System and method for selecting rotation angle for reduced field of view imaging

Patent metadata

Field	Value
Publication number	US-11486952-B2
Application number	US-202017065673-A
Country	US
Kind code	B2
Filing date	Oct 8, 2020
Priority date	Oct 9, 2019
Publication date	Nov 1, 2022
Grant date	Nov 1, 2022

Abstract

In a method for determining imaging parameters for a Magnetic Resonance (MR) image, a set of image sequence parameters of the imaging sequence is determined, a frequency offset of off-resonant tissue potentially present in the object under examination is determined, an allowed maximum position shift of the off-resonant tissue along a slice selection direction is determined, a rotation angle which leads to the allowed maximum shift for the off-resonant tissue is determined based on the determined set of image sequence parameters, and the determined rotation angle is provided to the MR imaging system to allow the MR imaging system to generate the MR image using the determined rotation angle in the imaging sequence.

First claim

The invention claimed is:

1. A method for determining imaging parameters for a Magnetic Resonance (MR) image generated by an MR imaging system of an object under examination with an imaging sequence in which a part of the object under examination is imaged with a reduced field of view obtained with 2-dimensional (2D) spatially selective radio frequency (RF) excitation pulses and magnetic field gradients applied to two different directions, wherein the 2D spatially selective RF excitation pulses are tilted by a rotation angle with regard to an imaging plane where the MR image is generated, the method comprising:
determining a set of image sequence parameters of the imaging sequence;
determining a frequency offset of off-resonant tissue potentially present in the object under examination;
determining an allowed maximum position shift of the off-resonant tissue along a slice selection direction;
determining the rotation angle which leads to the allowed maximum shift for the off-resonant tissue based on the determined set of image sequence parameters; and
providing the determined rotation angle to the MR imaging system to allow the MR imaging system to generate the MR image using the determined rotation angle in the imaging sequence.

2. The method according to claim 1 , wherein determining the set of image sequence parameters comprises determining a number of one of the magnetic field gradients applied along one of the two different directions having a smaller gradient moment than the magnetic field gradients applied along the other of the two different directions, wherein the rotation angle is determined based on the number.

3. The method according to claim 2 , wherein determining the set of image sequence parameters further comprises determining a duration of one of the magnetic field gradients applied along one of the two different gradient directions having a larger gradient moment than the magnetic field gradients applied along the other of the two different directions, wherein the rotation angle is determined based on the determined duration.

4. The method according to claim 1 , wherein determining the set of image sequence parameters comprises determining a duration of one of the magnetic field gradients applied along one of the two different gradient directions having a larger gradient moment than the magnetic field gradients applied along the other of the two different directions, wherein the rotation angle is determined based on the determined duration.

5. The method according to claim 1 , wherein determining the set of image sequence parameters comprises determining an extent of a field of view of the MR image in a phase encoding direction, the rotation angle being determined based on the determined extent of the field of view.

6. The method according to claim 1 , wherein the rotation angle is determined according to the following equation:
alpha= a sin(Shift/DeltaFrequency/ N blip/DurationLine/FOVphase*BWTP)
with Shift being the allowed maximum position shift, DeltaFrequency being the frequency offset, Nblip being a number of magnetic field gradients applied along one of the two different directions having a smaller gradient moment than the magnetic field gradients applied along the other of the two different directions, DurationLine being a duration of one of the magnetic field gradients along the gradient direction having a larger gradient moment, FOVphase being the field-of-view in a phase encoding direction, and BWTP being the product of a temporal duration of at least one RF pulse of the 2D spatially selective RF excitation pulses applied during application of the magnetic field gradients applied along one of the two different directions having the smaller gradient moment and a spectral width of the at least one RF pulse.

7. The method according to claim 1 , wherein the allowed maximum position shift along the slice selection direction is determined in multiples of a slice thickness in the slice selection direction.

8. The method according to claim 1 , wherein the frequency offset is determined based on a magnetic field strength of a static polarizing magnetic field.

9. The method according to claim 1 , wherein the frequency offset is determined from a homogeneity map generated for a static polarizing magnetic field.

10. The method according to claim 1 , further comprising:
checking whether the determined rotation angle is within a predefined angle range; and
in response to the determined rotation angle being outside the predefined angle range, amending the determined rotation angle to a new rotation angle lying in the predefined angle range.

11. The method according to claim 1 , wherein determining the set of image sequence parameters comprises determining a product of: (a) a temporal duration of at least one RF pulse of the 2D spatially selective RF excitation pulses applied during an application of the magnetic field gradients applied along one of the two different directions having a smaller gradient moment than the magnetic field gradients applied along the other of the two different directions, and (b) a spectral width of the at least one RF pulse.

12. A non-transitory computer program product which includes a program and is directly loadable into a memory of the MR imaging system, when executed by a process of the MR imaging system, causes the processor to perform the method as claimed in claim 1 .

13. A non-transitory computer-readable storage medium with an executable program stored thereon, that when executed, instructs a processor to perform the method of claim 1 .

14. A Magnetic Resonance (MR) imaging system configured to determining imaging parameters for an MR image to be generated by the MR imaging system of an object under examination with an imaging sequence in which a part of the object under examination is imaged with a reduced field of view obtained with 2-dimensional (2D) spatially selective radio frequency (RF) excitation pulses and magnetic field gradients applied to two different directions, the 2D spatially selective RF excitation pulses being tilted by a rotation angle with regard to an imaging plane where the MR image is generated, the MR system comprising:
a memory that stores instructions; and
a processor configured to execute the instructions to:
determine a set of image sequence parameters of the imaging sequence;
determine a frequency offset of off-resonant tissue potentially present in the object under examination;
determine an allowed maximum position shift of the off-resonant tissue along a slice selection direction;
determine the rotation angle which leads to the allowed maximum shift for the off-resonant tissue based on the determined set of image sequence parameters; and
control the MR imaging system to generate the MR image using the imaging sequence.

Assignees

• Siemens Healthcare Gmbh

Inventors

• Benkert Thomas

Classifications

• A61B5/055

  involving electronic [EMR] or nuclear [NMR] magnetic resonance, e.g. magnetic resonance imaging · CPC title

• G01R33/4833

  using spatially selective excitation of the volume of interest, e.g. selecting non-orthogonal or inclined slices · CPC title

• G01R33/56554

  caused by acquiring plural, differently encoded echo signals after one RF excitation, e.g. correction for readout gradients of alternating polarity in EPI · CPC title

• G01R33/243

  Spatial mapping of the polarizing magnetic field · CPC title

• G01R33/5616

  using gradient refocusing, e.g. EPI · CPC title