Local SAR Constrained Parallel Transmission RF Pulse in Magnetic Resonance Imaging

• US 20130063143A1
• Filed: 08/31/2012
• Published: 03/14/2013
• Est. Priority Date: 09/01/2011
• Status: Abandoned Application

• Current Assignee: Massachusetts General Hospital (Mass General Brigham, Inc.), Siemens AG
• Original Assignee: Siemens AG
• Inventors: Adalsteinsson, Elfar, Gebhardt, Matthias, Wald, Lawrence L., Lee, Joonsung
• Application Number:
  US13/601,385
• Publication Number:
  US 20130063143A1
• Time in Patent Office: Days
• US Class Current: 324/307
• CPC Class Codes: G01R 33/288 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 Class Codes: G01R 33/5612 Parallel RF transmission, i.e. RF pulse transmission using a plurality of independent transmission channels

Abstract

A method of designing a parallel transmission radio frequency (RF) pulse for a magnetic resonance imaging (MRI) system includes compressing a model for a subject to be scanned by the MRI system into a plurality of virtual observation points within the model based on comparisons of peak sensitivity to local specific absorption rate (SAR), and defining the parallel transmission RF pulse that minimizes a weighted average of local SAR values with an iterative procedure that optimizes a set of weighting factors for the plurality of virtual observation points to maximize the minimized weighted average.

Claims

1. A method of designing a parallel transmission radio frequency (RF) pulse for a magnetic resonance imaging (MRI) system, the method comprising:

compressing a model for a subject to be scanned by the MRI system into a plurality of virtual observation points within the model based on comparisons of peak sensitivity to local specific absorption rate (SAR); and

defining, with a processor, the parallel transmission RF pulse for antenna of the MRI system that minimizes a weighted average of local SAR values with an iterative procedure that optimizes a set of weighting factors for the plurality of virtual observation points to maximize the minimized weighted average.

2. The method of claim 1, wherein the iterative procedure is nested within an outer iterative procedure configured to minimize a peak local SAR value for the parallel transmission RF pulse being defined.

3. The method of claim 1, wherein defining the parallel transmission RF pulses comprises determining a direction to change each weighting factor that increases the minimized weighted average after each iteration of the iterative procedure.

4. The method of claim 1, further comprising, before implementing the iterative procedure, initializing the set of weighting factors to equal values that sum to unity.

5. The method of claim 1, wherein:

the model comprises a number of voxels;

the method further comprises calculating a spatial matrix for each voxel of the model, the spatial matrix being indicative of absorption sensitivity; and

compressing the model comprises defining an upper bound matrix for finding the virtual observation points as a sum of the spatial matrix of the virtual observation point and a global SAR matrix scaled by an overestimation factor.

6. The method of claim 5, wherein compressing the model further comprises selecting the overestimation factor.

7. The method of claim 5, wherein compressing the model further comprises iteratively evaluating the voxels to determine whether the absorption sensitivity of a respective one of the voxels is upper bounded by the absorption sensitivity of at least one previously evaluated voxel.

8. A method of applying a parallel transmission radio frequency (RF) pulse in a magnetic resonance imaging (MRI) system, the model being defined via a number of voxels, the method comprising:

calculating a spatial matrix for each voxel of a model for a subject to be scanned by the MRI system, the spatial matrix being indicative of absorption sensitivity;

designating a subset of the voxels as a plurality of virtual observation points for the model by iteratively evaluating the spatial matrices of the voxels to determine whether the absorption sensitivity of a respective one of the voxels is upper bounded by a global SAR-based overestimation of the absorption sensitivity of at least one previously evaluated voxel;

defining the parallel transmission RF pulse that minimizes a weighted average of local specific absorption rate (SAR) over the virtual observation points with an iterative procedure that optimizes a set of weighting factors for the weighted average to maximize the minimized weighted average of local SAR over the virtual observation points; and

transmitting the defined parallel transmission RF pulse.

9. The method of claim 8, wherein the iterative procedure is nested within an outer iterative procedure configured to minimize a peak local SAR value for the parallel transmission RF pulse being defined.

10. The method of claim 8, wherein defining the parallel transmission RF pulses comprises determining a direction to change each weighting factor that increases the minimized weighted average after each iteration of the iterative procedure.

11. The method of claim 8, further comprising, before implementing the iterative procedure, initializing the set of weighting factors to equal values that sum to unity.

12. The method of claim 8, wherein designating the subset of the voxels comprises defining an upper bound matrix for finding the virtual observation points as a sum of the spatial matrix of the virtual observation point and a global SAR matrix scaled by an overestimation factor that tunes the designating step.

13. The method of claim 12, wherein designating the subset of the voxels further comprises selecting the overestimation factor.

14. A magnetic resonance imaging (MRI) system comprising:

a data storage unit to store calibration data for a model for a subject to be scanned, the model having a number of voxels;

a coil array for transmitting a parallel transmission radio frequency (RF) pulse to the subject; and

a control system in communication with the data storage unit and the coil array;

wherein the control system is configured to design the parallel transmission RF pulse to control local specific absorption rate (SAR) based on the model, a model compression in which the model is compressed into a plurality of virtual observation points within the model based on comparisons of peak sensitivity to SAR, and an iterative procedure applied to pulses configured to minimize a weighted average of local SAR values, the iterative procedure being configured to optimize a set of weighting factors for the plurality of virtual observation points to maximize the minimized weighted average.

15. The magnetic resonance imaging (MRI) system of claim 14, wherein the iterative procedure is nested within an outer iterative procedure configured to minimize a peak local SAR value for the parallel transmission RF pulse being defined.

16. The magnetic resonance imaging (MRI) system of claim 14, wherein the control system is configured to determine a direction to change each weighting factor that increases the minimized weighted average after each iteration of the iterative procedure.

17. The magnetic resonance imaging (MRI) system of claim 14, wherein the control system is configured to initialize the set of weighting factors to equal values that sum to unity.

18. The magnetic resonance imaging (MRI) system of claim 14, wherein:

the model comprises a number of voxels;

the control system is configured to calculate a spatial matrix for each voxel of the model, the spatial matrix being indicative of absorption sensitivity; and

the control system is configured to define an upper bound matrix for finding the virtual observation points as a sum of the spatial matrix of the virtual observation point and a global SAR matrix scaled by an overestimation factor.

19. The magnetic resonance imaging (MRI) system of claim 18, wherein the control system is configured via user selection of the overestimation factor.

20. The magnetic resonance imaging (MRI) system of claim 18, wherein the control system is configured to iteratively evaluate the voxels to determine whether the absorption sensitivity of a respective one of the voxels is upper bounded by the absorption sensitivity of at least one previously evaluated voxel.