Electricity: measuring and testing – Particle precession resonance – Using a nuclear resonance spectrometer system
Reexamination Certificate
2002-12-11
2004-10-19
Shrivastav, Brij B. (Department: 2859)
Electricity: measuring and testing
Particle precession resonance
Using a nuclear resonance spectrometer system
C324S307000
Reexamination Certificate
active
06806709
ABSTRACT:
CROSS-REFERENCES TO RELATED APPLICATIONS
NOT APPLICABLE
REFERENCE TO A “SEQUENCE LISTING,” A TABLE, OR A COMPUTER PROGRAM LISTING APPENDIX SUBMITTED ON A COMPACT DISK.
NOT APPLICABLE
BACKGROUND OF THE INVENTION
This invention relates generally to magnetic resonance imaging (MRI), and more particularly, the invention relates to flow imaging using a phase contrast Steady State Free Precession MRI sequence.
Magnetic resonance imaging (MRI) requires placing an object to be imaged in a static magnetic field, exciting nuclear spins in the object within the magnetic field, and then detecting signals emitted by the excited spins as they precess within the magnetic field. Through the use of magnetic gradient and phase encoding of the excited magnetization, detected signals can be spatially localized in three dimensions.
FIG. 10A
is a perspective view partially in section illustrating conventional coil apparatus in an NMR imaging system, and
FIGS. 10B-10D
illustrate field gradients which can be produced in the apparatus of FIG.
10
A. This apparatus is discussed by Hinshaw and Lent “An Introduction to NMR Imaging: From the Block Equation to the Imaging Equation.”
Proceedings of the IEEE
, Vol. 71, No. 3, March 1983, 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 would be positioned within the saddle coil
14
.
In
FIG. 10B
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 ideally does not vary with distance along the Y or Z axes.
FIGS. 10C and 10D
are similar representation of the Y gradient and Z gradient fields, respectively.
FIG. 11
is a functional block diagram of conventional imaging apparatus as disclosed in NMR-A Perspective in Imaging, General Electric company. 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
for impressing an rf magnetic moment at the Larmor frequency are controlled by the transmitter
24
. After the selected nuclei have been flipped, the rf coil
26
is employed to detect the FID signal which is passed to the receiver
28
and thence through digitizer
30
for processing by computer
20
.
Investigation of blood flow in the heart and vessels can provide insight in the function of the cardiovascular system. Magnetic resonance imaging with multidirectional CINE velocity mapping can be used to study relationships between aortic and left ventricular blood flow patterns and the geometry of the thoracic aortic aneurysms and grafts. Recognizable altered flow patterns were found to be associated with altered vessel geometry. The results of velocity mapping of aortic wall motion as well as pulse wave velocities can be combined with distensibility and stiffness index in order to find potential correlations of these parameters.
Fully balanced Steady State Free Precession (SSFP) imaging has recently gained increased importance due to its high signal-to-noise ratio (SNR). The gradient waveforms have zero net area in each repetition time (TR) interval and are first order moment nulled along read and slice direction at each rf excitation.
BRIEF SUMMARY OF THE INVENTION
The inventors recognized, however, that during data acquisition, SSFP sequences are motion sensitive and the MR-signal exhibits a motion related phase depending on the first moments of the gradient activity between rf-excitation and data readout. Flow or motion quantification can thus be accomplished but, as in conventional phase contrast (PC) MRI, a method to remove other phase shifts (e.g. due to off-resonance) is needed. In accordance with the invention, a novel technique for velocity measurements (PC-SSFP) combines CINE phase contrast (PC) MRI and balanced Steady State Free Precession. Sensitivity to through plane velocities is established by inverting (i.e. negating) the slice select gradient for consecutively executed balanced SSFP pulse sequences. Velocity sensitivity (venc) can be adjusted by increasing the first moments of the slice select gradients. Comparison of measurements on phantoms with those from established 2D CINE MRI demonstrated excellent correlation between both modalities. Advantages of PC-SSFP include the intrinsic high signal to noise ratio of balanced SSFP and consequently low phase noise in encoded velocities.
Velocity encoding in the slice direction is performed by data acquisition with two SSFP sequences which differ only in the sign of all gradients along the slice selection direction. First moments M
1
+
and M
1
−
associated with the slice selection gradients are thus altered accordingly. As a result phase difference images &Dgr;&phgr; can be calculated which are directly related to the velocity v
2
. Similarly, velocity in the readout direction can be encoded by performing two sequences which differ only in the sign of all the gradients along the readout direction. Velocity effects in the phase encoded direction can be achieved by applying a bipolar lobe in this direction prior to readout whose effect is undone by a bipolar lobe after readout. Velocity encoding in the phase encoded direction is achieved by data acquisition with two SSFP sequences which differ only in the amplitude or sign of the bipolar lobes.
Different velocity sensitivities in the slice select direction can be realized by lengthening the plateau of the slice selection gradient and adjusting refocusing and prefocussing gradients accordingly while keeping the rf-pulse width constant and thus keeping TR at a minimum, or by changing the rf-bandwidth and gradient strength. Similarly, different velocity sensitivities in the readout direction can be achieved by controlling the length and amplitude of the gradients in this direction in conjunction with appropriate control of the readout bandwidth. Different velocity sensitivities in the phase encoded direction are achieved in a straight forward manner by control of the bipolar lobes.
The invention and objects and features thereof will more readily apparent from the following description and appended claims when taken with the drawings.
REFERENCES:
patent: 4973906 (1990-11-01), Bernstein
patent: 5225779 (1993-07-01), Parker et al.
patent: 5391989 (1995-02-01), Takane et al.
Markl et al., “Balanced Phase Contrast Steady State Free Precession (PC-SSFP): Velocity Encoding by Gradient Inversion,” ISMRM, May 2002.
Grant Proposal: Kyle Mann Grant, “Assessment fo Morphology and Function in Patients with Marfan Syndrome with Magnetic Resonance Imaging (MRI)” Unpublished.
Fellowship application, Applications for a DFG (Deutsche Forschungsgemeinschaft) postdoctoral fellowship, filed in Germany, Jun. 2000, not publicly available per German Funding Agency based on information and belief of Michael Markl, Applicant.
Markl et al., “Balanced Phase Contrast Steady State Free Precession (PC-SSFP): Velocity Encoding by Gradient Inversion,” Magnetic Resonance in Medicine 49: 945-952 (2003).
Alley Marcus
Markl Michael
Pelc Norbert J.
Beyer Weaver & Thomas LLP
Shrivastav Brij B.
The Board of Trustees of the Leland Stanford Junior University
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