Asymmetric spin‐echo (ASE) spiral improves BOLD fMRI in inhomogeneous regions

Abstract

Functional MRI (fMRI) is of limited use in areas such as the orbitofrontal and inferior temporal lobes due to the presence of local susceptibility-induced field gradients (SFGs), which result in severe image artifacts. Several techniques have been developed to reduce these artifacts, the most common being the dual-echo spiral sequences (spiral-in/out and spiral-in/in). In this study, a new multiple spiral acquisition technique was developed, in which the later spiral acquisitions are acquired asymmetrically with the peak of a spin-echo causing increased R(2)-weighting but matched R(2)'-weighting. This sequence, called asymmetric spin-echo (ASE) spiral, has demonstrated significant improvements in minimizing the signal loss and increasing the image quality as well as optimal blood-oxygen-level-dependent (BOLD)-weighting. The ASE spiral is compared to conventional spiral-out using both signal-to-noise ratio (SNR) and whole brain fMRI activation volumes from a breath-hold task acquired at 4 Tesla. The ASE dual spiral has exhibited SNR increases of up to 300% in areas where strong SFGs are present. As a result, the ASE spiral is highly efficient for recovering lost activation in areas of SFGs, as demonstrated by a 16% increase in the total number of activated voxels over the whole brain. Post spin-echo ASE spiral images have decreasing SNR due to R(2) signal losses, however the increase in R(2)-weighting leads to a higher percentage of signal changes producing ASE spiral images with equivalent contrast-to-noise ratio (CNR) for each echo. The use of this sequence allows for recovery of BOLD activation in areas of SFG without sacrificing the CNR over the whole brain.

Functional MRI activation in white matter during the Symbol Digit Modalities Test

“…Functional MRI was conducted using an asymmetric spin-echo (ASE) spiral sequence that collects three images per slice per volume (Brewer et al, 2009). The three ASE spiral images have equal blood-oxygen level-dependent (BOLD) contrast, but increasing T 2 -weighting.…”

“…The three ASE spiral images have equal blood-oxygen level-dependent (BOLD) contrast, but increasing T 2 -weighting. Prior work has shown that increased T 2 -weighting improves sensitivity to WM fMRI activation (Brewer et al, 2009; Gawryluk et al, 2009). Accordingly, the three ASE images were combined using an inverted signal weighted averaging algorithm.…”

Functional mapping in the corpus callosum: A 4T fMRI study of white matter

“…Statistical analyses were done with motion parameters as covariates. 2 Except where otherwise stated, Z statistic images were first developed using a threshold for clusters determined by Z > 2.0 and a (corrected) cluster significance threshold of P = 0.05 (Worsley et al, 1992). Subsequently, activation maps were displayed in MRIcro (Z > 2.5).…”

Confirming white matter fMRI activation in the corpus callosum: Co-localization with DTI tractography

“…Seventeen axial slices (4 mm thick, no gap) were prescribed to cover the corpus callosum, as well as the region extending superiorly. Other parameters for functional imaging were: 64 × 64 matrix (220 × 220 mm), 1 shot, 170 volumes, TR/TE/TE = 2000/68/27 ms (TE/TE are the spin-echo centre and asymmetric echo times, respectively; Brewer et al, 2009). …”