Local SAR reduction in multi-slice pTx via SAR-hopping between excitations

Patent metadata

Field	Value
Publication number	US-9417298-B2
Application number	US-201313827700-A
Country	US
Kind code	B2
Filing date	Mar 14, 2013
Priority date	May 9, 2012
Publication date	Aug 16, 2016
Grant date	Aug 16, 2016

Abstract

Described here are a system and method for designing radio frequency (“RF”) pulses for parallel transmission (“pTx”) applications, and particularly pTx applications in multislice magnetic resonance imaging (“MRI”). The concept of “SAR hopping” is implemented by framing the concept between slice-selective excitations as a constrained optimization problem that attempts designing multiple pulses simultaneously subject to an overall local SAR constraint. This results in the set of RF waveforms that yield the best excitation profiles for all pulses while ensuring that the local SAR of the average of all pulses is below the regulatory limit imposed by the FDA. Pulses are designed simultaneously while constraining local SAR, global SAR, and peak power, and average power explicitly.

First claim

The invention claimed is:

1. A method for designing parallel transmission (pTx) radio frequency (RF) pulses for use in multislice magnetic resonance imaging (MRI), the steps of the method comprising:
a) determining a target magnetization to be achieved in a plurality of slice locations in a subject;
b) selecting a set of compressed specific absorption rate (SAR) matrix points at which SAR can be evaluated;
c) designing a plurality of RF pulses for achieving the target magnetization by determining a set of RF waveforms that minimize an average local SAR of the plurality of RF pulses evaluated at the compressed SAR matrix points; and d) making the plurality of RF pulses designed in step c) accessible to an MRI system to perform multislice MRI.

2. The method as recited in claim 1 in which the selected set of compressed SAR matrix points define a sparse sampling of SAR matrices.

3. The method as recited in claim 1 in which step c) includes minimizing the average local SAR of the plurality of RF pulses evaluated at the compressed SAR matrix points using a constraint defined as,

∑

i
=
1

N
T

⁢

⁢

x
⁡

(

i
⁢

⁢
Δ
⁢

⁢
t

)

H

⁢

S
j

⁢

x
⁡

(

i
⁢

⁢
Δ
⁢

⁢
t

)

≤

SAR
local

;

wherein x(iΔt) is a vector including a concatenation of the RF waveforms evaluated at sample points, iΔt; { . . . } H indicated a Hermitian transpose operation; S j is a matrix of SAR kernels; N T is a number of sample points in the concatenation of the RF waveforms; and SAR local is a local SAR threshold.

4. The method as recited in claim 3 in which the matrix of SAR kernels, S j , is a block diagonal matrix.

5. The method as recited in claim 1 in which the set of RF waveforms determined in step c) also minimize a global SAR of each RF pulse evaluated at the compressed SAR matrix points.

6. The method as recited in claim 1 in which the set of RF waveforms determined in step c) also minimize a peak power in each of a plurality of transmit channels.

7. The method as recited in claim 1 in which the set of RF waveforms determined in step c) also minimize an average power in each of a plurality of transmit channels.

8. The method as recited in claim 1 in which the set of RF waveforms determined in step c) also minimize a global SAR of each RF pulse evaluated at the compressed SAR matrix points, a peak power in each of a plurality of transmit channels, and an average power in each of a plurality of transmit channels.

9. The method as recited in claim 1 in which step b) includes selecting the set of compressed SAR matrix points using a method that defines the compressed SAR matrix points as virtual observation points (VOPs).

10. A method for designing parallel transmission (pTx) radio frequency (RF) pulses for use in multislice magnetic resonance imaging (MRI), the steps of the method comprising:
a) determining a target magnetization to be achieved in a plurality of slice locations in a subject;
b) selecting a set of compressed specific absorption rate (SAR) matrix points at which SAR can be evaluated;
c) designing a plurality of RF pulses for achieving the target magnetization by determining a set of RF waveforms that minimize an average local SAR of the plurality of RF pulses evaluated at the compressed SAR matrix points;
in which step c) includes minimizing an objective function that comprises a least squares difference between the determined target magnetization and the set of RF waveforms after applying to the set of RF waveforms, a system matrix that describe magnetic resonance signals generated in response to RF energy; and in which the plurality of RF pulses is stored on a storage medium to be executed by an MRI system to perform the multislice MRI.

11. The method as recited in claim 10 in which the system matrix includes a small flip angle magnetic resonance signal model.

12. A magnetic resonance imaging (MRI) system, comprising:
a magnet system configured to generate a polarizing magnetic field about at least a portion of a subject arranged in the MRI system;
a plurality of gradient coils configured to apply at least one magnetic gradient field to the polarizing magnetic field;
a radio frequency (RF) system including at least one RF coil configured to apply an RF field to the subject and to receive magnetic resonance signals therefrom;
a computer system programmed to:
determine a target magnetization to be achieved in a plurality of slice locations in the subject;
select a set of compressed specific absorption rate (SAR) matrix points at which SAR can be evaluated;
determine a set of RF waveforms that minimize an average local SAR of a plurality of RF pulses evaluated at the compressed SAR matrix points using a constrained optimization; and
direct the RF system to produce the plurality of RF pulses using the determined set of RF waveforms, such that the determined target magnetization is generated in the plurality of slice locations in the subject.

13. The MRI system as recited in claim 12 in which the computer system is programmed to select the set of compressed SAR matrix points to define a sparse sampling of SAR matrices.

14. The MRI system as recited in claim 13 in which the computer system is programmed to compute the compressed SAR matrix points as virtual observation points (VOPs).

15. The MRI system as recited in claim 12 in which the computer system is programmed to determine the set of RF waveforms by constraining the constrained optimization using a constraint defined as:

Assignees

• Guerin Bastien
• Wald Lawrence L
• Adalsteinsson Elfar
• Massachusetts Gen Hospital

Inventors

• Guerin Bastien
• Wald Lawrence L
• Adalsteinsson Elfar

Classifications

• G01R33/4835

  of multiple slices · 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

• G01R33/5612

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