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Simultaneous Fat, T1 and Stiffness Quantification Using Multi-Echo, Variable Flip Angle, Spoiled-Gradient-Echo, Magnetic Resonance Elastography1Radiology, Mayo Clinic Arizona, Scottsdale, AZ, United States, 2Radiology, Mayo Clinic, Rochester, MN, United States, 3Siemens Healthcare, Salt Lake City, UT, United States
Synopsis
Hepatic fat fraction measurements using Dixon MRI and stiffness measurements using MRE provide important liver fat and fibrosis biomarkers for evaluating liver diseases. We have developed Dixon MR Elastography to measure fat fraction and stiffness in a single acquisition. In addition, by also using variable flip angles this new technique can simultaneously measure stiffness, T1, and fat fraction, thus providing even more clinically useful information in one scan. Promising results were obtained in a fat/water/gel phantom and six healthy volunteers.
Introduction
Several MRI parameters, , such as stiffness, fat fraction, and T1, have shown utility for diagnosing and grading various liver diseases. Measuring multiple parameters simultaneously could lead to a more cost-effective and time-efficient exam. Previous work has shown that it is possible to simultaneously measure hepatic stiffness and perform fat-water separation using a dual-echo, Dixon, spoiled-gradient-echo1 MR Elastography (MRE) acquisition. The purpose of this work is to extend this methodology to improve the water-fat separation with multi-echo Dixon and to use variable-flip-angle (VFA) to measure T1, making it possible to measure hepatic stiffness, T1 and fat fraction2, 3 in one sequence. This new technique circumvents the need for image registration while reducing scan time, which is clinically important for hepatic MRI exams4, 5.Materials and Methods
The pulse sequence timeline is shown in Fig. 1, which uses three readout echoes for Dixon analysis. This approach improves the water-fat separation with the long TEs and complicated phase maps that are typical of the MRE acquisitions2. A cylindrical water-fat-gel phantom with 4 layers with different concentrations was constructed as indicated in Fig. 26, 7. For each layer, (1) a bovine (Gelatin from bovine skin, Sigma Life Science, St Louis, MO, USA) and/or agar (Bacto-Agar, Spectrum Chemical MFG Corp, USA) solution, and (2) a vegetable oil/emulsifying wax (emulsifying wax pastilles, Milliard, Lakewood, NJ) mixture were made separately, and mixed at 40°C using a magnetic stirrer. At 30°C the phantom was placed in the refrigerator to set. After the previous layer had set, the next layer was prepared and poured on top of the previous layer.
All scans were performed on a clinical 1.5T scanner (Magnetom Aera, Siemens Healthcare, Erlangen, DE). The phantom was scanned with the full 3D VFA multi-echo GRE MRE:TR/TE1/TE2/TE3=33.34/20.3/22.6/24 ms, flip angle=5°, 10°, 15°, 20°, 25°, 30°, 35, 40° for each phase offset respectively. In the phantom test, the fat fraction was calculated from the images acquired with flip angle=5°. Six volunteers were scanned with a shorter 2D single-flip-angle acquisition with 4 slices: TR/TE1/TE2/TE3=33.34/21.4/23.8/26 ms, MRE phase offsets=3, flip angle=15°. All MRE data were acquired with through-plane motion encoding. Fat and water images were calculated from the three echoes with flexible echo time and averaged over the motion-encoding directions8, 9. T1 mapping was performed via linearized regression10, 11.
The fat fraction and T1 of the phantom was compared to that from a Turbo Spin Echo (TSE) sequence with Dixon (TR=77-3000 ms, TE=10 ms). The stiffness was compared to that from a prototype spin-echo EPI MRE sequence (TR/TE=4800/39 ms). In the volunteer study, fat and water images were acquired using a Q-Dixon sequence (TR/TE=15.6/2.38/4.76/7.14/9.52/11.90/14.28 ms, flip angle = 4°) and stiffness maps were compared to a prototype spin-echo EPI MRE sequence (TR/TE=1000/70 ms).
Results
Layers 2 and 3 of the phantom were analyzed to avoid slice-direction aliasing. Figure 3 shows magnitude, wave and stiffness images from layer 2. Table 1 summarizes the fat fraction, water T1, and stiffness of the phantom measured using the proposed and standard methods. Figure 4 shows the water and fat images and the wave and stiffness images for 1 of the volunteers obtained with the proposed and standard methods. The fat and water maps from the two methods look quite similar except in the areas near the spleen and skin. In these healthy volunteers there was not much hepatic or visceral fat signal. Table 2 summarizes the liver stiffness measured for each volunteer using the proposed and standard MRE acquisitions.
Discussions
The phantom and liver stiffnesses measured using the proposed sequence was very close to those measured using the standard MRE sequence. The phantom fat signal fraction measured with the proposed sequence was slightly lower than that from TSE, which may be the typical ‘bright fat’ phenomenon common with TSE, but the water T1s were close to each other. In the volunteer study, the fat water separation worked well in the liver but not as well near complicated interfaces, probably due to the long TE and B0 inhomogeneity. In future studies, more robust water-fat separation will be applied 2, 3, 12, and the full 3D acquisition with VFA will be implemented for in vivo acquisitions.Conclusion
This study demonstrates the feasibility of a VFA multi-echo GRE MRE sequence that is capable of simultaneously measuring stiffness and water T1 while also performing water-fat separation, which may become a valuable tool for routine, rapid, clinical hepatic imaging.Acknowledgements
No acknowledgement found.References
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