magnet
• Cryogen


  Cooling agent used to achieve superconductivity, Liquid helium can cool to 47 K, Liquid nitrogen can cool to 97 K, 0 K = -273 degrees Celsius or -459 degrees Fahrenheit
• Dewer / Cryostat


  Double-walled flask of metal with vacuum between walls, The inner chamber contains the cryogen, Similar to an Aquaflask / Hydroflask vacuum-insulated tumbler
• Fringe Magnetic Field


  Magnetic field outside of the patient aperture
• Quenching


  Rapid expulsion of liquid cryogen as gas
• Humans contain a large amount of water, hence a lot of hydrogen.
• Hydrogen has the highest gyromagnetic ratio (42.6 MHz / T)
• The hydrogen nucleus functions as a magnetic dipole since it spins on its own axis
• What happens when a person is put inside an MRI machine

  1. The hydrogen atom precess at the Larmor Frequency

  2. More hydrogen atoms are in the parallel configuration (low-energy) than in anti-parallel configurations (high energy)

  3. There is a net magnetization (sum of all magnetic fields of each proton) pointing in the same direction as the MRI machine's magnetic field

  4. All of these are part of the MRI Machine's initial phase – Magnetization
• Excitation


  1. The MRI machine performs a pre-scan to detect the frequency of the spinning protons (the Larmor Frequency)

  2. The MRI machine then sends a radio frequency pulse that matches the frequency of the spinning protons

  3. The protons that resonate with the frequency then change vectors and starts pointing away from Bo

  4. This change in vector is called "flipping," hence there is a parameter in sequences called "flip angle," which can be anywhere from 0o to 180o
• Relaxation Phase


  1. The protons will keep the RF pulse they received for a time, putting them in a "high energy state"

  2. Eventually the protons will revert back to a lower energy state, They do this by releasing the stored RF pulse along with some of it converted into heat

  3. Eventually the net magnetization of the once-resonant protons return to point at Bo

  4. This is what happens in T1 Relaxation, also called Spin-Lattice Relaxation or Longitudinal Relaxation
• T2 Relaxation


  • After an excitation pulse, the protons begin to all spin towards the same direction, This is called being "in-phase"
  • As time passes and they lose energy, some protons start to spin in other directions, This is called "de-phasing"
  • Eventually with enough time, none of the protons spin in the same direction, Hence why T2 is called "spin-spin relaxation" or "transverse relaxation"
• Acquisition


  1. The RF signal released by the protons are picked up by the Receive Coil

  2. It can only be done at 90o angles from Bo; otherwise, the current induced by Bo overwhelms the current produced by the RF signals from tissue

  3. Signals are strong in the beginning, and quickly becomes weaker, This is called Free Induction Decay
• Computing and Display


  The received signal is fed into the computer and converted into images
• Coils


  • Gradient Coils
  • Radiofrequency Coils
• Gradient Coils


  Uses resistive electromagnets, Creates additional magnetic fields and allows slice selection
• Gradient Coils


  • X-Coil - Sagittal plane
  • Y-Coil - Coronal plane
  • Z-Coil - Axial/Transverse plane