Biomedical Engineering Reference
In-Depth Information
not only did precessing nuclei emit a radiofrequency (rf) signal, but a radiofrequency could
also be used to control precession at the Larmor resonant frequency, and, once stopped, the
nuclei would emit a detectable RF signal at the same frequency. They won a Nobel Prize in
1951 for their work.
Interest shifted to determining composition of materials through unique frequency shifts
associated with different chemical compounds. Eventually, biological NMR experiments
were under way, and soon detailed spectral information from phosphorus, carbon, and
hydrogen nuclei were obtained. Specialized magnets were designed to accommodate parts
of the body for study.
Paul Lauterbur was one of the first to realize that images could be made using NMR
principles. He published an image of a heterogeneous object in 1973 [4]. Using the rf signals
from NMR, he was able to localize them in space by changing the magnetic field gradient.
By the mid- and late 1970s, early MR images were produced of animals and the human
body. At first, because the signals were so weak, these results were regarded as a laboratory
curiosity. In 1971, Raymond Damadian demonstrated that the relaxation constants,
T
1
and
T
2
, differed for malignant tumors and normal tissue. Peter Mansfield developed a mathe-
matical model to analyze signals from within the human body in response to a strong
magnetic field, as well as a very fast imaging method. Continuous research spurred the evo-
lution of modern MRI instruments with high signal to noise and generated a considerable
knowledge base of how to apply MRI to diagnostic imaging. Lauterbur and Mansfield
shared the 2003 Nobel Prize for medicine for their MRI discoveries.
16.3.2 Magnetic Fields and Charges
To understand how MRI works, several relevant characteristics of magnetic fields are
reviewed here, in particular, the interactions between electrical charges and magnetic fields.
Einstein pointed out that it is useful to consider electricity and electromagnetic fields as
aspects of the same energy.
Four cases will be covered, each one useful for providing insights into aspects of MRI
processes. In the first case, a magnetic field is generated when an alternating current travels
along a wire. For an infinitely long wire, the Biot-Savart law reveals that a circular or cir-
cumferential magnetic field flux is generated by the current, as shown in Figure 16.27,
m
0
I
2
B
f
¼
ð
16
:
48
Þ
p
r
B
f
r
I
FIGURE 16.27
Circular magnetic field generated by an electrical current flowing down an infinitely long wire.
