Improved solvent suppression and increased spatial excitation bandwidths for three-dimensional PRESS CSI using phase-compensating spectral/spatial spin-echo pulses

Abstract

Dual phase-compensating spectral/spatial echo-planar (EP) spin-echo (SE) pulses were incorporated into the point resolved spectroscopy (PRESS) excitation sequence to improve water and lipid suppression for 1H chemical shift imaging (CSI) and to decrease the dependence of the PRESS box location upon chemical shift. The asymmetric EPSE pulses (either minimum or maximum phase in the chemical shift domain) were substituted for the two PRESS SE pulses to yield zero phase spectra. Three different pulses were designed and tested at 1.5 T. Pulse 1, targeted for brain CSI (TE > 85 msec), passed choline to lipid resonances, suppressed water, and rephased the methyl lactate doublet independently of TE. Pulse 2, targeted for general purpose shorter TE PRESS, possessed both high chemical shift and spatial domain bandwidths. Pulse 3, designed for prostate CSI, passed choline to citrate resonances while suppressing lipids and water. The three pulses possessed spatial bandwidths ranging between 3.3 and 5.0 kHz, more than three times higher than that offered by one-dimensional SE pulses of equivalent maximum B1 amplitude. Phantom and in vivo experimental results demonstrated that, for EPSE pulses 1 and 2, suppression factors higher than 10(4) were achieved. The increased spatial bandwidths resulted in less contamination by signals from outside the designated PRESS excited region and a significant improvement in the uniformity of metabolite intensities for voxels located near edges of the PRESS box.