MR imaging using variable gradient forms

Variable rate selective excitation was proposed by Conolly et al. as a method for the reduction of rf power (SAR: specific absorption rate). The author generated variable gradient forms for given rf pulses under user defined constraints for the maximum of the rf amplitude. In most cases the resultant gradient forms showed sharp transitions in their course, which can lead to disturbed slice profiles in case of even small deviations of calculated and realized gradient shapes. We have therefore decided to generate the gradient forms interactively and to design them with smooth amplitude changes and finally to modulate the rf pulse amplitude in such a way that the maximum rf amplitude and the SAR is lower compared with the original one. In simulations and experiments it is demonstrated that the proposed procedure of the reduction of the rf power is applicable to MR imaging. Using this technique, improved slice profiles could be obtained without exceeding the actual SAR limits. For a representative rf pulse the transmitter voltage has been reduced by 50%, the SAR by 40%.