Feasibility of spiral fMRI based on an LTI gradient model

Abstract

Highlights• This work investigates the feasibility of using a one-time system calibration (called GIRF) based on a linear time-invariant gradient model to account for k-space trajectory deviations in spiral fMRI.• We show that the image quality and the spatial specificity of the fMRI activation are substantially improved when using the GIRF-prediction for trajectory correction while the nominal reconstructions suffer from artifacts and mis-placed fMRI activation.• We demonstrate that system characterization via the GIRF can enable spiral fMRI in situations when concurrent monitoring is not available. AbstractPurpose: Spiral imaging is very well suited for functional MRI, however its use has been limited by the fact that artifacts caused by gradient imperfections and B 0 inhomogeneity are more difficult to correct compared to EPI and requires accurate knowledge of the traversed k-space trajectory. With the goal of making spiral fMRI more accessible we have evaluated image reconstruction using trajectories predicted by the gradient impulse response function (GIRF), which can be determined in a one-time calibration step. Methods:GIRF-predicted reconstruction was tested for high-resolution (0.8mm) fMRI at 7T. Image quality and functional results of the reconstructions using GIRF-prediction were compared to reconstructions using the delay corrected nominal trajectory and concurrent field monitoring. Results:The reconstructions using nominal spiral trajectories contain substantial artifacts and activation maps contain mis-placed activation. Image artifacts are substantially reduced when using the GIRF-predicted reconstruction and the activation maps for the GIRF-predicted and monitored reconstructions largely overlap. The GIRF reconstruction provides a large increase in the spatial specificity of the activation compared to the nominal reconstruction. Conclusion:The GIRF-reconstruction generates image quality and fMRI results similar to using a concurrently monitored trajectory. The presented approach does not prolong or complicate the fMRI acquisition. Using GIRF-predicted trajectories has the potential to enable high-quality highresolution fMRI in situations where concurrent monitoring is not available.

Calibration of concomitant field offsets using phase contrast MRI for asymmetric gradient coils

“…Spiral readouts provide high sampling efficiency with flexible TE and are well suited for fast acquisitions such as cardiac imaging, 40 and CBF measurements 32 and are considered a promising alternative to EPI readouts for fMRI. 41,42 However, without concomitant field compensation spatial blurring and signal loss were evident in this study, limiting usability as artifact-free images with high spatial conspicuity are paramount for clinical use.…”

“…Concomitant field effects, if uncorrected, manifest as spatial blurring in spiral acquisitions, residual phase errors in phase contrast MRI, global shifts in EPI based sequences and inadequate slice compensation. Spiral readouts provide high sampling efficiency with flexible TE and are well suited for fast acquisitions such as cardiac imaging, 40 and CBF measurements 32 and are considered a promising alternative to EPI readouts for fMRI 41,42 . However, without concomitant field compensation spatial blurring and signal loss were evident in this study, limiting usability as artifact‐free images with high spatial conspicuity are paramount for clinical use.…”

Fast measurement of the gradient system transfer function at 7 T

“…We can therefore rule out temperature-related LTI violations as a cause for the observed discrepancies. Temperature effects might become relevant in imaging applications with high gradient duty cycles and long readout durations, for example fMRI 16 or DWI. 35 Shifts of the mechanical resonances with rising temperature 21,33,34 could compromise the long-term trajectory predictions of the GSTF.…”

“…Its first-order terms can be used to calculate actual k-space trajectories and apply them in image-reconstruction algorithms, [11][12][13] or to achieve a superior gradient pre-emphasis. 14,15 The GSTF-based correction methods have, for example, recently been shown to enable the use of spiral trajectories for functional 16 or real-time imaging. 17,18 The GSTF can be determined from the field response to a broadband input waveform.…”