Crushed rephased orthogonal slice selection (CROSS) for simultaneous acquisition of two orthogonal proton resonance frequency temperature maps

Purpose To evaluate a novel imaging sequence termed crushed rephased orthogonal slice selection (CROSS) that uses the available time in long echo time (TE) gradient echo (GRE) imaging—as employed for proton resonance frequency (PRF) shift thermometry—to simultaneously acquire two orthogonal magnetic resonance imaging (MRI) temperature maps around the target region. Materials and Methods The CROSS sequence encodes a second orthogonal slice between excitation and data readout in long‐TE imaging and applies dedicated crusher (CR) gradients to separate the signals from the two slices. Numerical simulations of the Bloch equations and phantom experiments were performed to analyze the MR signal. In phantom and in vivo experiments with two domestic pigs, the applicability of the CROSS sequence for temperature mapping of thermal therapies with focused ultrasound and laser was studied. Results A successful separation of the signals from the two slices was achieved for CR dephasing lengths approaching the in‐plane resolution. In the two animal experiments, CROSS temperature mapping could be successfully demonstrated at a temporal resolution of 2–3 seconds and a temperature uncertainty of 3–4K. Conclusion At the expense of a reduced signal in the overlap of the two slices, the CROSS sequence achieves an improvement of temporal resolution by 50%, without requiring further acceleration techniques such as parallel imaging, over conventional sequential GRE sequences employing the same repetition time as the CROSS sequence acquires two slices within one repetition interval. J. Magn. Reson. Imaging 2013;38:1510–1520. © 2013 Wiley Periodicals, Inc.