Effects of Relaxation During RF Pulses on the Homogeneity of Signal Intensity in Parallel Transmission

Masaki Sekino^1,2, Nicolas Boulant¹, Michel Luong³, Alexis Amadon¹, Hiroyuki Ohsaki², Denis Le Bihan¹

Dynamic RF shimming techniques in combination with parallel transmission have been uniquely based on the spinsf coherent dynamics, and do not account for relaxation during the RF pulses. We numerically investigated at 7 T the degradation of the spoiled gradient echo signal homogeneity when designing a transmit-SENSE pulse aimed at homogenizing the flip angle over an axial slice of a human head. Although the final uniformity may still be very good, we show that the signal inhomogeneity can sometimes deteriorate by a factor of 10 because of relaxation and should worsen even more at higher fields. The effect seems mostly noticeable around the Ernst flip angle.