Iterative minimization procedure with uncompressed local sar estimate

Patent metadata

Field	Value
Publication number	US-2016128574-A1
Application number	US-201414539552-A
Country	US
Kind code	A1
Filing date	Nov 12, 2014
Priority date	Nov 12, 2014
Publication date	May 12, 2016
Grant date	—

Abstract

A method for providing an image of a subject in a magnetic resonance imaging MRI system with parallel transmission (pTx) is provided. A localizer scan of the subject is provided. Body anatomy is determined from the localizer scan. The body anatomy is matched with at least one body model of a plurality of body models. B 1 + and B o maps are calculated from the body anatomy. The electric fields from the at least one body model and constraints are used to simultaneously determine a plurality of excitation pulses for a pTx. MRI is performed on the subject using the determined excitation pulses. Global SAR is monitored in real time to ensure all time-averaged constraints are satisfied simultaneously.

First claim

What is claimed is:

1 . A method for providing an image of a subject in a magnetic resonance imaging MRI system with parallel transmission (pTx), comprising:
a) providing a localizer scan of the subject; b) determining body anatomy from the localizer scan; c) matching the body anatomy with at least one body model of a plurality of body models; d) calculating B 1 + and B o maps from the body anatomy; e) using the electric fields from the at least one body model and constraints to simultaneously determine a plurality of excitation pulses for a pTx; f) performing MRI on the subject using the determined excitation pulses; g) monitoring global SAR in real time to ensure all time-averaged constraints are satisfied simultaneously.

2 . The method, as recited in claim 1 , wherein step e) is performed as an optimization problem with a single objective term consisting of all time-averaged quantities including local SAR, global SAR, and average power and constraints on instantaneous quantities including flip angle inhomogeneity resulting from each pulse and instantaneous power applied to each channel.

3 . The method, as recited in claim 2 , wherein local SAR matrices at all voxels are included in the objective term without need for compression.

4 . The method, as recited in claim 1 , wherein more than one body model is used for SAR estimation, performed by concatenating SAR matrices from the voxels across all body models.

5 . The method, as recited in claim 2 , wherein only the magnitude of excitation and not the phase is of interest.

6 . The method, as recited in claim 2 , wherein the definition for flip angle inhomogeneity is nonconvex allowing for freedom in choice of spokes locations and target phase.

7 . The method, as recited in claim 2 , wherein the optimization problem is decomposed with the alternating direction method of multipliers algorithm resulting in two subproblems to be solved iteratively.

8 . The method, as recited in claim 7 , wherein the a first subproblem consists of an unconstrained minimization of a piecewise positive semidefinite function.

9 . The method, as recited in claim 7 , wherein a second subproblem consists of a projection operation onto the nonconvex set of feasible pulses.

10 . The method, as recited in claim 8 , wherein the first subproblem is solved with Kelley's cutting plane algorithm.

11 . The method, as recited in claim 10 , wherein the subgradient and objective value in each iteration of Kelley's cutting plane algorithm is computed with a vectorized oracle.

12 . The method, as recited in claim 11 , wherein the vectorized oracle exists on a GPU for additional acceleration.

13 . The method, as recited in claim 9 , wherein the projection operation is performed in parallel for each RF pulse.

14 . The method, as recited in claim 9 , wherein the nonconvex projection operation is accomplished through sequential convex projections onto ellipsoids with fixed k-space locations and target phases.

15 . The method, as recited in claim 1 , wherein B1+/B0 mapping on the scanner and body model selection on a workstation are performed in parallel.

16 . The method, as recited in claim 1 , wherein RF pulse design is performed in parallel with non-pTx scanning.

17 . The method, as recited in claim 1 , wherein electric fields of at least one body model of the plurality of body models has been precalculated, and wherein using the electric fields, comprises the precalculated electric fields of the at least one body model.

Assignees

• Univ Leland Stanford Junior

Inventors

• Rutt Brian K
• Pendse Mihir R

Classifications

• G01R33/5612

  Parallel RF transmission, i.e. RF pulse transmission using a plurality of independent transmission channels · CPC title

• G01R33/246

  Spatial mapping of the RF magnetic field B1 · CPC title

• G01R33/243

  Spatial mapping of the polarizing magnetic field · CPC title

• A61B5/055

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

• G01R33/288 Primary

  Provisions within MR facilities for enhancing safety during MR, e.g. reduction of the specific absorption rate [SAR], detection of ferromagnetic objects in the scanner room · CPC title