Understanding MRI

Learning Objectives
Flash Interface
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The following outline provides a basic overview of the topics covered.

It is not a substitute for the presentation.

I.  General Review of Crucial Physics Concepts

a.       Precession – Conservation of Angular Momentum

                                         i.      To understand how hydrogen atoms spin as they relax back in line with the magnetic field, it is necessary to understand the concept of precession.  This is best explained through the example of a gyroscope.

b.      Constructive/Destructive Interference

                                         i.      To understand how the summing current induced by varying hydrogen precessions at different field strengths leads to interference.  A 2D Fourier transform is used to process this data.

c.       Lenz’s Law 

                                         i.     To understand how  precession of hydrogen atoms is observed through frequency of induced current oscillation in a solenoid pick-up coil.

           II.      MRI Basics

a.       Why hydrogen atoms?

                                         i.      Hydrogen is abundant in the water molecules in human tissue, however different tissue types have different hydrogen concentrations which allows different tissue types to be distinguished.

b.       Strong magnetic field causes H atoms to line-up with magnetic field.

c.       RF Pulse, specific to hydrogen atoms, knock them out of alignment.   Pulses are usually 90 degrees or 180 degrees.

Basic MRI info primarily from:  

Schaffer, Chris.   “Introduction to Biomedical Engineering.”  BME 131 Lecture.   Spring 2007

         III.      Spatial Encoding - Signal is received by pickup coils along three axis

a.       Gradient slice selection – First Dimension

                                        i.      Caused by magnetic field gradient.

b.      Phase Encoding – Second Dimension

                                         i.      Apply a second orthogonal magnetic gradient briefly causing hydrogen atoms to progress at different Lamore Frequencies.

c.       Frequency Encoding – Third Dimension

                                        i.      Applying a third gradient gives us a third dimension of frequency encoding.

           V.     Fourier Transform and Image Formation

                      a.           “The Fourier transform ( FT ) process is like the musician hearing a tone (time domain signal) and determining what note             `                   (frequency) is    being played. The inverse Fourier Transform ( IFT ) is like the musician seeing notes (frequencies)                                   on a sheet of  music and converting them to tones (time domain signals).”  (Quoted from Hornak).

                      b.            Fourier Transform helps to take a very complex interference pattern and break it down into individual precessional frequencies.

                      c.           Fourier Transform must be taken along two orthogonal axis, in a "2D Fourier Transform" to account for phase encoding gradient.

                      c.           A 2D Fourier Transform results in a "K Space"

                                    i.   The central part of the "K space" is where the most important spatial information is held.

                                    ii.  Peripheral areas of the K-space contribute to the edge contrast of the image.

                    d.           Taking the inverse 2D Fourier Transform of the K-Space yields the final image.

            IV.   Why is an MRI scan so loud?

                    a.            Strong magnetic field leads to stretching of coils at each pulse.

                    b.            Just for fun - can tune pulses to play music (very expensive musical instrument...)

            V.    Modifications to MRI Machines

                    a.            Open MRI - solves problem of claustrophobia, but magnet is not as strong.  This leads to lower image resolution

                    b.           Functional MRI -  distinguishes deoxyhemoglobin and oxyhemoglobin to identify areas of increased oxygen consumption in brain.



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 ©2008 Scott Hayes

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Last updated: 05/06/08.