Information Technology Reference
In-Depth Information
4 Magnetic Resonance Imaging (MRI)
Nuclear magnetic resonance (NMR) initially found widespread use in analyzing the
spectral characteristics of organic compounds, based on their magnetic properties.
In September of 1971, Paul Lauterbur had an idea for the application of three
dimensional magnetic
first images of
two spatially separate tubes containing water was published in Nature in March of
1973 [
6
]. This quantum advance of the invention of instruments to examine
material characteristics in a spatially distributed manner, led to the design of the
magnetic resonance imaging (MRI) scanner. Body tissues have intrinsic and
varying magnetic properties. These characteristics are exploited in MRI to create
spatially distributed images of the internal anatomy of the body, which represent
cross sectional anatomic maps of the magnetic characteristics of the tissues at each
spatial point in the regions being imaged.
As heuristic model for magnetic properties of tissue and the basis of MRI,
consider a magnetic dipole, or for a more physically conceptual macroscopic
model, a bar magnet. Recall that magnetic
field gradients to produce NMR images. The
field lines emanate from the north pole of
the bar magnet, with the south pole of the magnet acting as a sink for magnetic
eld
lines. A characteristic quantity associated with the magnetic dipole is the magnetic
moment, which is expressed with the magnetic
field strength and orientation of the
dipole. At the atomic level, the nuclei of atoms constituting the tissue are made up
of neutrons and protons. These nucleons individually have small magnetic
moments, and when they occur unpaired in the nucleus they give the nucleus a net
magnetic moment. The hydrogen atom, with its unpaired proton nucleus is a
constituent many of molecules in the body including water and fatty tissues, and is
of keystone importance in most current clinical imaging applications.
Now, when a macroscopic
“
”
magnetic dipole
is placed into an external magnetic
field (as in the
conceptual macroscopic analogy of a directional compass needle). Energy must be
applied to turn the compass needle (
“
dipole
”
) into a different direction, or higher
energy state. Now, as we are on the atomic scale, quantum considerations come into
play, and the dipole moment actually precesses about the direction of the applied
field, it tends toward its lowest energy state, aligning with that
field. The dipole moment of the proton nucleus will then tend to align with an
applied magnetic
field within quantum limits. The application of energy is achieved
by the application of an electromagnetic energy in the form of a radiofrequency
pulse [
1
]. This energy pulse is absorbed by the tissues, with resultant rotation of the
magnetic moments into a higher energy state. The magnetic moments then give off
energy as they relax toward lower energy states, at rates related to their local
molecular environment, or tissue characteristics. Spatial variances in this relaxation
rate by tissue type are detected and used as a basis for creating a spatial map of the
magnetic properties of body tissue cross sections, or magnetic resonance images.
Armed with an understanding of the conceptual basis for MR imaging, we now
consider the clinical application. MR imaging is performed by placing the patient
into the center of a large magnet, typically structured in the overall geometry of a
