0317

Real-Time Water Fat Imaging at 0.55T with Spiral Out-In-Out-In Sampling

Ye Tian¹, Yongwan Lim¹, and Krishna S. Nayak¹
¹Ming Hsieh Department of Electrical and Computer Engineering, University of Southern California, Los Angeles, CA, United States

Synopsis

Real-time MRI is a powerful tool to study organ function in a dynamic environment. Many of these regions of interest have both water and fat present, such as the heart and joints. Water/fat separation usually requires multi-echo sequences that utilize chemical shift to distinguish water and fat signals. At 0.55T, the chemical shifts are smaller, making it possible to use long readouts (e.g. spirals) to more efficiently acquire images. Here, we demonstrate spiral out-in-out-in bSSFP imaging combined with region-growing field map estimation for water/fat separation. We demonstrate high quality water/fat separated real-time movies of cardiac function and wrist motion.

Introduction

Real-time MRI has the unique ability to reveal tissue movements and interactions in a non-invasive, radiation-free fashion, and has important diagnostic values in areas that often undergo irregular motions such as the heart and joints¹. Recently, 0.55T MRI with high-performance gradients has shown promising results for the heart and speech imaging by leveraging efficient spiral trajectories^2,3. Such systems are also desirable for real-time MRI (RT-MRI) due to reduced SAR, reduced susceptibility at air-tissue interfaces, shorter T₁ and longer T₂*, which will allow for long-readout real-time acquisition with more flexible sequence design and higher acquisition efficiency⁴.

In many regions of interest, both water and fat are present. This results in chemical-shift or blurring artifacts and poses a demand for water/fat separated imaging, which is often done by multi-echo acquisitions. At 0.55T, water and fat have smaller frequency difference, making it possible to use long readouts to efficiently acquire multiple echoes. bSSFP imaging provides high contrast, high SNR efficiency, and is preferred for many RT-MRI applications. RT-MRI with water/fat separation is difficult at high field strengths due to off-resonance and banding artifacts.

In this work, we demonstrate a real-time water/fat imaging by using a bSSFP sequence with a spiral out-in-out-in trajectory at 0.55T. This sequence acquires four images at three echo times within each TR, enabling water/fat separation. High-quality real-time water/fat images were obtained in the heart during free breathing and in the wrist during voluntary movements designed to test joint stability.

Methods

Pulse Sequence
Figure 1 illustrates the spiral out-in-out-in bSSFP sequence. A base spiral-out trajectory was first designed. After the first spiral-out, the trajectory continues to sample along the trajectory ($$$k_x(t'),k_y(t')$$$), where $$$k_x(t),k_y(t)$$$ denotes the initial spiral-out trajectory and $$$t'$$$ indicates the reversed time⁵. The second out-in spiral has a 180^o rotation to the first out-in spiral.

Imaging System
Experiments were performed using a whole body 0.55T system (prototype Magnetom Aera, Siemens Healthineers, Erlangen, Germany) equipped with high-performance shielded gradients (45mT/m amplitude, 200T/m/s slew rate). Imaging was performed with RTHawk system (HeartVista Inc., Menlo Park, CA). Data collection used a 6-channel body array and a table-integrated 18-channel spine array.

Cardiac Scans
Four volunteers (1 male, 3 females) were scanned for the heart during free breathing with parameters: voxel size=1.5x1.5x8mm³, T_read=8.14ms, TR=10ms, TE=0.93/5.0/9.07ms, flip angle=80^o. ROI shimming was performed for the ventricles, excluding the chest wall.

Wrist Scan
One volunteer was scared during a wrist maneuver, the radial-to-ulnar deviation, relevant to evaluation for dynamic scapholunate instability. The arm was placed above the spine array and below the body-6 array (supported by sandbags), in order to not restrict the range of motion. The parameters were: voxel size=1.4x1.4x8mm³, T_read=8.16ms, TR=10.3ms, TE=1.09/5.17/9.25ms, flip angle=70^o. ROI shimming was performed over the base of the wrist and covering all relevant bones and ligaments of the wrist.

Image Reconstruction
Reconstruction was performed using gradient impulse response function predicted trajectories⁶. Four echo images were reconstructed separately by spatiotemporal constrained reconstruction (STCR)⁷ at a temporal resolution of 40ms/frame for the heart and 82.4ms/frame for the wrist. Sensitivity maps were estimated from temporally combined first echo image using the Walsh method⁸. The off-resonance map $$$\varDelta{f}$$$ was estimated using a region-growing iterative method⁹frame-by-frame, and then filtered by a [7,7,11] median filter in [x,y,t]. This method assumes a smoothness in $$$\varDelta{f}$$$ and iteratively fit for water, fat, and $$$\varDelta{f}$$$ from echo images.

Results

Figure 2 shows fitting results for one representative time frame. The approach achieved good water/fat separation, evidenced in a clear depiction of epicardial fat in the fat-only image and overall low NRMSE. The corresponding free-breathing real-time water/fat movie is shown in Figure 3. The proposed sequence was able to capture detailed structures such as the cross section of the right coronary artery and left circumflex artery in all scans, as shown in Figure 4.

