Biomedical Engineering Reference
In-Depth Information
the beam with the surface results in signals that have different energies
and properties. This translates into a significant “depth of field” which is
not possible with TEM. SEM can have resolutions of a few nm, and are
generally used to identify surface features and topography of cells and
tissues.
E.
Magnetic resonance imaging (MRI)
MRI is a non-invasive method for imaging tissues in 3 dimensions in
living subjects. It is used for a variety of medical purposes, such as identi-
fying solid and metastatic tumors, evaluating joint structure in individuals
with autoimmune diseases or sports related injuries, or examining blood
flow in the brain, etc. The development of MRI revolutionized diagnos-
tic medicine, since many things that were only revealed in the past by
exploratory surgery can now be imaged in a non-invasive, completely
harmless manner. In addition to diagnostics, the MRI is also used as a
powerful research tool. For example, functional MRI can be used to com-
pare blood flow and brain functioning in normal subjects compared with
patients with various forms of dementia, including Alzheimer's disease,
as a means to identify and understand changes that may occur during
the development of these diseases. For their roles in the development
of MRI technology, Paul Lauterbur and Peter Mansfield were awarded
the Nobel Prize in Medicine in 2003.
The design for the MRI developed by Lauterbur and Mansfield was
directly applicable to human studies and diagnostics, and proved to be
a very workable and adaptable MRI system. However, it was not the first
MRI design. The first prototypic MRI instrument and patents were de-
veloped by Raymond Damadian, who published his work in 1971 (40),
2 years before Lauterbur (41) and Mansfield (42) published their first pa-
pers. Consequently, when the Nobel Prize for the MRI was announced,
it generated an unusual amount of controversy, more so than normally
accompany a Nobel Prize. It was the public nature of the controversy
that set this Nobel Prize apart from others. Full page advertisements
protesting the decision were taken out on behalf of Damadian, but to no
avail.
MRI is based on principles similar to NMR (see Chapter 1). The sam-
ple (i.e., person) is placed within a strong magnetic field, and a radiofre-
quency field is then passed through the sample at a 90° angel to the
magnetic field. The energy of the protons is increased when they absorb
radio energy of the frequency. Radio waves are emitted by the atomic
nuclei when they return to their previous energy level. Paul Lauterbur
realized that by introducing gradients to the magnetic fields that were
used for MRI, the source of the emitted waves could be traced, resulting
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