Electricity: measuring and testing – Particle precession resonance – Using a nuclear resonance spectrometer system
Reexamination Certificate
2000-11-06
2002-06-11
Arana, Louis (Department: 2862)
Electricity: measuring and testing
Particle precession resonance
Using a nuclear resonance spectrometer system
C324S300000
Reexamination Certificate
active
06404198
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates generally to the field of magnetic resonance imaging (MRI). More particularly, the present invention relates to the field of MRI chemical-shift excitation.
BACKGROUND OF THE INVENTION
In a typical magnetic resonance imaging MRI) system, a subject such as a human body is placed in a static magnetic field such that selected nuclear magnetic dipoles of the subject preferentially align with the magnetic field. The MRI system then applies radio frequency (RF) pulsed magnetic fields to cause magnetic resonance of the preferentially aligned dipoles and detects RF magnetic resonance (MR) signals from the resonating dipoles for reconstruction into an image representation. The MRI system typically scans the region to be imaged by applying RF pulse sequences to the subject while imposing time-varying magnetic field gradients with the static magnetic field.
In imaging most tissues with MRI, the hydrogen protons from water are preferably detected as most soft tissues are composed of greater than approximately eighty percent water. Unfortunately, fat is also largely composed of hydrogen protons and may therefore appear as an unwanted or unnecessary component in many hydrogen MR images. A variety of methods have been developed to help eliminate the effect of fat magnetization from hydrogen MR images and thereby improve the contrast between normal and pathologic tissue in a variety of anatomic locations such as, for example, the liver and pancreas, the orbits, the breast, bone marrow, and the coronary arteries. Water excitation methods apply an RF pulse sequence to tip water magnetization and not fat magnetization for detection. Fat suppression methods apply an RF pulse sequence to tip fat magnetization and not water magnetization, eliminate the fat magnetization, and then excite the water magnetization for detection. Such methods are able to tip water and fat magnetization in a selective manner because of the chemical shift difference in resonant frequency between water protons and protons in the methylene (—CH
2
) groups of fat molecules.
The chemical shift difference between two chemical species in which excitation of one and elimination of the other is desired is given by &dgr; in parts per million (ppm). For water and fat protons, the chemical shift difference is approximately 3.5 ppm in accordance with the following equations:
ω
water
=
γ
B
0
=
~
2
π
(
64.05
megaHertz
(
MHz
)
)
at
B
0
=
~
1.5
Tesla
or
=
~
2
π
(
8.5
MHz
)
at
B
0
=
~
0.2
Tesla
Δ
ω
water
-
fat
=
~
2
π
(
64.05
MHz
)
δ
=
~
2
π
(
224
Hz
)
at
B
0
=
~
1.5
Tesla
or
=
~
2
π
(
8.5
MHz
)
δ
=
~
2
π
(
29.75
Hz
)
at
B
0
=
~
0.2
Tesla
where &ohgr; is the Larmor frequency of the nuclei of interest, &ggr; is the gyromagnetic ratio of the nuclei of interest, and B
0
is the applied static magnetic field.
One common fat suppression method applies binomial sets of RF pulses at specific amplitudes and specific interpulse intervals to tip fat magnetization into the transverse or detection plane while restoring water magnetization to the longitudinal axis. The amplitudes of the RF pulses are set such that their sum is approximately zero when observed by a water molecule (i.e., on resonance), and the duration of each interpulse interval is set, for example, to &pgr;/&Dgr;&ohgr;=~1/(448 Hz) at B
0
=1.5 Tesla=~2.2 milliseconds (ms) such that the water and fat protons precess by approximately 180° or &pgr; radians with respect to one another. Once in the detection plane, the fat magnetization may be spoiled or destroyed. A selective RF pulse may then be applied to tip the remaining longitudinal magnetization into the detection plane. As the time interval between the tipping of fat magnetization into the detection plane and spoiling is relatively short, the remaining longitudinal magnetization tipped by the selective RF pulse is substantially all water magnetization. Exemplary prior art binomial RF pulse sequences include 1-(-1), 1-(-2)-1, and 1-(-3)-3-(-1) sequences.
The application of a prior art binomial 1-(-1) RF pulse sequence for fat suppression is illustrated in graph form in
FIGS. 1A
,
1
B,
1
C,
1
D, and
1
E. As illustrated in
FIG. 1A
, water magnetization
11
and fat magnetization
12
are initially aligned with the static magnetic field B
0
along the z-axis at equilibrium. A first RF pulse in the 1-(-1) sequence tips both water magnetization
11
and fat magnetization
12
by approximately 45° as illustrated in FIG.
1
B. During an interpulse interval of approximately 2.2 ms for B
0
=~1.5 Tesla, fat magnetization
12
precesses by rotating approximately 180° about the z-axis such that water magnetization
11
and fat magnetization
12
are approximately 180° out of phase as illustrated in
FIG. 1C. A
second RF pulse in the 1-(-1) sequence tips both water magnetization
11
and fat magnetization
12
by approximately −45°, restoring water magnetization
11
to the z-axis while tipping fat magnetization
12
into the detection plane as illustrated in FIG.
1
D. Fat magnetization
12
is then spoiled by a magnetic field gradient pulse as illustrated in
FIG. 1E
, and water magnetization
11
may then be tipped from the z-axis into the detection plane by a selective RF pulse.
Adding more RF pulses in a binomial sequence helps improve the spectral width of the saturation in an inhomogeneous magnetic field. At B
0
=~1.5 Tesla, a binomial 1-3-3-1 RF pulse sequence, for example, may be used for fat suppression.
Applying binomial sets of RF pulses in relatively lower magnetic fields, however, incurs relatively longer repetition times TR and therefore scan times as the duration of each interpulse interval is inversely proportional to the strength of the magnetic field B
0
. At B
0
=~0.2 Tesla, for example, a binomial RF pulse sequence requires an approximately 16.8 ms interpulse interval as compared to the approximately 2.2 ms interpulse interval required at B
0
=~1.5 Tesla. For longer pulse sequences that are required for adequate suppression in an inhomogeneous magnetic field, the time penalty incurred is too great for many imaging applications. A binomial 1-3-3-1 RF pulse sequence, for example, requires approximately 50 ms in total interpulse interval time at B
0
=~0.2 Tesla.
Also, the effectiveness of binomial RF pulse sequences in suppressing fat may be compromised in relatively lower magnetic fields as the relatively longer interpulse intervals together with the reduced relaxation time T
1
for fat in the lower magnetic field allow significant fat magnetization regrowth. Relatively longer interpulse intervals also allow greater water magnetization decay as determined by the relaxation time T
2
for water.
Another common fat suppression method relies upon the regrowth of fat magnetization. Fat and water magnetization regrow at different rates as determined by their respective relaxation times T
1
. Following application of an inverting RF pulse, regrown magnetization will effectively cancel the inverted magnetization after a certain time period TI=ln(2)*T
1
=~0.693*T
1
.
The application of a prior art inversion recovery RF pulse sequence for fat suppression is illustrated in graph form in
FIGS. 2A
,
2
B, and
2
C. As illustrated in
FIG. 2A
, water magnetization
21
and fat magnetization
22
are initially aligned with the static magnetic field B
0
along the z-axis at equilibrium. An inverting RF pulse tips both water magnetization
21
Duerk Jeffrey L.
Lewin Jonathan S.
Wendt Michael
Wu Dee H.
Arana Louis
Case Western Reserve University
Fay Sharpe Fagan Minnich & McKee LLP
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