SLOW: A novel spectral editing method for whole‐brain MRSI at ultra high magnetic field

PURPOSE: At ultra‐high field (UHF), B(1) (+)‐inhomogeneities and high specific absorption rate (SAR) of adiabatic slice‐selective RF‐pulses make spatial resolved spectral‐editing extremely challenging with the conventional MEGA‐approach. The purpose of the study was to develop a whole‐brain resolved spectral‐editing MRSI at UHF (UHF, B (0) ≥ 7T) within clinical acceptable measurement‐time and minimal chemical‐shift‐displacement‐artifacts (CSDA) allowing for simultaneous GABA/Glx‐, 2HG‐, and PE‐editing on a clinical approved 7T‐scanner. METHODS: Slice‐selective adiabatic refocusing RF‐pulses (2π‐SSAP) dominate the SAR to the patient in (semi)LASER based MEGA‐editing sequences, causing large CSDA and long measurement times to fulfill SAR requirements, even using SAR‐minimized GOIA‐pulses. Therefore, a novel type of spectral‐editing, called SLOW‐editing, using two different pairs of phase‐compensated chemical‐shift selective adiabatic refocusing‐pulses (2π‐CSAP) with different refocusing bandwidths were investigated to overcome these problems. RESULTS: Compared to conventional echo‐planar spectroscopic imaging (EPSI) and MEGA‐editing, SLOW‐editing shows robust refocusing and editing performance despite to B (1) ( + )‐inhomogeneity, and robustness to B (0)‐inhomogeneities (0.2 ppm ≥ ΔB (0) ≥ −0.2 ppm). The narrow bandwidth (∼0.6–0.8 kHz) CSAP reduces the SAR by 92%, RF peak power by 84%, in‐excitation slab CSDA by 77%, and has no in‐plane CSDA. Furthermore, the CSAP implicitly dephases water, lipid and all the other signals outside of range (≥ 4.6 ppm and ≤1.4 ppm), resulting in additional water and lipid suppression (factors ≥ 1000s) at zero SAR‐cost, and no spectral aliasing artifacts. CONCLUSION: A new spectral‐editing has been developed that is especially suitable for UHF, and was successfully applied for 2HG, GABA+, PE, and Glx‐editing within 10 min clinical acceptable measurement time.