Fast Localized Calibration for Spatial-Spectral Excitation Without Fly-Back Gradients

Standard spatial-spectral excitation pulses apply a fly-back gradient between sub-pulses, limiting, for example, how thin the slices can be (> 4 mm). Without fly-back gradients, slices can be as thin as 1.7 mm but require a phase calibration for the sub-pulses with inverted gradients due to system imperfections. Here we propose and test a fast (< 1 s), localized pre-scan enabling thin-slice frequency-selective excitation.

The calibration pre-scan determines the correction phase between even and odd sub-pulses by measuring the phase difference of two FIDs excited with either the last or second-to-last RF sub-pulse of the spatial-spectral pulse. Non-localized and localized versions of the calibration pre-scan are tested and validated for water-only excitation in phantoms and the human heart. As an example, breath-held coronary MR angiography is performed with a spiral multi-slice sequence employing the proposed pulses with a slice thickness of 1.7 mm.

The proposed calibration pre-scan takes 11.1 ms per slice, whether it is localized or not. Localized calibration phases are validated with an image-based calibration map. Slice profiles are visualized for pulses with different time-bandwidth products. Fat is suppressed to below 20% and 25% of the surrounding water signal in both phantoms and the human heart, respectively. High-quality coronary angiograms are demonstrated in all test subjects.

Spatial-spectral pulses without fly-back gradients can be calibrated quickly (up to 90 slices in 1 s), enabling thinner slices, higher excitation angles, or pulses with sharper slice profiles using waveforms with higher time-bandwidth products.