Book contents
- Frontmatter
- Contents
- Preface
- Introduction
- PART I AN OVERVIEW OF FUNCTIONAL MAGNETIC RESONANCE IMAGING
- PART II PRINCIPLES OF MAGNETIC RESONANCE IMAGING
- IIA The Nature of the Magnetic Resonance Signal
- 7 Basic Physics of Magnetism and NMR
- 8 Relaxation and Contrast in MRI
- 9 Diffusion and the MR Signal
- IIB Magnetic Resonance Imaging
- PART III PRINCIPLES OF FUNCTIONAL MAGNETIC RESONANCE IMAGING
- Appendix: The Physics of NMR
- Index
8 - Relaxation and Contrast in MRI
from IIA - The Nature of the Magnetic Resonance Signal
Published online by Cambridge University Press: 05 September 2013
- Frontmatter
- Contents
- Preface
- Introduction
- PART I AN OVERVIEW OF FUNCTIONAL MAGNETIC RESONANCE IMAGING
- PART II PRINCIPLES OF MAGNETIC RESONANCE IMAGING
- IIA The Nature of the Magnetic Resonance Signal
- 7 Basic Physics of Magnetism and NMR
- 8 Relaxation and Contrast in MRI
- 9 Diffusion and the MR Signal
- IIB Magnetic Resonance Imaging
- PART III PRINCIPLES OF FUNCTIONAL MAGNETIC RESONANCE IMAGING
- Appendix: The Physics of NMR
- Index
Summary
INTRODUCTION
The contrast between one tissue and another in a magnetic resonance (MR) image varies over a wide range depending just on the pulse sequence used for imaging. This dramatic soft-tissue contrast makes magnetic resonance imaging (MRI) sensitive to subtle differences in anatomy. In this chapter we will consider the basic factors that affect the MR signal and determine the contrast characteristics of an image. How this signal is mapped to produce an MR image is taken up in Chapter 10, but it is helpful to remember that an MR image is essentially a snapshot of the distribution of the MR signal, and we will illustrate some of the signal characteristics with images. The flexibility of MRI comes about largely because the MR signal depends on a number of tissue properties. The utility of MRI for clinical studies stems from the variability of the relaxation times between one tissue type and another and between healthy and diseased tissue. The important questions for clinical imaging then tend to focus around issues of static signal contrast, and pulse sequences for best emphasizing the tissue differences (Hendrick, Nelson, and Hendee, 1984). For functional imaging, however, physiological changes such as altered blood oxygenation have more subtle effects on the MR signal, and the goal is to detect dynamic changes in the signal over time.
In the following sections, the sources of contrast in an MR image are discussed in terms of how the intrinsic nuclear magnetic resonance (NMR) properties of the tissue, such as the relaxation times and the proton density, interact with different pulse sequence parameters to affect the signal intensity.
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- Information
- Introduction to Functional Magnetic Resonance ImagingPrinciples and Techniques, pp. 155 - 184Publisher: Cambridge University PressPrint publication year: 2002