Magnetic Resonance Imaging

Jim Kelly

Contents

· Introduction

· What is NMR ?

· A Medical Marvel

· Vocabulary

· The Technique I & II

· Magnet Parameters

· The Applications

· Summary

· References

Introduction

· NMR Imaging was developed by Lauterbur & Mansfield in the early 1970’s

· Initial poor image quality meant No Medical Relevance, with development seen as a

New Medical Revolution

What is NMR ?

· A technique developed over 40 years ago as an analytical chemical tool

· It works because some nuclei are weakly magnetic and respond to magnetic fields

· The nuclei are then excited by an applied radio frequency ( R.F. ) pulse and radiate energy

· This yields information about the physical and chemical composition of the sample

· Varying the magnetic field gives spatial NMR images of slices of the object

· Most non metallic materials are transparent to to magnetic fields and radio frequencies

A Medical Marvel

· Non invasive imaging capability sensitive to subtle differences in water content detects local flow characteristics of onset of for example: multiple sclerosis, cancer, cardio-vascular conditions etc.

· routine brain scans possible

· novel applications with arthritis

· whole body imaging with non ionising radiation possible

Vocabulary

· Gyro; To rotate, a gyroscope can precess

· Gyromagnetic ratio; G = 42.571 MHz T^-1 for H

· Larmor frequency; f = (G/ 2 p) B, the rate of precession of nuclei in a magnetic field

· Precession; To rotate the axis of a spinning body at a angle about a given direction

· Relaxation time; The time T₁ for spins to re-align themselves in the direction of the field, for

T₂ the spin-spin interaction to decay

· Spin; The angular momentum e.g. of a proton

· Tesla; unit of magnetic field strength B

The Technique I

· The hydrogen nucleus has a charge and angular spin so it acts like a tiny bar magnet

· Nuclei in water will therefore align themselves in an external magnetic field

· A radio frquency pulse at the Larmor frequency f _L will tilt the nuclei from this direction

· They will then precess about the direction of the field losing energy as radio waves

· Coils around the sample detect an induced voltage as the strength of the signal decays

· The T₁ relaxation time for a compass needle to come to rest

The Technique II

· The T₂ relaxation time (spin-spin interaction) for compass needles to come to rest

· The spins exchange energy resulting in a longer relaxation time than the spin-lattice T₁

· Signals in the coil are detected and analysed into frequency components

· Spatial information obtained using a magnetic field gradient, built up in 2D slices

· This gives a map of water content in tissue, bone appears dark and soft tissue bright

· Both T₁ & T₂ are longer in diseased tissue

Magnet Parameters

· Superconducting electromagnet of Ni-Ti Alloy used

· Uniform Field strengths of between 2 to 5 Tesla, Field gradients stable to @ 1%

· Machines cost about £1 Million

· Whole body machines are becoming smaller

The Applications

· Imaging tumours in the brain

· Measurement of the fat content of foods

· Determination of the chemical composition of substances

Summary

· How does MRI work and what use is it ?

· Calculate the precession frequency of hydrogen in a magnetic field of 2 Tesla

· At what temperature are the coils in the electromagnet maintained ?

· What is the difference between a sagital and transverse scan ?

References

· “The Industrial Applications of MRI” The Royal Society New Frontiers in Science 1996

· “Medical Physics” by Martin Hollins p186, published by Nelson

· MRI resource pack produced by the University of Surrey, Physics Dept.

· Chemistry in Britain vol 32 no 6 p33-45 June 1996