Purpose A method using parallel transmission to mitigate B1+ inhomogeneity while explicitly constraining the temperature rise is reported and compared with a more traditional SAR-constrained pulse design. Methods Finite difference time domain simulations are performed on a numerical human head model and for a 16-channel coil at 10.5 Tesla. Based on a set of presimulations, a virtual observation point compression model for the temperature rise is derived. This compact representation is then used in a nonlinear programming algorithm for pulse design under explicit temperature rise constraints. Results In the example of a time-of-flight sequence, radiofrequency pulse performance in some cases is increased by a factor of two compared with SAR-constrained pulses, while temperature rise is directly and efficiently controlled. Pulse performance can be gained by relaxing the SAR constraints, but at the expense of a loss of direct control on temperature. Conclusion Given the importance of accurate safety control at ultrahigh field and the lack of direct correspondence between SAR and temperature, this work motivates the need for thorough thermal studies in normal in vivo conditions. The tools presented here will possibly contribute to safer and more efficient MR exams.
Bibliographical noteFunding Information:
The research leading to these results has received funding from the European Research Council under the European Union?s Seventh Framework Program (FP7/2013? 2018) / ERC Grant Agreement n. 309674 and from the National Institute of Health (NIH PR41 EB015894). The authors thank various colleagues at NeuroSpin and at CMRR for valuable discussions.
- parallel transmission
- ultrahigh field
- virtual observation points