Flip angle and SAR maps induced by a head displacement in parallel transmission and with Cartesian feedback

Introduction: Taming the specific absorption rate (SAR) when using parallel transmission is one of the most challenging problems in MRI at high fields. Using sophisticated simulation tools, it has been shown that the peak local SAR varies with anatomy, but also with the resolution, size and other parameters of the numerical head model [1, 2]. When using 8 transmit channels, the parameter space at one instant in time is 16-dimensional (amplitude and phase for each channel) and hence grows as 16×n, where n is the number of sub-pulses, or discretization steps, used in the RF pulse design and played on each channel. In such a large space, the maximum local SAR value found over the irradiated volume can show considerable variations [3]. There remains, however, the question of how robust both SAR and flip angle (FA) landscapes remain with respect to small perturbations. A head displacement for instance not only changes the solutions of Maxwell’s equations but also alters the tuning, matching and interactions of the coils and thereby the currents flowing in them. Now Cartesian feedback [4] has been proposed as a solution to the latter problem. We therefore present the results of simulations showing the effects of small rotations of the head upon the FA and SAR landscapes with and without Cartesian feedback being applied.