Gradient offset independent adiabatic pulses for high-field MR spectroscopy on clinical scanners

Adiabatic pulses are necessary to mitigate chemical shift artifact (CSA) and rf inhomogeneity at high-field MRS, but often they need longer durations (≥ 5 ms) and higher rf strengths (> 1 kHz) that limits their application in-vivo. Gradient offset independent adiabatic (GOIA) pulses have been proposed [1] as an elegant solution to reduce these requirements while effectively increasing the excitation bandwidth (> 20 kHz). Although GOIA has similar principles with VERSE (variablerate selective excitation) pulses [2], GOIA has the advantage of a constant adiabatic factor over the entire spectrum. Despite of the benefits, their use on clinical scanners has not been widespread. Here we report on a new class of GOIA pulses derived from WURST [3] adiabatic pulses. We present numerical simulations and experimental results obtained on phantoms and human volunteers on whole-body 3T and 7T Tim Trio clinical scanners (Siemens, Erlangen). Introduction: Chemical shift artifact is inversely proportional to the bandwidth (BW) of the rf pulse used for signal localization, thus increasing pulse bandwidths is highly desirable for MRS and MRSI applications. In general, a 5% displacement is considered acceptable for MRS/MRSI [4]. To achieve this at 7T over the 3.8 ppm 1H spectral range of interest a BW of 20 kHz is necessary. This would translate to only 5 mm displacement for a typical VOI of 10 cm in MRSI compared to 2 cm or more using conventional adiabatic pulses that are practically limited to BW of 5-6 kHz. We sought GOIA pulses that have BW = 20 kHz, are shorter than 5 ms and require less than 1 kHz peak rf amplitude. In particular, we designed pulses that are 3.5 ms (R = 70) and 4ms (R = 80) long and require 0.8 kHz or 0.7 kHz strengths, respectively. WURST (W8) [3] and HypSech (HS8) [5] have been used as starting classes. Localization with time varying gradients has been employed using inverse WURST (W4) and HypSech (HS4) modulation functions. These pulses (named GOIA-W84 and GOIA-HS84) compare favorably to FOCI [6] pulses of the same bandwidth and length which instead require 1.4 kHz maximum rf strength. In addition, constant adiabaticity is insured over the entire bandwidth for GOIA pulses while for FOCI and conventional adiabatic pulses the adiabatic condition degrades away from the center of the bandwidth. Numerical simulations: Performance of the GOIA-W84, GOIA-HS84 and FOCI pulses has been simulated using GAMMA [7] environment. Phase modulation of GOIA has been implemented by numerical integration of the constant adiabaticity equation [1]: ( ) ( ) ( ) ( ) ( ) ( ) ( ) 2 1 d t dt t G t d G t dt B t K ω ω Δ − Δ = ⎡ ⎤