Iterative correction of RF envelope distortion with GRATER‐measured waveforms

Abstract

Purpose To develop and evaluate a method for RF envelope correction without extra hardware or synchronization. Methods Transmitted RF waveforms are measured through a simple pulse sequence called the gradient reversal approach to evaluate RF (GRATER). The measured RF waveforms are used to compute predistorted RF waveforms. This process is repeated until a stopping criterion is met, for example, based on pulse performance or a maximum number of iterations. Excitation profiles and simultaneous multi‐slice (SMS) image quality are compared before and after RF predistortion. Results The proposed method improved the accuracy of multiband RF pulses, reducing normalized root mean squared error (NRMSE) by >12‐fold, and reducing spurious side‐lobe excitation by >6‐fold. The reduction in unwanted side‐lobe signal is demonstrated using SMS bSSFP imaging at 3T in phantoms and in the heart. Conclusion Iterative GRATER‐based predistortion is a practical, hardware‐free way to boost performance of short duration, low flip angle RF pulses, such as those used in SMS bSSFP imaging. Because of its efficiency, this technique could be included as part of an initial scan setup or for use with subsequent scans.

Simultaneous multi-slice T1 mapping using MOLLI with blipped CAIPIRINHA bSSFP

“…The second condition would require extra calibration data corresponding to side-lobes positions at the beginning of the SMS acquisition, increasing the potential for misalignment between the extended calibration acquisition and the accelerated imaging acquisition [27], as well as lengthening the total acquisition time, thus reducing the benefits from the SMS acceleration. Fourth, Landes et al proposed more recently [45] to tune the RF design to purposely reduce side-lobes excitation by an iterative process between RF exact shape measurement using GRATER [44] and RF design. This solution is tedious and can only fix the design for one RF pulse at a time, but it has the potential to reduce even further the side-lobe excitation than the proposed AM-only RF design.…”

Intra‐scan RF power amplifier drift correction

“…Here, we utilized DICO feedbacks and introduced two approaches for prospective corrections of RFPA drift. Utilizing RF current and voltage sensors, 11 pick-up coils, 27,28 gradient reversal approach to evaluate RF (GRATER), 29 and magnetic field monitoring using NMR sensors 30 are alternative methods proposed in the literature for RF monitoring and potentially for measuring RFPA drift. When compared to DICOs, pick-up coils may offer certain advantages, such as providing additional information that might not be necessary for drift correction and offering a more direct approach.…”