Basic Physics of MR Signal and Image Generation - Advance Image Processing in Magnetic Resonance Imaging - page 3

1.10 Acquiring MR Signals in the K-Space .....................................................30
1.10.1 K-Space Trajectories .....................................................................33
1.11 Imaging Methods.......................................................................................33
Acronyms ............................................................................................................36
References ...........................................................................................................36

1.1 INTRODUCTION

The nuclear magnetic resonance (NMR or MR) phenomenon in bulk matter was
first demonstrated by Bloch and associates [1] and Purcell and associates [2] in
1946. Since then, MR has developed into a sophisticated technique with applications
in a wide variety of disciplines that now include physics, chemistry, biology, and
medicine. Over the years, MR has proved to be an invaluable tool for molecular
structure determination and investigation of molecular dynamics in solids and
liquids. In its latest development, application of MR to studies of living systems
has attracted considerable attention from biochemists and clinicians alike. These
studies have progressed along two parallel and perhaps complementary paths. First,
MR is used as a spectroscopic method to provide chemical information from
selected regions within an object (magnetic resonance spectroscopy [MRS]). Such
information from a localized area in living tissue provides valuable metabolic data
that are directly related to the state of health of the tissue and, in principle, can be
used to monitor tissue response to therapy. In the second area of application, MR
is used as an imaging tool to provide anatomic and pathologic information.
The rapid progress of MR to diverse fields of study can be attributed to the
development of pulse Fourier transform techniques in the late 1960s [3]. Additional
impetus was provided by the development of fast Fourier transform algorithms,
advances in computer technology, and the advent of high-field superconducting
magnets. Then, the introduction of new experimental concepts such as two-dimen-
sional MR has further broadened its applications [4] to the magnetic resonance
imaging (MRI) technique.
MRI is a tomographic imaging technique that produces images of internal phys-
ical and chemical characteristics of an object from externally measured MR signals.
Tomography is an important area in the ever-growing field of imaging science.
The Greek term ) means “cut,” but tomography is concerned with

creating images of the internal (anatomical or functional) organization of an object
without physically cutting it open.
Image formation using MR signals is made possible by the spatial information
encoding principles, originally named

tomos ( τοµοσ

[4,5]. As it will be briefly
described in this chapter, these principles enable one to uniquely encode spatial
information into the activated MR signals detected outside an object.
As with any other tomographic imaging device, an MRI scanner outputs a
multidimensional data array (or image) representing the spatial distribution of
some measured physical quantity. But unlike many of them, MRI can generate
two-dimensional sectional images at any orientation, three-dimensional volumetric

zeugmatography