15
Magnetic Resonance
Imaging
15.1 INTRODUCTION AND OVERVIEW
Magnetic resonance imaging (MRI) is an imaging technique that makes use of the
phenomenon of nuclear spin resonance. The principle idea of MRI is based on the
fact that when a magnet is exposed to an external magnetic field, it tries to orient
itself to align with the external magnetic field. This idea explains that the spin axis
of the protons in the atoms of the biological tissues exposed to the MRI’s large
magnetic field will orient itself in a direction parallel to the magnetic field lines. In
the majority of cases, the magnetic resonance is tuned to imaging of the magnetic
spin of the hydrogen nuclei in water molecules. Measuring the spin information for
a group of molecules provides the means of creating an MR image of the biological
tissue under study.
The possibility of using the nuclear magnetic resonance (NMR) was discovered
in the 1940s when the discovery was made that certain substances absorb and emit
radiofrequency electromagnetic radiation when placed in an alternating magnetic
field. The initial applications were in the molecular analysis of the chemical and
physical properties of single-element liquids or solids. In the early 1970s, the biological applications of NMR imaging were first proposed by Raymond V. Damadian. He
recognized a difference in nuclear magnetic relaxation times of healthy and cancerous tissues. This was the first indication that MRI could be applied to collect information about the tissue physiology.
Further improvements were incorporated by the chemist Paul Lauterbur. His
technique was still able to generate only two-dimensional (2-D) images. Later
on, Peter Mansfield extended MRI capabilities by introducing the techniques to
produce a full three-dimensional (3-D) rendering of biological tissues. The latest major discovery in MRI was functional magnetic resonance imaging (f MRI).
Unlike MRI that focuses on the anatomical details of the tissue, f MRI is designed
to create images showing the functional activities of the tissue. For instance, f MRI
techniques measure the blood oxygenation levels in the brain to monitor the brain
functions.
Since the discovery of MRI, this technology has been used for very many medical applications. Due to the resolution of MRI and the fact that as far as we know
this technology is essentially harmless, MRI has emerged as the most accurate and
desirable imaging technology. It is anticipated that as the price of MRI technology
reduces, many currently used technologies such as x-ray CT scans will be replaced
283
Magnetic Resonance
Imaging
15.1 INTRODUCTION AND OVERVIEW
Magnetic resonance imaging (MRI) is an imaging technique that makes use of the
phenomenon of nuclear spin resonance. The principle idea of MRI is based on the
fact that when a magnet is exposed to an external magnetic field, it tries to orient
itself to align with the external magnetic field. This idea explains that the spin axis
of the protons in the atoms of the biological tissues exposed to the MRI’s large
magnetic field will orient itself in a direction parallel to the magnetic field lines. In
the majority of cases, the magnetic resonance is tuned to imaging of the magnetic
spin of the hydrogen nuclei in water molecules. Measuring the spin information for
a group of molecules provides the means of creating an MR image of the biological
tissue under study.
The possibility of using the nuclear magnetic resonance (NMR) was discovered
in the 1940s when the discovery was made that certain substances absorb and emit
radiofrequency electromagnetic radiation when placed in an alternating magnetic
field. The initial applications were in the molecular analysis of the chemical and
physical properties of single-element liquids or solids. In the early 1970s, the biological applications of NMR imaging were first proposed by Raymond V. Damadian. He
recognized a difference in nuclear magnetic relaxation times of healthy and cancerous tissues. This was the first indication that MRI could be applied to collect information about the tissue physiology.
Further improvements were incorporated by the chemist Paul Lauterbur. His
technique was still able to generate only two-dimensional (2-D) images. Later
on, Peter Mansfield extended MRI capabilities by introducing the techniques to
produce a full three-dimensional (3-D) rendering of biological tissues. The latest major discovery in MRI was functional magnetic resonance imaging (f MRI).
Unlike MRI that focuses on the anatomical details of the tissue, f MRI is designed
to create images showing the functional activities of the tissue. For instance, f MRI
techniques measure the blood oxygenation levels in the brain to monitor the brain
functions.
Since the discovery of MRI, this technology has been used for very many medical applications. Due to the resolution of MRI and the fact that as far as we know
this technology is essentially harmless, MRI has emerged as the most accurate and
desirable imaging technology. It is anticipated that as the price of MRI technology
reduces, many currently used technologies such as x-ray CT scans will be replaced
283