Phase relaxed localized excitation pulses for inner volume fast spin echo imaging

Abstract

Purpose: To design multidimensional spatially selective radiofrequency (RF) pulses for inner volume imaging (IVI) with three-dimensional (3D) fast spin echo (FSE) sequences. Enhanced background suppression is achieved by exploiting particular signal properties of FSE sequences. Theory and Methods: The CPMG condition dictates that echo amplitudes will rapidly decrease if a 90° phase difference between excitation and refocusing pulses is not present, and refocusing flip angles are not precisely 180°. This mechanism is proposed as a means for generating additional background suppression for spatially selective excitation, by biasing residual excitation errors toward violating the CPMG condition. 3D RF pulses were designed using this method with a 3D spherical spiral trajectory, under-sampled by factor 5.6 for an eight-channel PTx system, at 3 Tesla. Results: 3D-FSE IVI with pulse durations of approximately 12 ms was demonstrated in phantoms and for T2-weighted brain imaging in vivo. Good image quality was obtained, with mean background suppression factors of 103 and 82 ± 6 in phantoms and in vivo, respectively. Conclusion: Inner Volume Imaging with 3D-FSE has been demonstrated in vivo with tailored 3D-RF pulses. The proposed design methods are also applicable to 2D pulses. Magn Reson Med 76:848–861, 2016. © 2015 The Authors. Magnetic Resonance in Medicine published by Wiley Periodicals, Inc. on behalf of International Society for Magnetic Resonance in Medicine.