At high fields, the SAR is proportional to (i) the square of the static magnetic field strength and (ii) the square of the RF pulse flip angle being used.
For these reasons fast spin echo sequences give the greatest concern for RF effects. They use a train of 180° refocussing RF pulses.
Further reading on this topic:
Books: MRI From Picture to Proton p190, Q&A in MRI p304-305
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