Figure 5 shows a water/fat separated real-time movie during radial-to-ulnar deviation maneuver. The fat image depicts the bone marrow clearly as well as all the ligament interval, especially the scapholunate interval that is often important for assessing dynamic wrist instability during the active wrist motion¹⁰.

Discussions

In this study, we have demonstrated the feasibility of using a spiral out-in-out-in bSSFP sequence for water/fat separated real-time MRI at 0.55T. The sequence achieved high-quality separation of water and fat in the heart during free breathing and the wrist during voluntary maneuvers. We used a two-step reconstruction approach to obtain water/fat separated images, STCR followed by region-growing $$$\varDelta{f}$$$ estimation. There is potential to improve the performance by directly reconstructing water/fat images such as described in Tan et al.¹¹ and Benkert et al.¹²

Water/fat separation using multiple-echo spirals has been previously demonstrated at higher field strengths^13-15. The innovation in this study is the efficient spiral out-in-out-in trajectory that can sample three echoes within one TR and its combination with bSSFP imaging for a substantial boost in SNR efficiency. This is uniquely possible at low field strengths such as 0.55T, and enables real-time characterization of water/fat components in the ROI with high spatio-temporal resolution.

Conclusion

We have demonstrated feasibility of water/fat separation using an innovative bSSFP spiral in-out-in-out sequence at 0.55T. High quality real-time water/fat separation is achieved in the heart and wrist.

Acknowledgements

We acknowledge grant support from the National Science Foundation (#1828736) and National Institute of Health (R01-HL130494) and research support from Siemens Healthineers.

References

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2. Restivo MC, Ramasawmy R, Bandettini WP, Herzka DA, Campbell-Washburn AE. Efficient spiral in-out and EPI balanced steady-state free precession cine imaging using a high-performance 0.55T MRI. Magnetic Resonance in Medicine 2020;84(5):2364-2375.

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10. Shaw CB, Foster BH, Borgese M, Boutin RD, Bateni C, Boonsri P, Bayne CO, Szabo RM, Nayak KS, Chaudhari AJ. Real-time three-dimensional MRI for the assessment of dynamic carpal instability. PLoS One 2019;14(9):e0222704.

11. Tan Z, Voit D, Kollmeier JM, Uecker M, Frahm J. Dynamic water/fat separation and B1 inhomogeneity mapping—joint estimation using undersampled triple-echo multi-spoke radial FLASH. Magn Reson Med 2019;82(3):1000-1011.

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Figures

Figure 1. Spiral Out-In-Out-In trajectory. (left) spiral out-in-out-in trajectory in k_x,k_y,t space. (right) spiral out-in-out-in trajectory in the k_x-k_y plane. The readout spirals out first, and when the k-space edge is reached it continues to spiral in. This is followed by the same trajectory but with a 180^o rotation in the k_x-k_y plane, forming a spiral out-in-out-in readout. Three echoes are acquired, and four images can be reconstructed from data collected by this trajectory.

Figure 2. Echo images and water, fat, $$$\varDelta f$$$ fitting results. The top row shows the four images at three echo times, reconstructed from the spiral out-in-out-in data. The bottom row shows the fitting results. Water component, fat component, and off-resonance map are jointly estimated from the four echo images. NRMSE map of the fitting is also shown. The fitting estimates a spatially smooth $$$\varDelta f$$$ within the shimming ROI (ventricles) and small NRMSE (<=20%) as expected. Note the high NRMSE occurs at air-tissue boundaries and banding areas (white arrows).

Figure 3. Short-axis cardiac water/fat separated RT-MRI from one volunteer. Shown is a mid short-axis slice acquired during free breathing. Epicardial fat is clearly seen on the fat-only image. A banding artifact moving with respiration in the stomach (outside the ROI) is also visible in the water-only image, which often appears in bSSFP imaging when TR is long (TR = 10 ms in this case). Some water/fat swapping is also visible at the chest wall.

Figure 4. Four-chamber cardiac water/fat separated RT-MRI from four volunteers. One cardiac cycle (5x slowed) is looped for display. Water/fat images obtained by spiral out-in-out-in offer clear epicardial fat depiction and reveal detailed structure. The right coronary artery (blue arrows) and left circumflex artery (red arrows) are clearly seen in cross-section from the water images. The surrounding fat is also clearly visible in the fat images. We notice some artifacts in the blood pool and aorta, likely due to phase from through-plane flow.

Figure 5. Water/fat separated RT-MRI of the wrist during radial to ulnar deviation maneuver. The bone merrow has fat as its main component, which appear bright in the fat-only image. The synovial fluid is better depicted by the water-only image. The ligament gaps and their interactions are clearly visible during real-time motion on both water (bright) and fat (dark) images.

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

0317

DOI: https://doi.org/10.58530/2022/0317