MRI Techniques E-Book

Vincent Perrin

• Herausgeber: John Wiley & Sons
• Kategorie: Fachliteratur
• Sprache: Englisch

Seitenzahl: 221

Veröffentlichungsjahr: 2013

Beschreibung

The rapid developments in magnetic resonance imaging (MRI) over the past 20 years have affirmed its supremacy over most other means of non-invasive exploration of the human body. This progress has had other consequences for imaging physicists: having knowledge about only one of the sides of MRI is nowadays no longer enough to develop new sequences or even to learn more about those that already exist. It is necessary to have a clear and precise view of all the fields explored today by this imaging technique, such as rapid imaging, flows, diffusion, perfusion or even functional MRI. This book aims to allow readers with the basics of physics and mathematics within the field MRI to easily immerse themselves in techniques that are not familiar to them. Pragmatic in approach, moving between the physics underlying the techniques being studied and the clinical examination of images, it will also be of interest to radiologists looking to define protocols or make better use of the images obtained. Contents 1. Flow. 2. Diffusion. 3. Perfusion. 4. Functional MRI. About the Authors Vincent Perrin is a specialist teacher in the fields of physics and chemistry.

Leseprobe

First published 2013 in Great Britain and the United States by ISTE Ltd and John Wiley & Sons, Inc.

Apart from any fair dealing for the purposes of research or private study, or criticism or review, as permitted under the Copyright, Designs and Patents Act 1988, this publication may only be reproduced, stored or transmitted, in any form or by any means, with the prior permission in writing of the publishers, or in the case of reprographic reproduction in accordance with the terms and licenses issued by the CLA. Enquiries concerning reproduction outside these terms should be sent to the publishers at the undermentioned address:

ISTE Ltd27-37 St George’s RoadLondon SW19 4EUUK

www.iste.co.uk

John Wiley & Sons, Inc.111 River StreetHoboken, NJ 07030USA

www.wiley.com

© ISTE Ltd 2013

The rights of Vincent Perrin to be identified as the author of this work have been asserted by him in accordance with the Copyright, Designs and Patents Act 1988.

Library of Congress Control Number: 2013946298

British Library Cataloguing-in-Publication Data

A CIP record for this book is available from the British Library

ISBN: 978-1-84821-503-0

Table of Contents

Chapter 1 Flow

1.1. Blood

1.2. Basic phenomena in angiography

1.3. Artifacts relating to the flow

1.4. MRA sequences

1.5. Study of two clinical problems

Chapter 2 Diffusion

2.1. General points

2.2. Principle behind diffusion imaging and the associated sequence

2.3. Study of the obtained signal

2.4. Diffusion sequence and diffusion images

2.5. The different clinical applications for diffusion

2.6. Artifacts frequently encountered in diffusion

2.7. Diffusion-tensor imaging

2.8. Tractography

Chapter 3 Perfusion

3.1. General points

3.2. Exogenous tracers

3.3. Endogenous tracers: ASL technique

3.4. Medical applications

Chapter 4 Functional MRI

4.1. Introduction

4.2. Principle

4.3. Introduction to statistical analysis

4.4. Statistical analysis and fMRI

4.5. Pre-processing and limitations of fMRI

4.6. Application of fMRI

Bibliography

Index

Chapter 1

Flow

1.1. Blood

1.1.1. Characteristics of bloodflow

Bloodflow is characterized by four parameters:

− its velocity;

− its acceleration;

− its direction;

− its orientation.

Arterial flow has the highest velocity.

This velocity varies with the phases of the cardiac cycle: it may be very high (150-175 cm/s during systole, the period of contraction of the myocardium) but also almost null (at the end of diastole, the period of relaxation of the myocardium).

The flow in the cerebral arteries is much slower: 40-70 cm/s.

Finally, venous flow is the slowest: generally less than 20 cm/s.

Bloodflow runs from the organs to the heart in venous circulation and from the heart to the organs in arterial circulation.

1.1.2. Laminar flow and turbulent flow

We can distinguish two main types of flow:

− A laminar flow is found when the velocities are relatively low. It is characterized by a distribution of velocities which are all parallel and whose profile is parabolic: the velocity of the fluid is maximum at the center and almost null when against the walls.

Figure 1.1.Profile of velocities in a laminar flow

− A turbulent flow is a chaotic form of flow. The velocity then exhibits a vortical nature: local circular motions arise.

Figure 1.2.Profile of velocities in a turbulent flow

There are many reasons why a laminar flow could become a turbulent flow:

− the velocity of the flow surpasses a critical value;

− the structure of the vessels changes (see Figure 1.2).

Venous flow is laminar with a velocity that is constant overall.

Arterial flow is an intermediary case: it is laminar in diastole and turbulent during systole.

A turbulent flow causes significant artifacts in images of the flow, which we shall now go on to discuss in detail.

1.2. Basic phenomena in angiography

The flow phenomena which we are going to describe in this section enable us to spontaneously view blood vessels (without the injection of a contrast-enhancing product); the effect they have on the image depends on the sequence used (spin echo (SE) or gradient echo (GE)), on the parameters of that sequence (TR, TE…) but also on the particular parameters of the flow itself: its velocity, the orientation of the vessel in relation to the slice, etc.