Short echo time ¹H‐MRSI of the human brain at 3T with minimal chemical shift displacement errors using adiabatic refocusing pulses

Abstract

MR scanners with a magnetic field strength of 3T are becoming more and more available to the scientific and clinical community. The intrinsic increase in signal-tonoise ratio (SNR) when moving to higher field strengths creates better possibilities for using MR spectroscopy in characterizing brain pathologies within clinically acceptable measurement times. Moreover, the chemical shift dispersion (in Hertz) between different resonances increases linearly with field strength.As the distribution of metabolites can be heterogeneous, especially in local pathologies (e.g., brain tumors), twodimensional (2D) or three-dimensional (3D) 1 H-MR spectroscopic imaging (MRSI) (1) is the preferred spectroscopic examination method. MRSI can cover a large part of the brain and simultaneously localize metabolite signals into small voxels.One of the important markers in brain pathologies is the lipid content. To prevent contamination of signals from voxels within the brain with lipid signals from subcutaneous tissue outside the brain, an accurate suppression of lipid signals from subcutaneous tissue around the skull is required. This can be achieved in different ways. First of all, with a point-resolved spectroscopy (PRESS) pulse sequence (2) a volume of interest (VOI) is excited with three orthogonal slice-selective pulses excluding the skull. Second, remaining undesired signals can be suppressed with outer volume saturation slabs before excitation of the VOI. Third, in MRSI a combination of weighted elliptical kspace sampling and apodization of k-space before Fourier transformation can reduce voxel bleed to a minimum, while sensitivity is maintained (3,4).An often-underestimated problem with proton MRSI is the chemical shift artifact or chemical shift displacement error (CSDE). Signals with different chemical shifts experience different slice selections: the combination of the bandwidth of the radio frequency (RF) pulse, the chemical shift of the metabolite of interest, and the strength of the selective gradient define the exact slice selection for every individual signal. With a trend toward signal excitation with large body coils (and signal reception with multichannel array coils), bandwidths of conventional RF pulses become quite small. As the chemical shift (in Hertz) increases with magnetic field, the CSDE at 3T can become very large, if unaccounted for. One solution to this problem is using OVERPRESS (General Electric terminology), also known as fully-excited VOI (Siemens terminology): the excited volume is deliberately chosen to be in the order of 20% larger than the VOI (5), and signals from outside the VOI are suppressed with outer volume saturation pulses, e.g., very selective suppression pulses with large bandwidths (6). A different solution to this problem can be found in using RF pulses with large bandwidths to excite and refocus the MR signals of interest themselves, instead of suppressing unwanted signals from outside the VOI. Adiabatic RF pulses have large bandwidths, and were originally introduced in the MR fiel...

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