Electricity: measuring and testing – Particle precession resonance – Using a nuclear resonance spectrometer system
Reexamination Certificate
2000-08-29
2002-10-15
Lefkowitz, Edward (Department: 2862)
Electricity: measuring and testing
Particle precession resonance
Using a nuclear resonance spectrometer system
C324S309000
Reexamination Certificate
active
06466014
ABSTRACT:
BACKGROUND OF THE INVENTION
The field of the invention is nuclear magnetic resonance imaging methods and systems. More particularly, the invention relates to the separation of fat and water signals in MR images produced using the Dixon method.
When a substance such as human tissue is subjected to a uniform magnetic field (polarizing field B
0
), the individual magnetic moments of the spins in the tissue attempt to align with this polarizing field, but precess about it in random order at their characteristic Larmor frequency. If the substance, or tissue, is subjected to a magnetic field (excitation field B
1
) which is in the x-y plane and which is near the Larmor frequency, the net aligned moment, M
z
, may be rotated, or “tipped”, into the x-y plane to produce a net transverse magnetic moment M
t
. An NMR signal is emitted by the excited spins, and after the excitation signal B
1
is terminated, this signal may be received and processed to form an image.
Materials other than water, principally fat, are found in biological tissue and have different gyromagnetic ratios. The Larmor frequency of protons in fat is shifted approximately 225 Hz from those of protons in water in a 1.5 Tesla polarizing magnetic field B
0
. The difference between the Larmor frequencies of such different species of the same nucleus, viz., protons, is termed chemical shift, reflecting the differing chemical environments of the two species.
Often it is desired to “decompose” the NMR image into its several chemical shift components. In the exemplary case of protons, which will be used hereafter for illustration, it may be desired to portray as separate images the water and fat components of the subject. One method of accomplishing this is to acquire two images S
0
and S
−1
with the fat and water components of the images in phase, and out of phase by &pgr; radians, respectively (the “Dixon” technique). Adding and subtracting these images provides separate fat and water images. The phase shift between the fat and water components of the images may be controlled by timing the RF pulses of the NMR sequence so that the signal from the fat image evolves in phase with respect to the water by the proper angle of exactly &pgr;, before the NMR signal is acquired.
In the ideal case above, the frequency of the RF transmitter is adjusted to match the Larmor frequency of the water. If the polarizing magnetic field B
0
is uniform, this resonance condition is achieved through out the entire subject. Similarly, the out-of-phase condition (&pgr; radians) for the fat component is achieved for all locations in the subject under homogeneous field conditions. In this case, the decomposition into the separate images is ideal in that fat is completely suppressed in the water image, and vice versa.
When the polarizing field is inhomogeneous, however, there are locations in the subject for which the water is not on resonance. In this case, the accuracy of the decomposition breaks down and the water and fat images contain admixtures of the two species. Field inhomogeneities may result from improper adjustment or shimming of the polarizing magnetic field B
0
, but are more typically the result of “demagnetization” effects caused by the variations in magnetic susceptibility of the imaged tissue, which locally distort the polarizing magnetic field B
0
. These demagnetization effects may be of short spatial extent but of conventional linear or higher order shimming techniques.
The influence of demagnetization may be accommodated, however, by a three-point Dixon imaging technique that uses three acquired images S
0
, S
1
and S
−1
, with the phase evolution times adjusted so that the fat and water components of the images are in phase, out of phase by &pgr;, and out of phase by −&pgr; respectively. The complex pixels in each of the three images after conventional reconstruction may be processed as described, for example, in U.S. Pat. No. 5,144,235 to produce a separate water and a separate fat image.
An important assumption in Dixon imaging is that the spectral composition of living tissues is made of two distinct &dgr;-peaks, one corresponding to the water proton resonance and the other corresponding to a loosely termed “fat” resonance peak. The latter is approximately 3.35 ppm, or 225 Hz at 1.5 Tesla field strength, apart from the water resonance frequency. In reality, the “fat” is composed of multiple spectral components. Table 1, lists the major spectral components of corn oil that was measured at 1.5 Tesla.
TABLE 1
Amplitude, T1 and T2 of the corn oil sample at 1.5 Tesla
(assuming water chemical shift is 4.7 ppm.)
Chemical
Freq Shift
Amplitude
T1(n)
T2(n)
Shift
From Water
No.
Component
A(n)
(ms)
(ms)
(ppm)
Signal (Hz)
1
CH
2
0.26
577
227
0.8
−250
2
(CH
2
)
n
1.00
223
107
1.2
−220
3
O═C—CH
2
CH
2
0.10
185
43
1.5
−200
4
C═C—CH
2
0.21
209
67
1.9
−180
5
O═C—CH
2
0.11
210
71
2.15
−160
6
C—CH
2
—C═
0.05
245
183
2.6
−135
7
CH
2
O(right)
0.04
237
36
3.95
−48
8
CH
2
O(left)
0.04
242
38
4.15
−35
9
CH═CH AND CHO
0.15
204
137
5.2
30
As illustrated in Table 1, the loosely-termed “fat” peak is actually composed of a series of peaks (Peaks 1-6) dominated by Peak #2 that corresponds to the methylene protons. In addition, there is actually another group of peaks (Peaks 7-9) whose frequencies fall more closely to the water resonance frequency. The signals from these protons that generate these latter peaks intermix with the “water” signals and are not separated properly by the Dixon method.
SUMMARY OF THE INVENTION
The present invention is an improved method for producing separate water and fat images. More specifically, the invention includes acquiring MRI data with an MRI system using a pulse sequence in which separate water and fat images may be reconstructed, producing a pixel shifted fat image from a reconstructed fat image which indicates fat signal components intermixed with a reconstructed water image, multiplying the pixel shifted fat image by a factor a, and subtracting the result from the water image. Fat signal components that are not separated from the water signal are emulated by the pixel shifted fat image and subtracted from the water image to remove them therefrom.
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