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 = Y B

•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

•T1 and T2 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