Composite Localization with adiabatic slice selective excitation and refocusing (cLASER) for improved 1H MRSI in non uniform B1 fields

INTRODUCTION: H magnetic resonance spectroscopic imaging (MRSI) can be used as a diagnostic tool to distinguish abnormal from normal tissue by obtaining metabolic information which can be used as biomarkers. MRSI can be improved by going to higher field strengths like 7 Tesla which offer increased spectral resolution and SNR. However, at these field strengths the non uniformity of the B1 field makes MRSI very difficult to obtain, particularly from a relatively large region of interest (ROI). Furthermore, strong chemical shift displacement artifacts occur due to the limited B1 field. Adiabatic RF pulses for localization (i.e. LASER sequence (1)) have already shown to overcome both these limitations. However, these sequences result in high RF power deposition and have relatively long echo times (TE) because they require 6 adiabatic RF refocusing pulses for localization. Another approach is a semi adiabatic LASER (sLASER) sequence (2), which results in a much shorter TE and less RF power deposition, but is still sensitive to the B1 non uniformities especially in combination with surface coil transceivers. Recently it was shown that spectral-spatial (composite) pulses can be used to obtain slice selective adiabatic excitation (3). Therefore, we propose to include a composite slice selective adiabatic excitation in a sLASER sequence (cLASER) to benefit from the reduced SAR and TE while preserving the adiabaticity of the sequence. The full potential of the proposed cLASER sequence is shown with MRSI results in the human prostate in-vivo using an endorectal transceiver at 7T.