Introduction to Diffusion Tensor Imaging (eBook)

Cover 1
Introduction to Diffusion Tensor Imaging 4
Copyrightpage 5
Contents 6
Preface 10
ACKNOWLEDGEMENTS 14
Chapter 1: Basics of diffusion measurement 16
1.1 NMR SPECTROSCOPYANDMRI CAN DETECT SIGNALS FROM WATER MOLECULES 16
1.2 WHAT IS DIFFUSION? 18
1.3 HOW TO MEASURE DIFFUSION? 19
Chapter 2: Anatomy of diffusion measurement 28
2.1 A SET OF UNIPOLAR GRADIENTS AND SPIN-ECHO SEQUENCE IS MOST WIDELY USED FOR DIFFUSION WEIGHTING 28
2.2 THERE ARE FOUR PARAMETERS THAT AFFECT THE AMOUNT OF SIGNAL LOSS 28
2.3 THERE ARE SEVERAL WAYS OF ACHIEVING A DIFFERENT DEGREE OF DIFFUSION WEIGHTING 32
Chapter 3: Mathematics of diffusion measurement 34
3.1 WE NEED TO CALCULATE DISTRIBUTION OF SIGNAL PHASES BY MOLECULAR MOTION 34
3.2 SIMPLE EXPONENTIAL DECAY DESCRIBES SIGNAL LOSS BY DIFFUSION WEIGHTING 42
3.3 DIFFUSION CONSTANT CAN BE OBTAINED FROM THE AMOUNT OF SIGNAL LOSS BUT NOT FROM THE SIGNAL INTENSITY 42
3.4 FROM TWO MEASUREMENTS, WE CAN OBTAIN A DIFFUSION CONSTANT 45
3.5 IF THERE ARE MORE THAN TWO MEASUREMENT POINTS, LINEAR LEAST-SQUARE FITTING IS USED 46
Chapter 4: Principle of diffusion tensor imaging 48
4.1 NMR/MRI CAN MEASURE DIFFUSION CONSTANTS ALONG AN ARBITRARY AXIS 48
4.2 DIFFUSION SOMETIMES HAS DIRECTIONALITY 48
4.3 SIX PARAMETERS ARE NEEDED TO UNIQUELY DEFINE AN ELLIPSOID 50
4.4 DIFFUSION TENSOR IMAGING CHARACTERIZES THE DIFFUSION ELLIPSOID FROM MULTIPLE DIFFUSION CONSTANT MEASUREMENTS ALONG DIFFERENT DIRECTIONS 52
4.5 WATER MOLECULES PROBE MICROSCOPIC PROPERTIES OF THEIR ENVIRONMENT 54
4.6 HUMAN BRAIN WHITE MATTER HAS HIGH DIFFUSION ANISOTROPY 55
Chapter 5: Mathematics of diffusion tensor imaging 56
5.1 OUR TASK IS TO DETERMINE THE SIX PARAMETERS OF A DIFFUSION ELLIPSOID 56
5.2 WE CAN OBTAIN THE SIX PARAMETERS FROM SEVEN DIFFUSION MEASUREMENTS 58
5.3 DETERMINATION OF THE TENSOR ELEMENTS FROM A FITTING PROCESS 60
Chapter 6: Practical aspects of diffusion tensor imaging 64
6.1 TWO TYPES OF MOTION ARTIFACTS: GHOSTING AND COREGISTRATION ERROR 64
6.2 WE USE ECHO-PLANAR IMAGING TO PERFORM DIFFUSION TENSOR IMAGING 66
6.3 THE AMOUNT OF DIFFUSION-WEIGHTING IS CONSTRAINED BY THE ECHO TIME 68
6.4 THERE ARE VARIOUS k-SPACE SAMPLING SCHEMES 68
6.5 PARALLEL IMAGING IS GOOD NEWS FOR DTI 72
6.6 IMAGE DISTORTION BY EDDY CURRENT NEEDS SPECIAL ATTENTION 75
6.7 DTI RESULTS MAY DIFFER IF SPATIAL RESOLUTION AND SNR ARE NOT THE SAME 76
6.8 SELECTION OF b-MATRIX 78
Chapter 7: New image contrasts from diffusion tensor imaging: theory, meaning, and usefulness of DTI-based image contrast 84
