Basic Physics of MR Signal and Image Generation - Advance Image Processing in Magnetic Resonance Imaging - page 2

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Basic Physics of MR
Signal and Image
Generation

Maria Filomena Santarelli

CONTENTS

1.1 Introduction..................................................................................................4

1.2 Nuclear Spin ................................................................................................6

1.3 Nuclei in a Magnetic Field .........................................................................7

1.3.1 Notes on Larmor Frequency: Changes

Due to Dishomogeneities ................................................................9

1.3.2 Bulk Magnetization .......................................................................10

1.4 RF Excitation for the Resonance Phenomenon Generation .....................11

1.5 MR Signal Generation and Acquisition ....................................................14

1.5.1 Free Induction Decay and the Fourier Transform ........................14

1.6 MR Signal Characteristics.........................................................................16

1.6.1 Relaxation ......................................................................................16

1.6.1.1 The Decay of Transverse Magnetization: T2 ................16

1.6.1.2 The Recovery of Longitudinal Magnetization: T1........16

1.6.1.3 Pseudo-Relaxation: T2

*

..................................................17

1.6.2 Proton Density ...............................................................................17

1.6.3 The Bloch Equations .....................................................................18

1.6.3.1 Rotating Frame of Reference.........................................18

1.7 Multiple RF Pulses ....................................................................................18

1.7.1 Gradient Echo ................................................................................19

1.7.2 Inversion Recovery ........................................................................19

1.7.3 Spin Echo ......................................................................................20

1.8 Magnetic Field Gradients ..........................................................................23

1.9 Spatial Localization of MR Signals ..........................................................24

1.9.1 Slice Selection ...............................................................................24

1.9.2 Frequency Encoding ......................................................................26

1.9.3 Phase Encoding .............................................................................28

1.9.4 Phase History of Magnetization Vectors during

Phase Encoding .............................................................................28

1.9.5 Timing Diagram of an Imaging Sequence....................................29

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