Hadamard-encoded dual-voxel SPECIAL: Short-TE MRS acquired in two brain regions simultaneously using Hadamard encoding

Abstract

Purpose: The spin-echo, full-intensity acquired localized (SPECIAL) sequence is a method for single-voxel, localized MRS in vivo with short TEs. In this study we modified the SPECIAL sequence to simultaneously record spectra from two volumes of interest. This new technique is called Hadamard-encoded dual-voxel SPECIAL (HD-SPECIAL).

Methods: The SPECIAL sequence consists of a spin echo localized to a column of tissue, preceded by a slice-selective inversion pulse in alternating scans to invert a section of the column. Full localization is achieved by subtraction of the inversion-on scans from the inversion-off scans. In HD-SPECIAL, the two-step inversion scheme is replaced by a four-step Hadamard-encoded scheme involving single-band and dual-band inversion pulses to select two regions of the spin-echo column. By appropriate Hadamard combination of the four acquired shots, spectra can be reconstructed from both desired regions. This approach does not rely on parallel imaging reconstruction. Using a 3T scanner, HD-SPECIAL localization is demonstrated both in phantoms and in the human brain in vivo, and the performance of HD-SPECIAL is assessed by comparing with the conventional SPECIAL sequence.

Results: Phantom and in vivo measurements show excellent agreement between measures from HD-SPECIAL and SPECIAL sequences. Relative to consecutive SPECIAL measurements from two regions, HD-SPECIAL reduces the total scan time 2-fold with minimal penalty in terms of spectral quality or SNR.

Conclusion: The HD-SPECIAL sequence enables reliable acquisition of MR spectra simultaneously from two regions at 3 T, offering the potential to study interregional variations in metabolite concentrations.

Keywords: in vivo; Brain; Hadamard; SPECIAL sequence; dual-voxel; short TE MRS.