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Initial feasibility of a multi-band PSF-mapping based, reverse-gradient approach with geometric distortion correction for whole-brain fMRI
Myung-Ho In1, Daehun Kang1, Hang Joon Jo2, Uten Yarach1, Nolan K. Meyer1, Joshua D Trzasko1, Erin M Gray1, John III Huston1, Matt A Bernstein1, and Yunhong Shu1
1Department of Radiology, Mayo Clinic, Rochester, MN, United States, 2Department of Physiology, College of Medicine, Hanyang University, Seoul, Korea, Republic of

Synopsis

A point-spread-function mapping-based reverse-gradient approach was demonstrated as a viable method to correct severe susceptibility artifacts for deep-brain-stimulation fMRI in a pig model, but at the cost of reduced temporal resolution. Interleaved acquisition of the echo-planar-imaging was used with opposite phase-encoding polarities. In this work, feasibility was evaluated in in-vivo resting-state fMRI reliability in high-susceptibility regions. To compensate for the reduced temporal resolution, multi-band imaging was used, and the improved reliability in highly susceptible regions was evaluated on both standard whole-body and high-performance compact 3T scanners.

Introduction

Gradient-echo echo-planar imaging (EPI) is the most commonly-used method for fMRI acquisition. It is prone to signal dropout and geometric distortion artifacts in regions of rapid susceptibility change, including the frontal and temporal lobes. A recent study [3] demonstrated that a PSF mapping-based reverse-gradient (PSF-RG) approach can minimize susceptibility artifacts in a pig’s brain for fMRI during deep-brain-stimulation. In this work, the effectiveness of PSF-RG approach was evaluated in the human brain for fMRI, especially in high susceptibility brain regions. Since a pair of interleaved EPI acquisitions with opposite phase-encoding gradient polarity were employed to minimize the signal dropout, the temporal resolution of the fMRI is reduced by half. However, this loss can be fully compensated for using a multi-band acceleration technique [4]. The method was tested on both a clinical whole-body 3T (WB3T) and a high-performance compact 3T (C3T) scanner [4-6].

Methods

After informed consent, three subjects were scanned using a 32-channel coil (Nova medical, USA) on a clinical 3T scanner, GE 750 (GE Healthcare, Waukesha, WI). One of subjects was scanned on both the WB3T and the C3T. With the multi-band implementation, the PSF and the corresponding EPI fMRI pair with opposite PE polarity was acquired with 5 different isotropic resolutions (2.0, 2.2, 2.5, 2.7, and 3.0 mm). A relatively high multi-band factor of 6 was applied without in-plane acceleration. The imaging protocol details are provided in Table 1. After PSF mapping-based distortion correction [3,7], a total of five different variants of the EPI series including: forward and reverse phase-encoded EPIs without (NF and NR), and with distortion correction (DF and DR) and the weighted combination of the distortion-corrected EPI pair (DW) were obtained. To investigate the effectiveness of this approach for human fMRI studies, the signal recovery performance was evaluated both qualitatively and quantitatively. Quantitatively, temporal SNR (tSNR), mean coverage ratio, and the Dice similarity coefficient were computed. After co-registration between the functional and the anatomical images with rigid body transform using AFNI software [8], all functional data were interpolated to 2.0 mm resolution, and masked based on the temporal SNR map. Labeling of brain regions based on anatomical image was performed using FreeSurfer [9]. Both the Dice similarity coefficients (i.e., the Sorensen-Dice index) and the coverage ratio between EPI and anatomical volume, were evaluated. Finally, the results obtained on both the standard clinical and the high-performance compact 3T scanners were compared.

Results and Discussion

Severe susceptibility artifacts appeared as signal dropout and geometric distortions, especially in regions of temporal and frontal lobes of the human brain (Fig. 1). Signal dropout varied across the slices within a subject depending on the phase-encoding polarity. The geometric mismatch between the distortion-free reference and EPI without correction was significant in local areas. The complex pattern could easily mislead the interpretation of fMRI activations. With the proposed correction scheme, the distortion-corrected EPI pairs with opposite PE polarities were better geometrically matched with the distortion-free reference image. The signal dropout was reduced in the combined distortion-corrected EPI pairs (Fig. 1). The quantitative indices in the combined image are the best among all image datasets regardless of spatial resolution (Fig. 2). Temporal SNR tended to increase with the voxel size. Interestingly, high-resolution imaging offered a better coverage ratio on the C3T, which may be due to reduced signal dropouts associated with partial volume effects (Fig. 3B). In distinction, 2.5 mm resolution data excelled in the quantitative comparison on the WB3T (Figs. 2) since partial Fourier acquisition was required to keep the TE as 30 ms for high-resolution imaging (> 2.5 mm3) on the WB3T and leaded to signal dropout arising from the incorrect phase information [10] in the vendor reconstruction. The use of high-performance gradients greatly reduced the echo spacing on the C3T [11], which was advantageous to acquire 2.0 mm resolution data without partial Fourier acquisition (Table 1), and to reduce the level of distortion in EPI (Fig. 3A). Nevertheless, the PSF-RG scheme was still effective to further minimize the artifacts in local areas even on the C3T (Fig. 3A). With a relatively high multi-band factor of 6, the effective temporal resolution of the RG approach was 1.70 and 1.19 seconds, respectively on the WB3T and C3T, even at 2.5 mm isotropic resolution, whole-brain fMRI. Furthermore, the PSF scans required less than 30 seconds on both scanners (Table 1). Thus, the effective temporal resolution for the RG approach and the calibration time for distortion correction are quite practical for fMRI studies.

Conclusion

This feasibility study demonstrates that the PSF-based RG approach can be beneficial to improve human fMRI in high-susceptibility areas, on both a standard whole-body and a high-performance compact 3T. In conjunction with the multi-band imaging, the temporal resolution of the RG approach and the calibration scan for distortion correction are practical for fMRI.

Acknowledgements

This work was supported by NIH U01 EB024450 and NHI U01 EB026979.

References

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Figures

Table 1. Imaging protocols for PSF mapping and EPI scans with reversed gradient approach.

Figure 1. Qualitative evaluation of the proposed reverse gradient scheme in human brain: forward and reverse phase-encoded EPIs without and with distortion correction (DiCo) and the weighted combination of the distortion-corrected EPI pair. For comparison, a distortion-free reference image calculated from the PSF data is also shown. Yellow- and red-colored arrows indicate image intensity measured dominantly in the forward and reverse EPI, respectively.

Figure 2. Quantitative evaluation of the RG approach with different spatial resolutions on the whole-body 3T scanner: (A) Temporal SNR (TSNR), (B) Dice coefficient, and mean coverage ratios (C) for the entire brain cortices and (D) only for the frontal and temporal cortices.

Figure 3. Comparison of the RG approach on the standard whole-body (WB3T) and compact 3T (C3T) scanners. Qualitative (A) and quantitative comparisons (B) are displayed. Yellow- and red-colored arrows indicate image intensity measured dominantly in the forward and reverse EPI, respectively. For comparison, a sagittal slice from EPI volumes with 2.5 mm isotropic resolution and the final combined EPI data are chosen in (A) and (B), respectively.

Proc. Intl. Soc. Mag. Reson. Med. 28 (2020)
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