Local SAR, global SAR, and power-constrained large-flip- angle pulses with optimal control and virtual observation points

TY - JOUR

T1 - Local SAR, global SAR, and power-constrained large-flip- angle pulses with optimal control and virtual observation points

AU - Vinding, Mads Sloth

AU - Guérin, Bastien

AU - Vosegaard, Thomas

AU - Nielsen, Niels Christian

PY - 2017/1/1

Y1 - 2017/1/1

N2 - Purpose: To present a constrained optimal-control (OC) framework for designing large-flip-angle parallel-transmit (pTx) pulses satisfying hardware peak-power as well as regulatory local and global specific-absorption-rate (SAR) limits. The application is 2D and 3D spatial-selective 90° and 180° pulses. Theory and Methods: The OC gradient-ascent-pulse-engineering method with exact gradients and the limited-memory Broyden-Fletcher-Goldfarb-Shanno method is proposed. Local SAR is constrained by the virtual-observation-points method. Two numerical models facilitated the optimizations, a torso at 3 T and a head at 7 T, both in eight-channel pTx coils and acceleration-factors up to 4. Results: The proposed approach yielded excellent flip-angle distributions. Enforcing the local-SAR constraint, as opposed to peak power alone, reduced the local SAR 7 and 5-fold with the 2D torso excitation and inversion pulse, respectively. The root-mean-square errors of the magnetization profiles increased less than 5% with the acceleration factor of 4. Conclusion: A local and global SAR, and peak-power constrained OC large-flip-angle pTx pulse design was presented, and numerically validated for 2D and 3D spatial-selective 90° and 180° pulses at 3 T and 7 T. Magn Reson Med 77:374–384, 2017.

AB - Purpose: To present a constrained optimal-control (OC) framework for designing large-flip-angle parallel-transmit (pTx) pulses satisfying hardware peak-power as well as regulatory local and global specific-absorption-rate (SAR) limits. The application is 2D and 3D spatial-selective 90° and 180° pulses. Theory and Methods: The OC gradient-ascent-pulse-engineering method with exact gradients and the limited-memory Broyden-Fletcher-Goldfarb-Shanno method is proposed. Local SAR is constrained by the virtual-observation-points method. Two numerical models facilitated the optimizations, a torso at 3 T and a head at 7 T, both in eight-channel pTx coils and acceleration-factors up to 4. Results: The proposed approach yielded excellent flip-angle distributions. Enforcing the local-SAR constraint, as opposed to peak power alone, reduced the local SAR 7 and 5-fold with the 2D torso excitation and inversion pulse, respectively. The root-mean-square errors of the magnetization profiles increased less than 5% with the acceleration factor of 4. Conclusion: A local and global SAR, and peak-power constrained OC large-flip-angle pTx pulse design was presented, and numerically validated for 2D and 3D spatial-selective 90° and 180° pulses at 3 T and 7 T. Magn Reson Med 77:374–384, 2017.

KW - hard constraints

KW - local SAR

KW - multidimensional spatial-selective excitation

KW - optimal control

KW - virtual observation points

UR - http://www.scopus.com/inward/record.url?scp=84952845379&partnerID=8YFLogxK

U2 - 10.1002/mrm.26086

DO - 10.1002/mrm.26086

M3 - Journal article

C2 - 26715084

SN - 0740-3194

VL - 77

SP - 374

EP - 384

JO - Magnetic Resonance in Medicine

JF - Magnetic Resonance in Medicine

IS - 1

ER -