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Accelerated 3D EPI navigator for prospective motion correction

Yulin Chang¹, Daniel Nicolas Splitthoff², Wei-ching Lo¹, M. Dylan Tisdall³, and Andre van der Kouwe⁴
¹Siemens Medical Solutions USA Inc., Malvern, PA, United States, ²Siemens Healthcare GmbH, Erlangen, Germany, ³Radiology, University of Pennsylvania, Philadelphia, PA, United States, ⁴Athinoula A. Martinos Center for Biomedical Imaging, Department of Radiology, Harvard Medical School, Massachusetts General Hospital, Boston, MA, United States

Synopsis

We show that for navigator-based prospective motion correction MRI, acceleration of 3D EPI acquisition increases sequence flexibility and improves the navigator image quality without sacrificing the quality of motion correction.

Introduction

Navigator-based prospective motion correction (PMC) has become indispensable in MRI to manage motion artifact in both clinic and research^1-3. This approach is relatively easy to set up as it does not reply on additional hardware for motion tracking. However, the real-time navigator acquisitions require “dead time” in the MRI pulse sequence and this imposes limits on the sequence timing. For 3D EPI-based volumetric navigators (vNavs), in addition to the extended time needed for the full 3D volume acquisition, the prolonged echo train for each partition also results in signal loss and image distortion. Accelerated EPI acquisitions have shown improved imaging speed and image quality⁴. In this study, we explore the impact of accelerating the navigator on sequence timing, navigator image quality, and real-time motion correction for structural imaging of the brain.

Methods

The prototype MR pulse sequence used in this study consists of MPRAGE⁵ as the “parent” sequence and 3D-EPI as the navigator in each TR between the inversion RF pulse and the turbo-FLASH readout³. Details of the MPRAGE: 1 mm isotropic resolution, sagittal orientation, 240×210×160 matrix (with 10% oversampling along partition), phase-encoding acceleration factor of 2, phase partial Fourier 6/8; TR=2.5 s, 4-echo readout⁶ with TE=2.08/3.84/5.60/7.36 ms and bandwidth of 720 Hz/pixel. The navigator details: 256 mm cubic FOV and 32×32×32 matrix; a single-shot EPI readout for each partition; partition partial Fourier 6/8; binomial water excitation RF pulses of 2° or sinc RF of 1°; readout with ramp sampling and bandwidth of 5040 Hz/pixel.

An in-plane acceleration factor of 3 is used to reduce the single-shot EPI echo train from 32 echoes to 11 echoes for each navigator partition, which reduced TE from 6.68 ms to 3.60 ms and the total readout duration from 8.96 ms to 3.08 ms. An additional acceleration factor of 2 along partition was also used in selected scans to reduce the total navigator acquisition time. GRAPPA⁷ is used to reconstruct the accelerated navigators in real-time based on the external GRE reference scan at the beginning of the sequence. An anthropomorphic head phantom⁸ and two volunteers were scanned on a 3 T MAGNETOM Prisma (Siemens Healthcare, Erlangen, Germany) whole-body scanner with a 32-ch receive head coil. A written consent was obtained from each volunteer. Volunteers were instructed to move during the scan.

Results

With 3× in-plane acceleration, the total navigator acquisition is reduced from 272 ms to 125 ms (sinc pulse; from 355 ms to 208 ms for water excitation), and to just 65 ms when combined with 2× partition acceleration. The shortened navigator allows shorter TI (≤1100 ms) for MPRAGE to optimize the gray-white matter contrast⁶ compared to the minimum allowed TI (1260 ms) with no acceleration.

Figure 1 compares the image quality between fully sampled and 3× accelerated navigators. For the fully sampled navigators, signal dropout is visible for areas of high susceptibility, whereas the signals in these areas are visibly improved for the accelerated navigators (red arrows). In addition, due to the effective TI change (from 247 ms to 156 ms), the accelerated navigator exhibits enhanced gray-white matter contrast (yellow arrow). The utility of the accelerated navigator for PMC is shown in Fig. 2, where two scans with moderate motions were acquired. Clearly, PMC using the 3× accelerated navigator significantly improves image quality compared to the one without motion correction. As shown in Fig. 3, at 3× acceleration, different gray-white matter contrasts can be obtained over the extended range of effective navigator TIs allowed by partition partial Fourier of MPRAGE, even though the overall brain signals are suppressed (similar to FatNav⁹). These navigators, despite their different appearance, are all shown to be effective in reducing motion artifact with PMC. Figure 4 shows phantom and in vivo navigator images acquired with the additional 2× partition acceleration. Note at this level of acceleration (6×) there is nearly no discernible parallel imaging artifact or g-factor¹⁰ penalty.

Discussion

Acceleration of the navigator acquisition in PMC provides several benefits. Most importantly, the reduced temporal footprint requires less dead time in the sequence, rendering more flexibility in the timing of a sequence and potentially allowing vNav to be used in more sequences. Specific for MPRAGE, this translates into shorter TI for contrast optimization. Second, geometric distortion of the navigator is substantially reduced in proportion to the acceleration factor. Third, signal dropouts are reduced by the shorter navigator TE. Finally, since the navigator itself is acquired on the T1 recovery curve, a steady state can never be assumed. Therefore, a shorter acquisition window leads to less image and contrast blurring. For MPRAGE, our data showed that the navigator should be acquired closer to the inversion pulse in order to maximize SNR.

Conclusions

In this study we demonstrated that accelerated navigators allow for more flexibility of sequence timing and improve navigator image quality without compromising the motion correction functionality.

Acknowledgements

No acknowledgement found.

References

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Figures

Navigator image quality comparison for fully sampled (left) and 3× in-plane accelerated (right) acquisitions. The shorter TE and shorter echo-train result in less signal dropout in areas of high magnetic susceptibility, such as the orbitofrontal area of the brain (red arrows) and near the sinus and in the neck (blue arrows). Also, due to the overall shorter navigator acquisition, the effective inversion time is longer for accelerated navigator, which results in visibly higher gray-white matter contrast (yellow arrow).

In vivo validation of motion correction using 3× accelerated navigators. Two scans with moderate head motions were acquired and prospective motion correction was only performed on one of them using the 3× accelerated navigators . Motion artifacts were significantly reduced with motion corrected scan (right), demonstrating the utility of accelerated navigator acquisitions. The plots show the corresponding motions parameters for each scan.

The reduced acquisition time of the navigator with acceleration can be used to improve the contrast of the navigator images. Here the effective inversion time of the navigator was extended by using a partition partial Fourier factor of 6/8 for the MPRAGE. The white matter signal is severely suppressed relative to gray matter at TI = 462 ms (top) while at TI = 552 ms (bottom) the effect is almost the opposite. Despite these differences, they are similarly effective in correcting the motion in the MPRAGE (right, with similar level of motions to that in Fig. 2).

Phantom and in vivo navigator images with combined 3× in-plane and 2× partition acceleration. At an effective acceleration factor of 6, the total navigator acquisition time is reduced to only 65 ms. No obvious image quality penalty from parallel imaging reconstruction is visible at this level of acceleration.

Proc. Intl. Soc. Mag. Reson. Med. 30 (2022)

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DOI: https://doi.org/10.58530/2022/1954