Electricity: measuring and testing – Particle precession resonance – Using a nuclear resonance spectrometer system
Reexamination Certificate
1999-05-14
2001-10-23
Oda, Christine (Department: 2862)
Electricity: measuring and testing
Particle precession resonance
Using a nuclear resonance spectrometer system
C324S311000, C324S307000, C324S300000
Reexamination Certificate
active
06307368
ABSTRACT:
BACKGROUND OF THE INVENTION
This invention relates generally to magnetic resonance imaging, and more particularly the invention relates to spectrally selective steady-state free precession (SSFP) imaging using magnetic resonance imaging.
Magnetic resonance imaging (MRI), is a non-destructive method for the analysis of materials and represents a new approach to medical imaging. It is completely non-invasive and does not involve ionizing radiation. In very general terms, nuclear magnetic moments are excited at specific spin precession frequencies which are proportional to the local magnetic field. The radio-frequency signals resulting from the precession of these spins are received using pickup coils. By manipulating the magnetic fields, an array of signals is provided representing different regions of the volume. These are combined to produce a volumetric image of the nuclear spin density of the body.
Briefly, a strong static magnetic field is employed to line up atoms whose nuclei have an odd number of protons and/or neutrons, that is, have spin angular momentum and a magnetic dipole moment. A second RF magnetic field, applied as a single pulse traverse to the first, is then used to pump energy into these nuclei, flipping them over, for example to 90° or 180°. After excitation, the nuclei gradually return to alignment with the static field and give up the energy in the form of weak but detectable free induction decay (FID). These FID signals are used by a computer to produce images.
Referring to the drawings,
FIG. 1A
is a perspective view partially in section illustrating coil apparatus in an NMR imaging system, and
FIGS. 1B-1D
illustrate field gradients which can be produced in the apparatus of FIG.
1
A. This apparatus is discussed by Hinshaw and Lent. “An Introduction to NMR Imaging: From the Bloch Equation to the Imaging Equation.” Proceedings of the IEEE, Vol. 71, No. 3, March, 198, pp. 338-350. Briefly, the uniform static field B
0
is generated by the magnet comprising the coil pair
10
. A gradient field G (x) is generated by a complex gradient coil set which can be wound on the cylinder
12
. An RF field B
1
is generated by a saddle coil
14
. A patient undergoing imaging wold be positioned along the Z axis within the saddle coil
14
.
In
FIG. 1B
an X gradient field is shown which is parallel to the static field B
0
and varies linearly with distance along the X axis but does no vary with distance along the Y or Z axes.
FIGS. 1C and 1D
are similar representation of the Y gradient and Z gradient fields, respectively.
FIG. 2
is a functional block diagram of the imaging apparatus as disclose in
NMR
-
A Perspective in Imaging,
General Electric Company, 1982. A computer
20
is programmed to control the operation of the NMR apparatus and process FID signals detected therefrom. The gradient field is energized by a gradient amplifier
22
, and the RF coils
26
. After the selected nuclei have been flipped, the RF coils
26
are employed to detect the FID signal which is passed to the receiver
28
and thence through digitizer
30
for processing by computer
20
.
Magnetic resonance imaging is a major noninvasive method of medical diagnosis. However, two limitations against extending clinical applicability are cost and diagnostic sensitivity. If scan time is reduced without sacrificing SNR or contrast (sensitivity), the technique becomes more cost effective. Steady state free precession (SSFP) methods permit fast imaging with increased signal, but suffer from banding artifacts due to B
0
inhomogeneity. Additionally, SSFP techniques yield an undesirably intense lipid proton signal.
In accordance with the present invention, we present two related solutions addressing these issues. In one solution called fluctuating equilibrium magnetic resonance (FEMR); a novel pulse sequence is presented that produces a magnetization that fluctuates from excitation to excitation in the steady state, thus permitting the simultaneous acquisition of multiple images with differing contrast.
In accordance with the other solution to these two shortcomings of SSFP imaging, a novel postprocessing of linear combinations of Fourier data from several SSFP sequences yielding several images upon reconstruction, each with a different contrast. We call this method linear combination SSFP (LCSSFP).
SUMMARY OF THE INVENTION
In accordance with one aspect of the invention, a fast, spectrally-selective steady-state free precession (SSFP) imaging method. Combining k-space data from SSFP sequences with certain phase schedules of radiofrequency excitation pulses permits manipulation of the spectral selectivity of the image. For example, lipid and water can be rapidly resolved. The contrast of each image depends on both T
1
, and T
2
, and relative contribution of the two relaxation mechanisms to image contrast can be controlled by adjusting the flip angle. Several applications of the technique are presented, including fast musculoskeletal imaging, brain imaging, and angiography. The technique is referred to herein as linear combinations steady-state free precession (LCSSFP).
Briefly, the banding artifact of SSFP serves as a novel tissue contrast generation mechanism. With the correct choice of sequence repetition time (TR), all spins within a certain spectral range are suppressed while all other protons yield high signal. Furthermore, by acquiring a complete Fourier dataset multiple times, each time with an SSFP sequence with a different radiofrequency (RF) phase schedule, and then linearly combining the datasets before reconstruction, the spectral profile of the resulting image intensity is modified. Specifically, the band of suppressed Larmor frequencies can be translated and broadened. Additionally, the suppression can be eliminated altogether.
Thus, by taking different linear combinations of the datasets, multiple images can then be reconstructed, each with a different tissue contrast. Two goals are thus achieved. In particular, lipid and water can be resolved based on chemical shift differences. Since conventional lipid suppression techniques have been heretofore incompatible with an SSFP sequence, new applications of steady-state techniques are possible with this method. Secondly, SSFP is rendered immune to main field inhomogeneity artifacts.
In accordance with another aspect of the invention, certain phase schedules of radiofrequency excitation pulses produce an equilibrium magnetization that fluctuates between several values, thus permitting simultaneous acquisition of several images with different contrast features. For example, fat and water images can be rapidly acquired. The effective echo time can be adjusted using the flip angle, thus providing control over the T
2
contribution to the contrast.
Briefly, the banding artifact is again exploited as a novel tissue contrast generation mechanism. With an appropriate choice of sequence repetition time, all spins within a certain band of resonance frequencies can be suppressed while all other spins yield high signal. Furthermore, certain phase cycles of the radiofrequency pulse modify this spectral profile, and produce an equilibrium magnetization that fluctuates periodically from excitation to excitation. Thus, multiple images are acquired simultaneously, each with a different tissue contrast. In particular, lipid and water can be resolved based on chemical shift difference. Since conventional fat suppression techniques are incompatible with any SSFP sequence, new applications of steady-state techniques are possible with this method. Clinical applications of the technique include musculoskeletal applications, abdominal imaging, brain imaging, and fast MR angiography.
The invention and objects and features thereof will be more readily apparent from the following description and appended claims when taken with the drawings.
REFERENCES:
patent: 3824452 (1974-07-01), Freeman
patent: 4999580 (1991-03-01), Meyer et al.
patent: 5187369 (1993-02-01), Takane et al.
patent: 5256967 (1993-10-01), Foo et al.
patent: 5347216 (1994-09-01), Foo et al.
Zur e
Nishimura Dwight G.
Pauly John M.
Vasanawala Shreyas S.
Board of Trustees of the Leland Stanford Junior University
Oda Christine
Shrivastav Brij B.
Townsend and Townsend / and Crew LLP
Woodward Henry K.
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