Dynamic multi-coil technique (DYNAMITE) shimming for echo-planar imaging of the human brain at 7 Tesla

Abstract

Gradient-echo echo-planar imaging (EPI) is the primary method of choice in functional MRI and other methods relying on fast MRI to image brain activation and connectivity. However, the high susceptibility of EPI towards B0 magnetic field inhomogeneity poses serious challenges. Conventional magnetic field shimming with low-order spherical harmonic (SH) functions is capable of compensating shallow field distortions, but performs poorly for global brain shimming or on specific areas with strong susceptibility-induced B0 distortions such as the prefrontal cortex (PFC). Excellent B0 homogeneity has been demonstrated recently in the human brain at 7 Tesla with the DYNAmic Multi-coIl TEchnique (DYNAMITE) for magnetic field shimming (Juchem et al., J Magn Reson (2011) 212:280-288). Here, we report the benefits of DYNAMITE shimming for multi-slice EPI and T2* mapping. A standard deviation of 13 Hz was achieved for the residual B0 distribution in the human brain at 7 Tesla with DYNAMITE shimming and was 60% lower compared to conventional shimming that employs static zero through third order SH shapes. The residual field inhomogeneity with SH shimming led to an average 8 mm shift at acquisition parameters commonly used for fMRI and was reduced to 1.5-3 mm with DYNAMITE shimming. T2* values obtained from the prefrontal and temporal cortices with DYNAMITE shimming were 10-50% longer than those measured with SH shimming. The reduction of the confounding macroscopic B0 field gradients with DYNAMITE shimming thereby promises improved access to the relevant microscopic T2* effects. The combination of high spatial resolution and DYNAMITE shimming allows largely artifact-free EPI and T2* mapping throughout the brain, including prefrontal and temporal lobe areas. DYNAMITE shimming is expected to critically benefit a wide range of MRI applications that rely on excellent B0 magnetic field conditions including EPI-based fMRI to study various cognitive processes and assessing large-scale brain connectivity in vivo. As such, DYNAMITE shimming has the potential to replace conventional SH shim systems in human MR scanners.

In vivo B 0 field shimming methods for MRI at 7 T

“…In these experiments, MC shim settings were updated in under 1.5 ms during a 300 ms inter-slice delay without causing any apparent artifacts in the EPI slices (TR was 4.2 s). Simulations of shim performance in Juchem et al (2015a) show that the 48ch MC shim system can in principle outperform global 1st–5th order and dynamic 1st–4th order SH shimming. Other researchers have simulated the added benefit of using irregular optimized loop shapes (Zivkovic et al, 2016; While and Korvink, 2014).…”

“…Fig. 8 compares the efficacy of global 1st–3rd order shimming and 48ch MC dynamic shimming for improving EPI image quality and T 2 * maps (Juchem et al, 2015a). Global SH shimming removes smoothly-varying components of B 0 , but provides limited improvement in areas with steep B 0 variation such as the frontal lobes.…”

“…Several simulation studies have been performed to assess the benefit of even higher channel number shim arrays and SH order (Stockmann et al, 2016a; Juchem et al, 2015a; Han et al, 2013b). Fig.…”

“…12c and Table 2 display the frequency range containing 95%, 90%, and 80% of the residual ΔB 0 and σ B0 Global for simulated shimming across all 7 subjects (adopting the metrics used in de Graaf and Juchem (2016) and Juchem et al (2015a) to better characterize the typically non-gaussian distribution of off-resonance in vivo). Fig.…”

B0 magnetic field homogeneity and shimming for in vivo magnetic resonance spectroscopy

“…However, the applicability of such corrections is limited for severe spatial distortions; moreover, complete signal dropout cannot be recovered at all. MRI methods vary in their susceptibility to variations in background B 0 ; unfortunately, the work horses of MRI research and diagnostics, gradient-echo echo-planar imaging (EPI) sequences, tend to be particularly vulnerable to B 0 imperfections (17). More importantly for this topic, spatial misregistration for single-voxel MRS cannot be corrected by post-processing.…”