MRI System Using Nonuniform Magnetic Fields

Patent metadata

Field	Value
Publication number	US-2020003856-A1
Application number	US-201716465389-A
Country	US
Kind code	A1
Filing date	Dec 6, 2017
Priority date	Dec 6, 2016
Publication date	Jan 2, 2020
Grant date	—

Abstract

A method for magnetic resonance imaging uses an electromagnet [304], which may have open geometry, to generate a spatially nonuniform magnetic field within an imaging region [306]. The current through the electromagnet is controlled to repeatedly cycle the nonuniform magnetic field between a high strength for polarizing spins and a low strength for spatial encoding and readout. Using RF coils [308], excitation pulses are generated at a frequency that selects a non-planar isofield slice for imaging. The RF coils are also used to generate refocusing pulses for imaging and to generate spatial encoding pulses, which may be nonlinear. Magnetic resonance signals originating from the selected non-planar isofield slice of the nonuniform magnetic field in the imaging region [306] are detected using the RF coils [308] in parallel receive mode. MRI images are reconstructed from the parallel received magnetic resonance signals, e.g., using algebraic reconstruction.

First claim

1 . A method for magnetic resonance imaging, the method comprising:
using an electromagnet to generate a spatially nonuniform magnetic field within an imaging region; controlling current through the electromagnet to repeatedly cycle the nonuniform magnetic field between a first strength for polarizing spins and a second strength, lower than the first strength, for spatial encoding and readout; using RF coils to generate excitation pulses at a frequency that selects a non-planar isofield slice for imaging, and to generate refocusing pulses for imaging; using the RF coils to generate spatial encoding pulses, and using the RF coils in parallel receive mode to detect magnetic resonance signals originating from the selected non-planar isofield slice of the nonuniform magnetic field in the imaging region; and reconstructing MRI images from the parallel received magnetic resonance signals.

2 . The method of claim 1 wherein the nonuniform magnetic field has a spatial variation of more than 5 ppm within the imaging region.

3 . The method of claim 1 wherein the electromagnet is an open-geometry electromagnet that extends around the imaging region by no more than 270 degrees.

4 . The method of claim 1 wherein the first strength for polarizing spins is at least 0.2 T and the second strength for spatial encoding and readout is at most 0.1 T.

5 . The method of claim 1 wherein the first strength is at least 0.6 T and the second strength for spatial encoding and readout is at most 0.1 T.

6 . The method of claim 1 wherein the spatial encoding is nonlinear spatial encoding.

7 . The method of claim 1 wherein the spatial encoding is generated via the Bloch-Siegert shift or other spatial encoding pulses.

8 . The method of claim 1 wherein the RF coils comprise a phased array of coils.

9 . The method of claim 1 wherein using RF coils to generate refocusing pulses comprises inserting 180-degree RF pulses to refocus the effects of the residual static gradient fields.

10 . The method of claim 1 wherein using the RF coils to generate spatial encoding comprises using a first subset of the RF coils, and using RF coils to select a non-planar isofield slice and using the RF coils in parallel receive mode to detect magnetic resonance signals uses a second subset of the RF coils, where the first subset and second subset contain no common coils.

11 . The method of claim 1 wherein using the RF coils to generate spatial encoding comprises using a first subset of the RF coils, using RF coils to select a non-planar isofield slice comprises using a second subset of the RF coils, and using the RF coils in parallel receive mode to detect magnetic resonance signals uses a third subset of the RF coils, where the first subset, second subset, and third subset contain no common coils.

12 . The method of claim 1 wherein reconstructing MRI images from the parallel received magnetic resonance signals uses algebraic reconstruction.

Assignees

• Univ Yale

Inventors

• Constable Robert T
• Wan Yuqing

Classifications

• G01R33/02

  Measuring direction or magnitude of magnetic fields or magnetic flux (G01R33/20 takes precedence) · CPC title

• G01R33/24

  for measuring direction or magnitude of magnetic fields or magnetic flux · CPC title

• G01R33/445 Primary

  MR involving a non-standard magnetic field B0, e.g. of low magnitude as in the earth's magnetic field or in nanoTesla spectroscopy, comprising a polarizing magnetic field for pre-polarisation, B0 with a temporal variation of its magnitude or direction such as field cycling of B0 or rotation of the direction of B0, or spatially inhomogeneous B0 like in fringe-field MR or in stray-field imaging · CPC title

• G01R33/287

  involving active visualization of interventional instruments, e.g. using active tracking RF coils or coils for intentionally creating magnetic field inhomogeneities · CPC title

• G01R33/3808

  Magnet assemblies for single-sided MR wherein the magnet assembly is located on one side of a subject only; Magnet assemblies for inside-out MR, e.g. for MR in a borehole or in a blood vessel, or magnet assemblies for fringe-field MR · CPC title