Magnetic Resonance Imaging Magnetic resonance started out as

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

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

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=YB • For Hydrogen, Y = 42. 58 MHz/T

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

MRI Signal Intensity is Determined by Four Parameters • Proton Density – concentration of protons in the tissue in the form of water and macromolecules • T 1 and T 2 Relaxation Time – defines the way protons revert back to their resting states after the initial RF pulse • Flow – loss of signal

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

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

Techniques Used – Wrist Coil

The Research • Mesenchymal stem cells were magnetically tagged with ferumoxide • Rats underwent MRI to determine SI changes in the liver

MRI and Prussian Blue Staining of Liver

Signal intensity changes & Labeled Cell Migration

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