Consistent fat suppression with compensated spectral‐spatial pulses

Abstract

Reliable fat suppression is especially important with fast imaging techniques such as echo-planar (EPI), spiral, and fast spin-echo (FSE) T2-weighted imaging. Spectral-spatial excitation has a number of advantages over spectrally selective presaturation techniques, including better resilience to B0 and B1 inhomogeneity. In this paper, a FSE sequence using a spectral-spatial excitation pulse for superior fat suppression is presented. Previous problems maintaining the CPMG condition are solved using simple methods to accurately program radio-frequency (RF) phase. Next an analysis shows how B0 eddy currents can reduce fat suppression effectiveness for spectral-spatial pulses designed for conventional gradient systems. Three methods to compensate for the degradation are provided. Both the causes of the degradation and the compensation techniques apply equally to gradient-recalled applications using these pulses. These problems do not apply to pulses designed for high-speed gradient systems. The spectral-spatial FSE sequence delivers clinically lower fat signal with better uniformity than spectrally selective pre-saturation techniques.

Optic nerve diffusion measurement from diffusion-weighted imaging in optic neuritis

“…The optic nerve diffusion protocol consisted of three series of four interleaved sections each, acquired by using the ZOOM-EPI sequence with a spectralspatial 90°pulse for fat saturation (20) and an inversion recovery prepulse for CSF signal intensity suppression. The 12 ϫ 4 mm contiguous sections were combined into a single data set after reconstruction.…”

Spectrally selective pencil‐beam navigator for motion compensation of MR‐guided high‐intensity focused ultrasound therapy of abdominal organs

“…This is, however, of limited interest for a train of 12-turn spirals whose first radial side lobe occurs at a distance approximately 20 times the radius of the pencil beam. Furthermore, rotation of the k-space starting point would require a different gradient waveform for each 2D RF pulse, which might make the pulse train more sensitive to eddy currents and B 1 field inhomogeneities (23,25).…”

“…The diameter of the simulated pencil beam was 30 mm, and the total flip angle of the pulse train was 25 . Relaxation was omitted from the simulations.…”

“…Even if no compensation of the eddy currents would have been performed, the identical k-space trajectories and gradient waveforms of the individual 2D RF pulses will induce similar eddy currents for each pulse in the pulse train (25). This should in turn render the spectral selectivity of the proposed binomial 2D RF pulse train insensitive to eddy currents.…”

Spatiotemporal magnetic field monitoring for MR

“…If known with sufficient accuracy, gradient errors can often be addressed ex-post by suitable means of signal processing. Following this idea, various groups have proposed ways of measuring eddy current effects (13)(14)(15)(16) and effective k-space trajectories (17)(18)(19)(20)(21)(22)(23)(24)(25)(26). While quite successful, such calibration methods ultimately yield only approximations of the field evolution present in the eventual scan, for two main reasons.…”