Biomedical Engineering Reference
In-Depth Information
X
Magnetic
Field
Z
Y
B0 +
Δ
B
Gradient Coils
B
0
Object
Being Imaged
RF Coil
B
o
1
2
3
ω
ω
ω
B
1
Resonant frequency/spatial position
Patient
Figure 6.1-5 Spatial position caused by magnetic field.
the echo signal. The frequency of precession u
i
for each
proton depends upon its position. Frequencies measured
from the echo are mapped to the corresponding position.
Brain scans are carried out to detect tumors, infarcts,
aneurysms, or other pathological conditions. These are
normally easy to detect since both the proton densities
and relaxation times are markedly different from those
of healthy tissue.
Gradient Coils
Figure 6.1-4 RF and gradient coils construction.
frequency of protons in a reference compound that
makes NMR possible to be measured. This application of
magnetic resonance is generally referred to as high-res-
olution NMR spectroscopy, and is widely used in the
pharmaceutical and chemical industries.
NMR imaging, better known as MRI, is one of the
most important imaging technologies found in most
modern hospitals. In MRI, it is the protons in the water
molecules of a patient's tissue that are the source of the
signal. The spatial information needed to form images
from magnetic resonance is obtained by placing magnetic
field gradient coils on the inside of the magnet. These
coils, constructed from copper wire, create additional
magnetic fields that vary in strength as a linear function of
distance along the three spatial axes (
Figure 6.1-4
). This
means that the resonant frequencies of the water protons
within the patient's body are now spatially encoded.
The chief use of clinical MRI is for imaging the brain
and spine. However, the recent development of rapid
imaging methods has extended its application to the
chest and abdomen where motion previously caused
blurring of the image. The contrast in MRI images arises
from differences in the number of protons in a given
volume and in their relaxation times (the time taken for
the magnetization of a sample to return to equilibrium
after the RF pulse is turned off), which are related to the
molecular environment of the protons (see
Figure 6.1-5
).
Following excitation, each proton within the excited
volume precesses at the same frequency. During de-
tection of the echo, a gradient
B
0
is applied causing
a variation in the frequencies for the protons generating
6.1.3 A more detailed overview
of MRI
In clinical practice, MRI has been a great tool in the
medical community. Over the years, MRI has assisted
physicians in diagnosis, treatment, and presurgical
treatments. The main advantage of MRI when compared
with other imaging tools (e.g., x-ray computed tomog-
raphy or CTscan) is that it does not require exposure of
the human body to ionizing radiation; therefore it is very
safe. The MRI signals are also very sensitive to different
parts of tissues.
6.1.3.1 The physics of spin
Spin is a fundamental property of nature like electrical
charge or mass. Spin comes in multiples of
1
2
and can be
þ
or
. Protons, electrons, and neutrons all possess spin.
Individual unpaired electrons, protons, and neutrons
each possesses a spin of
2
:
In the deuterium atom (
2
H),
with one unpaired electron, one unpaired proton, and
one unpaired neutron, the total electronic spin is equal to
1
2
and the total nuclear spin is equal to 1. Two or more
particles with spins having opposite signs can pair up to
eliminate the observable manifestations of spin. An