7.1 TWO SCALAR MAPS (ANISOTROPY AND DIFFUSION CONSTANT MAPS) AND FIBER ORIENTATION MAPS ARE IMPORTANT OUTCOMES OBTAINED FROM DTI 84
7.2 SCALAR MAPS (ANISOTROPY AND DIFFUSION CONSTANT MAPS) AND FIBER ORIENTATION MAPS ARE TWO IMPORTANT IMAGES OBTAINED FROM DTI 85
7.3 THERE ARE TUBULAR AND PLANAR TYPES OF ANISOTROPY 87
7.4 DTI HAS SEVERAL DISADVANTAGES 90
7.5 THERE ARE MULTIPLE SOURCES THAT DECREASE ANISOTROPY 91
7.6 ANISOTROPY MAY PROVIDE UNIQUE INFORMATION 94
7.7 COLOR-CODEDMAPS ARE A POWERFUL VISUALIZATION METHOD TO REVEAL WHITE MATTER ANATOMY 98
Chapter 8: Limitations and improvement of diffusion tensor imaging 100
8.1 TENSORMODEL OVERSIMPLIFIES THE UNDERLYING ANATOMY 100
8.2 THERE ARE MORE SOPHISTICATED NON-TENSORŽ-BASED DATA PROCESSING METHODS, WHICH REQUIRE DIFFERENT DATA ACQUISITION PROTOCOLS 102
8.3 NON-TENSOR MODELS USUALLY REQUIRE HIGH VALUES 105
Chapter 9: Three-dimensional tract reconstruction 108
9.1 THREE-DIMENSIONALTRAJECTORIES CAN BE RECONSTRUCTED FROM DTI DATA 108
9.2 THERE ARE TWO TYPES OF RECONSTRUCTION TECHNIQUES 108
9.3 THERE ARE THREE STEPS IN THE TRACT PROPAGATION MODELS 109
9.4 SIMPLE STREAMLINE TRACKING CAN BE USED TO RECONSTRUCT A TRACT 110
9.5 THERE ARE MANY LIMITATIONS TO SIMPLE TRACT PROPAGATION METHODS 114
9.6 SEVERAL APPROACHES ARE PROPOSED TO TACKLE THE LIMITATIONS 115
9.7 TRACT EDITING USESMULTIPLE REGIONS OF INTEREST 121
9.8 BRUTE-FORCE APPROACH IS AN EFFECTIVE TECHNIQUE FOR COMPREHENSIVE TRACT RECONSTRUCTION 125
9.9 ACCURACY AND PRECISION ARE IMPORTANT FACTORS TO BE CONSIDERED 125
9.10 REPRODUCIBILITY OF TRACTOGRAPHY IS MEASURABLE 128
9.11 TRACTOGRAPHY REVEALS MACROSCOPIC WHITE MATTER ANATOMY 129
9.12 THERE ARE ROUGHLY THREE TYPES OF INFORMATION OBTAINED FROM TRACTOGRAPHY 130
9.13 HOWCAN WEVALIDATE TRACTOGRAPHY? 132
9.14 HOW SHOULD WE USE A TOOL WITH UNKNOWN ACCURACY? 134
9.15 QUANTIFICATION IS A KEY TO MANY TYPES OF TRACTOGRAPHY-BASED STUDIES 135
9.16 THERE ARE SEVERAL POSSIBLE REASONS THAT LEAD TO SMALLER (OR LARGER) RECONSTRUCTION RESULTS 136
Chapter 10: Quantification approaches 140
10.1 IMPROVEMENT OF CONVENTIONAL QUANTIFICATION APPROACHES 140
10.2 QUANTIFICATION OF ANISOTROPY AND TRACT SIZES BY DTI 145
Chapter 11: Application studies 164
11.1 BACKGROUND OF APPLICATION STUDIES OF DTI 164
11.2 EXAMPLES OF APPLICATION STUDIES 165
References and Suggested Readings 178
PREFACE 178
CHAPTER 3 179
CHAPTER 4 179
CHAPTER 5 180
CHAPTER 6 180
CHAPTER 7 181
CHAPTER 8 183
CHAPTER 9 183
CHAPTER 10 185
CHAPTER 11 186
Subject Index 190