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Biomedical Signal and Image Processing
sinusoidal wave train. The electromagnetic radiation pulse results in a magnetization
alignment that emits a RF pulse of its own, which is referred to as the free induction
decay (FID). This pulse can be used to identify the magnetic characteristics of the
tissue under investigation.
Before discussing the image formation in MRI, we briefly describe the chemical
elements that can be used as the basis for MRI. Every element with an odd number
of protons produces a small magnetic field around itself. The MRI technology uses
the nuclear spin of atoms with hydrogen ( 1 H) as the most common element. Other
odd number elements that can be used in NMR imaging are sodium ( 23 Na), fluorine
( 19 F), carbon ( 13 C), phosphorus ( 31 P), potassium (K), and lithium (Li). Each atom has
a nuclear spin, which is proportional to the mass and therefore the angular momentum of the nucleus. Even though the charges in larger atoms still respond to the
external magnetic field, their relatively large mass can restrict their agility needed for
the imaging applications. The proportionality between the charge and mass is often
measured by a quantity called the gyromagnetic ratio, γ. The gyromagnetic ratio is
defined as follows:
q
g =
(15.2)
2m
where q and m stand for the charge and the mass of the particle, respectively.
Equation 15.2 is used to describe the gyromagnetic properties for hydrogen in which
m is replaced by the mass of a single proton. The γ in hydrogen is relatively large,
which is another reason supporting the use of hydrogen for MRI.
Due to the small magnitude and short duration of the FID signals generated by the
static source, the previously described process cannot produce a practically useful
image of the biological tissues. In order to create a meaningful image, another source
of magnetic field that exploits the resonance phenomenon is deployed. This secondary magnetic field, which is an alternating field, creates a resonance state that allows
a tomographic imaging of the body based on the physical principles of resonance, as
discussed next.
15.2.1 RESONANCE
The static magnetic field discussed in the previous section is typically used as a
“bias” field to activate the molecular units and prepare them to be stimulated by an
alternating external field. When an external alternating electromagnetic field that
alternates with the Larmor frequency is applied, the precession motion of the proton
spin will come into resonance, and the microscopic magnetization vector will lose
the equilibrium that it would have reached if the units were exposed only to the static
external magnetic field.
When the alternating external electromagnetic field is lifted, the protons will
realign themselves again, as the static external magnetic field is still active. The
subsequent realignment will result in emission of a powerful FID signal that can be
easily recorded. The amplitude of the FID signal burst is an indication of the quantity of the protons that are present in every point inside the tissue.
Biomedical Signal and Image Processing
sinusoidal wave train. The electromagnetic radiation pulse results in a magnetization
alignment that emits a RF pulse of its own, which is referred to as the free induction
decay (FID). This pulse can be used to identify the magnetic characteristics of the
tissue under investigation.
Before discussing the image formation in MRI, we briefly describe the chemical
elements that can be used as the basis for MRI. Every element with an odd number
of protons produces a small magnetic field around itself. The MRI technology uses
the nuclear spin of atoms with hydrogen ( 1 H) as the most common element. Other
odd number elements that can be used in NMR imaging are sodium ( 23 Na), fluorine
( 19 F), carbon ( 13 C), phosphorus ( 31 P), potassium (K), and lithium (Li). Each atom has
a nuclear spin, which is proportional to the mass and therefore the angular momentum of the nucleus. Even though the charges in larger atoms still respond to the
external magnetic field, their relatively large mass can restrict their agility needed for
the imaging applications. The proportionality between the charge and mass is often
measured by a quantity called the gyromagnetic ratio, γ. The gyromagnetic ratio is
defined as follows:
q
g =
(15.2)
2m
where q and m stand for the charge and the mass of the particle, respectively.
Equation 15.2 is used to describe the gyromagnetic properties for hydrogen in which
m is replaced by the mass of a single proton. The γ in hydrogen is relatively large,
which is another reason supporting the use of hydrogen for MRI.
Due to the small magnitude and short duration of the FID signals generated by the
static source, the previously described process cannot produce a practically useful
image of the biological tissues. In order to create a meaningful image, another source
of magnetic field that exploits the resonance phenomenon is deployed. This secondary magnetic field, which is an alternating field, creates a resonance state that allows
a tomographic imaging of the body based on the physical principles of resonance, as
discussed next.
15.2.1 RESONANCE
The static magnetic field discussed in the previous section is typically used as a
“bias” field to activate the molecular units and prepare them to be stimulated by an
alternating external field. When an external alternating electromagnetic field that
alternates with the Larmor frequency is applied, the precession motion of the proton
spin will come into resonance, and the microscopic magnetization vector will lose
the equilibrium that it would have reached if the units were exposed only to the static
external magnetic field.
When the alternating external electromagnetic field is lifted, the protons will
realign themselves again, as the static external magnetic field is still active. The
subsequent realignment will result in emission of a powerful FID signal that can be
easily recorded. The amplitude of the FID signal burst is an indication of the quantity of the protons that are present in every point inside the tissue.