Creation and use of a moving reference frame for NMR imaging of flow
First Claim
1. A method for measuring movement of nuclei in a sample using a nuclear magnetic resonance technique, comprising the steps of:
- exciting a nuclear magnetic resonance signal;
measuring said nuclear magnetic resonance signal emitted by said nuclei; and
determining the nuclei moving in a given range of velocities;
wherein a moving reference frame adjustment parameter is effectively varied over time to provide a moving reference frame to which the motion of said nuclei is effectively compared in said measuring and determining steps so as to cause nuclei moving with the reference frame to have an NMR response substantially the same as the nuclei would have if both the reference frame and the nuclei were stationary; and
wherein resolution of velocity is independent of the thickness, in the direction of motion of the reference frame, of a volume in which nuclei are excited and from which the NMR response is sampled.
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Abstract
The present invention provides an NMR image in which the individual image element intensities are proportional to the amount of nuclei flowing within a window of velocities. The invention is predicated on the fact that static nuclei in a static reference frame provide the most intense portions of the NMR signal. A moving reference frame for image creation is created moving at the velocity of the nuclei to be imaged. Nuclei moving at the same velocity as the reference frame thus have zero velocity with respect to the reference frame, and hence provide maximum signal intensity. In the described embodiment, the moving reference frame is created by satisfaction of two conditions, which relate to tracking the Larmor frequency and phase of the nuclei of interest moving in a gradient. These conditions may be satisfied by variation of the main NMR magnetic field Ho as a function of time. The field may be varied simply by addition of an additional coil to conventional NMR equipment. The invention is applicable to image formation using either steady state free precession techniques or conventional spin echo techniques.
41 Citations
33 Claims
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1. A method for measuring movement of nuclei in a sample using a nuclear magnetic resonance technique, comprising the steps of:
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exciting a nuclear magnetic resonance signal; measuring said nuclear magnetic resonance signal emitted by said nuclei; and determining the nuclei moving in a given range of velocities; wherein a moving reference frame adjustment parameter is effectively varied over time to provide a moving reference frame to which the motion of said nuclei is effectively compared in said measuring and determining steps so as to cause nuclei moving with the reference frame to have an NMR response substantially the same as the nuclei would have if both the reference frame and the nuclei were stationary; and wherein resolution of velocity is independent of the thickness, in the direction of motion of the reference frame, of a volume in which nuclei are excited and from which the NMR response is sampled.
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2. A method of measuring the velocity of flow of a fluid, comprising the steps of:
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selecting a particular fluid velocity range to be studied; applying a radio frequency excitation signal to said fluid; applying a time varying magnetic field to a sample, said time varying magnetic field varying at a rate such that a reference frame moving at a velocity equal to the velocity of the fluid to be studied is created so as to cause nuclei moving with the reference frame to have an NMR response substantially the same as the nuclei would have if both the reference frame and the nuclei were stationary, wherein resolution of velocity is independent of the thickness, in the direction of motion of the reference frame, of a volume in which nuclei are excited and from which the NMR response is sampled; detecting a nuclear magnetic resonance (NMR) signal from said sample; and using said NMR signal to generate an image of fluid flowing in said velocity range, whereby static nuclei are prevented from making as great a contribution to the NMR signal as the nuclei flowing in the velocity range to be studied.
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3. A method for selectively imaging flowing nuclei using nuclear magnetic resonance (NMR) techniques, comprising the steps of:
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selecting a velocity for study; defining a frame of reference moving with a velocity substantially equal to the velocity to be studied so as to cause nuclei moving with the reference frame to have an NMR response substantially the same as the nuclei would have if both the reference frame and the nuclei were stationary; and applying successive pulses of radio frequency excitation energy and recording an NMR signal emitted by excited nuclei in intervals between applications of said radio frequency energy; wherein resolution of velocity is independent of the thickness, in the direction of motion of the reference frame, of a volume in which nuclei are excited and from which the NMR response is sampled.
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4. A method for measuring movement of flowing nuclei using nuclear magnetic resonance (NMR) techniques, comprising the steps of:
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selecting a velocity range for study; defining a reference frame moving with a velocity within the velocity range for study; exciting nuclei so as to cause nuclei moving with the reference frame to have an NMR response substantially the same as the nuclei would have if both the reference frame and the nuclei were stationary; and detecting the NMR response; wherein resolution of velocity is independent of the thickness, in the direction of motion of the reference frame, of a volume in which nuclei are excited and from which the NMR response is sampled. - View Dependent Claims (5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25)
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26. An apparatus for measuring movement of flowing nuclei using nuclear magnetic resonance (NMR) techniques, comprising:
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means for selecting a velocity range for study; means for defining a reference frame moving with a velocity within the velocity range for a study; means for exciting nuclei so as to cause nuclei moving with the reference frame to have an NMR response substantially the same as the nuclei would have if both the reference frame and the nuclei were stationary; and means for detecting the NMR response; wherein resolution of velocity is independent of the thickness, in the direction of motion of the reference frame, of a volume in which nuclei are excited and from which the NMR response is sampled. - View Dependent Claims (27, 28, 29, 30, 31, 32, 33)
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Specification