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Evaluation of Accelerated SEMAC at 0.55T using Hexagonal Sampling and Parallel Imaging
Bahadır Alp Barlas1, Kübra Keskin1, Bochao Li2, Brian A Hargreaves3,4,5, and Krishna S Nayak1,2
1Electrical and Computer Engineering, University of Sourthern California, Los Angeles, CA, United States, 2Biomedical Engineering, University of Southern California, Los Angeles, CA, United States, 3Radiology, Stanford University, Stanford, CA, United States, 4Electrical Engineering, Stanford University, Stanford, CA, United States, 5Bioengineering, Stanford University, Stanford, CA, United States

Synopsis

Keywords: Artifacts, Low-Field MRI

Motivation: Low-field and mid-field MRI systems have tremendous potential for imaging near metal implants with reduced artifacts, but reduced ability to do parallel imaging limits encoding options.

Goal(s): To achieve 50% scan time reduction in addition to conventional parallel imaging at 0.55T.

Approach: Hexagonal sampling (in ky-kf space) combined with GRAPPA-2 in SEMAC acquisitions was evaluated qualitatively and quantitatively via phantom and in vivo experiments at 0.55T.

Results: GRAPPA-2 hexagonal sampling achieved comparable image quality to conventional GRAPPA-2 SEMAC with slight SNR reduction and a modest increase in artifact area, while allowing a 50% decrease in scan time.

Impact: We evaluate the performance of hexagonal sampling combined with GRAPPA-2 at 0.55T where high parallel imaging factors are challenging via phantom and in vivo experiments. 50% additional scan time reduction is achieved with a modest increase in artifact area.

INTRODUCTION

Metallic implants cause significant distortion in the static magnetic field1,2. The resulting artifacts can be mitigated using multi-spectral imaging (MSI) techniques, namely, MAVRIC, MAVRIC-SL, and SEMAC3,4. MSI requires longer scan times that are partially compensated using parallel imaging, partial Fourier, and/or compressed-sensing5,6.

Imaging near metallic implants at lower-field strengths7,8 is promising due to substantially reduced off-resonance9,10. However, the parallel imaging factor is limited at low-to-mid field (<1.5T) because achieving body noise dominance limits the ability to use high channel-count arrays with small coils11.

In this work, we evaluate the use of hexagonal sampling combined with GRAPPA-2 to halve the scan time of conventional GRAPPA-2 SEMAC acquisitions at 0.55T. The technique was shown to achieve a 50% scan time reduction without introducing additional artifacts at 1.5T12. Performance is evaluated using phantom and in-vivo experiments.

METHODS

Experiments: Experiments were performed using a whole-body 0.55T MRI (prototype MAGNETOM Aera, Siemens Healthineers, Erlangen, Germany) with high-performance shielded gradients (45mT/m amplitude, 200T/m/s slew rate). The 6-channel body coil (anterior) and 6-elements of the table-integrated spine array were used for signal reception. Phantom and in-vivo experiments were performed with GRAPPA-2 SEMAC acquisitions in coronal orientation.

Phantom: A hip implant with a cobalt-chromium head and titanium stem was scanned with parameters: 300x244mm2 FOV, 320x260 matrix size, 20 slices, 4mm slice thickness, SEMAC factor 12.

In-Vivo: One volunteer with a hip implant consisting of a titanium stem and a high susceptibility head was scanned after providing written informed consent and under a protocol approved by our IRB. Scan parameters: 280x228mm2 FOV, 320x260 matrix size, 75% phase oversampling (to prevent aliasing from the other hip), 20 slices, 4mm slice thickness, SEMAC factor 8.

Image Reconstruction & Analysis: Figure 1 illustrates conventional GRAPPA-accelerated SEMAC and GRAPPA+hexagonal sampling. For the hexagonal case, 3D-GRAPPA is performed with a kernel size [5,5,3] along x-,y- and f- (bin) directions, respectively, to fill k-space in a checkerboard fashion (gray-dots reconstructed). The reconstructed k-space had the full hexagonal pattern, leading to aliasing replicas centered at the corners in y-f space. “Diamond” or “Cross” masks were applied to remove replicas while preserving the target y-f signal. The diamond and cross masks were designed to prioritize robustness against residual artifacts and sensitivity against implant position along y-direction. The performance of two masks with GRAPPA reconstruction is quantitatively compared by measuring the signal void artifact area and computing the noise levels as a function of y-position. After reconstruction, images were obtained by adaptive coil combination13 followed by the sum-of-squares (SOS) reconstruction of SEMAC images14.


