Across‐vendor standardization of semi‐LASER for single‐voxel MRS at 3T

Abstract

The semi-adiabatic localization by adiabatic selective refocusing (sLASER) sequence provides single-shot full intensity signal with clean localization and minimal chemical shift displacement error and was recommended by the international MRS Consensus Group as the preferred localization sequence at high-and ultra-high fields. Acrossvendor standardization of the sLASER sequence at 3 tesla has been challenging due to the B 1 requirements of the adiabatic inversion pulses and maximum B 1 limitations on some platforms. The aims of this study were to design a short-echo sLASER sequence that can be executed within a B 1 limit of 15 μT by taking advantage of gradientmodulated RF pulses, to implement it on three major platforms and to evaluate the between-vendor reproducibility of its perfomance with phantoms and in vivo. In addition, voxel-based first and second order B 0 shimming and voxel-based B 1 adjustments of RF pulses were implemented on all platforms. Amongst the gradient-modulated pulses considered (GOIA, FOCI and BASSI), GOIA-WURST was identified as the optimal refocusing pulse that provides good voxel selection within a maximum B 1 of 15 μT based on localization efficiency, contamination error and ripple artifacts of the Dinesh K Deelchand and Adam Berrington share equal authorship

Methodological consensus on clinical proton MRS of the brain: Review and recommendations

“…Data were obtained using volumetric (3D echo planar spectroscopic imaging [EPSI]) proton ( 1 H) MRSI with lipid inversion nulling at 3 T, TE/TR/TI = 17.6/1550/198 ms, 50 × 50 × 18 k-space points over 280 × 280 × 180 mm 3 , and total acquisition duration of 16 minutes B 1 strength of 13-15 µT. 82 In addition, gradient-modulated offset-independent adiabaticity semi-LASER has a reduced RF power deposition compared with hyperbolic secant adiabatic full-passage pulses of the same bandwidth, enabling the sequence to be used with a shorter TR and at higher field strengths within specific absorption rate limits. Although adiabatic pulse pairs offer improved resilience to B 1 inhomogeneity, their primary advantage for SVS at 3 T is increased bandwidth and slice-selection profiles, thereby reducing CSD to acceptable levels.…”

“…Higher‐order hyperbolic secant adiabatic full passage or gradient‐modulated offset‐independent adiabaticity pulses may be used for refocusing in semi‐LASER, with the latter using wideband, uniform rate and smooth truncation RF and gradient waveform modulation . The advantage of gradient‐modulated offset‐independent adiabaticity pulses compared with hyperbolic secant adiabatic full passage is a reduced maximum B 1 strength required for a given pulse bandwidth, enabling semi‐LASER to be used at short TE (~30 ms) on 3T systems with a maximum available B 1 strength of 13‐15 µT . In addition, gradient‐modulated offset‐independent adiabaticity semi‐LASER has a reduced RF power deposition compared with hyperbolic secant adiabatic full‐passage pulses of the same bandwidth, enabling the sequence to be used with a shorter TR and at higher field strengths within specific absorption rate limits.…”

High‐resolution echo‐planar spectroscopic imaging at ultra‐high field

“…The proposed MRSI sequence (Figure ) combines VAPOR water suppression and semi‐LASER spatial localization with a ramp‐sampled symmetric EPSI readout, which requires fewer repetitions to acquire a full k space, thus reducing the scan time and keeping the total acquisition within the SAR limits. GOIA‐WURST adiabatic full passage (AFP) RF pulses were used with a duration 4.5 ms, bandwidth 10 kHz, 16th‐order hyperbolic secant (HS) modulation ( B 1 ) and fourth‐order gradient modulation . For the slice‐selective excitation, an asymmetric sinc pulse was used with a duration of 4.2 ms and a bandwidth of 3.7 kHz.…”

“…GOIA-WURST adiabatic full passage (AFP) RF pulses were used with a duration 4.5 ms, bandwidth 10 kHz, 16th-order hyperbolic secant (HS) modulation (B 1 ) and fourth-order gradient modulation. 8,29 For the slice-selective excitation, an asymmetric sinc pulse was used with FIGURE 1 Pulse sequence diagram used for the high-resolution MRSI protocol. The sequence consists of VAPOR water suppression interleaved with OVS pulses, followed by the semi-LASER localization using GOIA-WURST AFP refocusing pulses and a ramp-sampled symmetric EPSI readout.…”

ECLIPSE utilizing gradient‐modulated offset‐independent adiabaticity (GOIA) pulses for highly selective human brain proton MRSI

“…23 However, a frequency offset (that manifests from chemical shift) on gradient-modulated AFP pulses results in an asymmetrical inversion profile in addition to a spatial displacement. 29,30 Bloch simulations for the AFP-HS4 and GOIA-HS4 pulses in Figure 3A,B were repeated for a 600 Hz offset (lipid-water chemical shift difference at 4 T), as illustrated in Figure 3C,D. The GOIA-HS4 pulse provides a smaller displacement of~1.2 mm ( Figure 3D) relative to~6 mm from AFP-HS4, with a notable asymmetrical inversion profile at low RF amplitudes.…”