Magnetic%20Resonance%20Imaging

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Magnetic Resonance Imaging

• Magnetic resonance started out as a tomographic
  imaging modality for producing NMR images of a
  slice through the human body

• The intensity of a pixel is proportional to the
  NMR signal intensity of the contents of the
  corresponding volume element or voxel of the
  object being imaged

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Magnetic Resonance Imaging cont

• MRI is based on the absorption and emission of
  energy in the radio frequency range of the
  electromagnetic spectrum

• The human body is primarily fat and water which
  makes the human body 63 hydrogen atoms

• Because Hydrogen nuclei have an NMR signal, MRI
  primarily images the signal from hydrogen nuclei

• Hydrogen proton possesses a property called spin

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Spin

• Spin comes in multiples of ½ and can be or -

• Individual unpaired electrons, protons and
  neutrons each possess a spin of 1/2

• When placed in a magnetic field of strength B, a
  particle with a net spin can absorb a photon, of
  frequency V, which is dependent on the
  gyromagnetic ratio, Y of the particle

V Y B

• For Hydrogen, Y 42.58 MHz/T

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Components of an MRI System

• A large magnet to generate the magnetic field
• Shim coils to make the magnetic field as
  homogeneous as possible
• A radiofrequency (RF) coil to transmit a radio
  signal into the body part being imaged
• A receiver coil to detect the returning radio
  signal
• Gradient coils to provide spatial localization of
  the signal
• Mini-computer to reconstruct the radio signal
  into the final image

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MRI Signal Intensity is Determined by Four
Parameters

• Proton Density concentration of protons in the
  tissue in the form of water and macromolecules
• T1 and T2 Relaxation Time defines the way
  protons revert back to their resting states after
  the initial RF pulse
• Flow loss of signal

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So What Happens?

• A magnetic field runs straight down the center of
  the tube in which the subject is placed
• The hydrogen protons either line up in the
  direction of the feet or the head
• Most of the protons cancel each other out
• An RF pulse specific for Hydrogen is directed at
  the area of interest
• The pulse causes the protons in that area to
  absorb the energy required to make them spin or
  precess, in a different direction Resonance
• The RF pulse forces the few unmatched protons to
  spin at a particular frequency, in a particular
  direction Larmour Frequency

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The Rest of the Story

• The RF pulses are applied through a coil specific
  designed for different parts of the body
• At the same time gradient magnets are applied
  Slice
• Resonance frequency of nuclei in external
  magnetic field is proportional to the strength of
  the field Position, Size of Volumes, Image
  Construction
• When the RF pulse is turned off, hydrogen protons
  return to their natural alignment within the
  magnetic field and release energy
• Coil picks up signal
• Fourier transform - Imaging

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Techniques Used Wrist Coil
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The Research

• Mesenchymal stem cells were magnetically tagged
  with ferumoxide

• Rats underwent MRI to determine SI changes in the
  liver

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MRI and Prussian Blue Staining of Liver
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Signal intensity changes Labeled Cell Migration
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Conclusions

• With the addition of magnetic field gradients,
  iron oxide labeled cells can be delivered and
  retained at a site of interest
• Opens new area of investigation for delivering
  genetically altered cells in cell therapy