RESULTS

Figure 2 shows reconstructed SEMAC images acquired from the central slice of the hip implant phantom. Qualitatively, GRAPPA+hexagonal with diamond and cross masks appear comparable to the GRAPPA only. Masks suffer from slight signal loss around the head (green arrows). Small residual artifacts are observed for the cross mask (pink solid line). While slight SNR reduction is observed for diamond and cross cases, the shape of the masks has minimal effect on SNR along y-direction.

Figure 3 illustrates y-f profiles from the phantom (from yellow-dashed lines). The replicas are centered at the diagonal corners, and SNR is slightly reduced for hexagonal sampling. Leakage from replicas is apparent (pink arrow) for “cross” mask due to its mask shape. “Diamond” mask eliminated the replicas while preserving the target profile.

Figure 4 shows the SEMAC images acquired from a volunteer. Both masks preserved the off-resonance profile (2% signal loss for both masks). “Cross” mask significantly suffered from residual artifact (pink arrow) due to the unsuppressed fat signal from the replicas. “Diamond” mask achieved comparable image quality to the GRAPPA-2 full sampling case.

Figure 5 illustrates y-f profiles for the in-vivo experiment. The targeted y-f profile is maintained for both masks, verifying in-vivo SEMAC reconstructed images. The residual artifact is shown by the pink arrow for “cross” mask case.

DISCUSSION

We demonstrate that hexagonal sampling can be combined with GRAPPA-2 for accelerated SEMAC at 0.55T, where high parallel imaging factors are challenging. Masks can be optimized to improve robustness to implant placement and residual leakage. We suspect that mask centralization (along the y-direction) will improve the performance of “diamond” mask. Automatic mask centralization15 will be investigated as a future work. The technique will benefit from testing in a larger number of THA subjects with different material compositions, and in subjects with different orthopedic implants, such as spinal-fixation or knee replacement.

CONCLUSION

Hexagonal sampling combined with GRAPPA in SEMAC achieves an additional 50% scan time reduction. The necessary tradeoff is SNR, based on reduced scan time and a small increase in artifact area.

Acknowledgements

We acknowledge grant support from the National Institutes of Health (R01-AR078912) and National Science Foundation (Award #1828736), and research support from Siemens Healthineers.

References

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Figures

Figure 1: Reconstruction pipeline for SEMAC with (a) GRAPPA and (b) GRAPPA + Hexagonal sampling. The hexagonal k-space data is reconstructed by 3D-GRAPPA with a kernel of size [5, 5, 3] along x-, y-, and f- (bin) directions to fill missing hexagonal data. The hexagonal fashion places the aliasing replicas in the corners of the y-f profile. “Diamond” or “cross” masks remove the replicas while maintaining the target y-f profile. The masks shown are theoretical, and they resemble a staircase in practice due to the small SEMAC factor.

Figure 2: Phantom Results. (a) Sum-of-squares SEMAC images of a hip implant in a water bath. GRAPPA with hexagonal sampling shows a slight increase in the signal void (green arrows) ((c) quantitatively 29% and 26% increase (green areas) for diamond and cross masks, respectively), while, particularly, the cross mask introduced small residual artifacts (pink solid line). (b) Noise variance along y-direction (from pink frame). Modest SNR reduction with hexagonal sampling is observed.

Figure 3: Phantom Results. y-f profiles from the hip implant phantom at the x-positions (yellow dashed lines). (a) The titanium stem had almost no effect on the distortion, while the head significantly distorted the profiles. (b,c,d) The replicas are centered at the diagonal corners, and SNR is slightly reduced for hexagonal sampling. The geometry of the cross mask caused some leakage of signal from replicas (pink arrows). The diamond mask eliminated the replicas while preserving the target profile.

Figure 4: In-vivo Results. (a) coronal SEMAC images acquired from a volunteer with a hip implant. “Diamond” mask achieved almost identical image quality to the GRAPPA full sampling. “Cross” mask suffers from residual artifacts (pink arrow). (b) Negligible signal loss (2% increase in signal void) is observed in the head of the implant for both masks, indicating that the masks successfully maintained the off-resonance information. Note that gradient nonlinearity correction was not applied.

Figure 5: In-vivo Results. y-f profile for the in-vivo experiment at corresponding x-positions (yellow dashed lines). (b,c,d) Both masks successfully preserved the target y-f profiles verifying the reconstructed images shown in Figure 4. A significant pile-up signal (pink arrow) is observed for the “cross” mask case, leading to the residual artifact shown in Figure 4. The location of the artifact indicates that the signal aliased from the fat tissue.

Proc. Intl. Soc. Mag. Reson. Med. 32 (2024)
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DOI: https://doi.org/10.58530/2024/2652