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Numerical analysis of different dual-tuned coils for head and knee imaging at 7T
Xinqiang Yan1,2, Rachelle Crescenzi1,2, and John C. Gore1,2

1Institute of Imaging Science, Vanderbilt University, Nashville, TN, United States, 2Radiology and Radiological Science, Vanderbilt University, Nashville, TN, United States

Synopsis

A recurring need in MRI and MRS is to acquire signals from multiple nuclei, 1H proton and 23Na sodium, in the same study. However, a dual-tune volume coil implemented in a single structure leads to SNR and B1-efficiency penalties for the X-nucleus. We numerically investigated how B1-fields interact between either a traveling-wave coil existing outside the bore, or a local dipole array, paired with a birdcage 23Na-coil. For applications to head and knee imaging at 7T, an easily-implemented traveling-wave coil may provide sufficient proton signal for anatomical localization and B0 shimming, without significantly reducing the performance of the X-nucleus coil.

Introduction

A recurring need in MRI and MRS is to acquire signals from multiple nuclei, such as 1H proton and 23Na sodium, in the same study. Specifically, applications for sodium MRI to diseases of hypertension1, inflammation2,3, and chronic kidney disease4,5, for instance, demand improved spatial resolution to resolve the skin as well as deep subcutaneous and muscle tissues. A dual-tune volume coil is best suited for these applications, typically designed as a single birdcage structure with four rings6. However, the single structure leads to SNR and B1 efficiency penalties for the X-nucleus, especially at ultra-high field imaging. Currently, at 3T and below, a single-tuned coil can be optimized for the X-nucleus while non-optimized proton images for anatomic reference and B0 shimming can be obtained using a body coil. Current 7T human scanners are not equipped with a built-in body coil, though a traveling-wave coil outside the bore may substitute7. Other complicated dual-tuned coil designs combine an X-nucleus coil with strip-line8 or local dipole/monopole arrays9,10. In this work, we numerically investigated the interaction between B1 fields from a traveling-wave coil or local dipole array and the X-nucleus coil, and whether these designs can be used without significantly reducing the performance of an X-nucleus coil.

Methods

The typical diameter/length of birdcage coils were simulated for sodium imaging of the head or the knee using 28/20 cm and 19.5/15 cm, respectively. These birdcage coils are usually a high-pass design with 8 rungs. A traveling-wave proton coil may be constructed from a pair of quadrature dipole antennas (length≈15cm) placed far away from a sodium coil (>30cm away, Figure 1a). We investigated how these pairs of coils interact at 7T, and also evaluated the combination of a local fractionated 1H-dipole array11 with a birdcage 23Na-coil (Figure 1b).

Simulations were performed using HFSS (Ansys HFSS, Canonsburg, PA, USA). Coil elements were tuned to the Larmor frequency of 7T (78.6MHz for sodium or 298MHz for protons) and matched to 50Ω. To make sure the birdcage coil resonates at the uniform mode, its distributed capacitor values were pre-calculated from Birdcage Builder12 and used as reference values in RF circuit optimizations. We calculated the B1 fields of each coil (driven in quadrature) and normalized results to 1-Watt input power.

Results

The resonant frequency of a 23Na-coil is lower compared to the proton Larmor frequency (78.6MHz vs. 298MHz). For proton imaging with a separate traveling-wave coil, it exhibits a shielding effect which blocks the RF field of the traveling-wave and thus reduces the proton B1 efficiency, as shown in Figure 2a and Figure 3a. It may be noted that the shielding effect is mainly caused by the passive current on the end rings of the birdcage coil, rather than the rungs. For sodium imaging, the B1 field from the birdcage coil is not affected by the traveling-wave proton coil as it is >30cm away, as shown in Figure 2b and Figure 3b.

Figures 4 and 5 show the simulated B1 fields using the combination of a local birdcage coil with a local dipole array. Compared to the single-tuned coil, this combination has only a small B1 efficiency penalty for both sodium and proton imaging. As a local coil, the B1 efficiency of the dipole array is approximately 10 times compared to that of a traveling-wave coil (0.37uT/$$$\sqrt{W}$$$ vs 0.04uT/$$$\sqrt{W}$$$ in head imaging and 0.5uT/$$$\sqrt{W}$$$ vs 0.05uT/$$$\sqrt{W}$$$ for knee imaging).

Discussions and Conclusion

In this work, we numerically investigated two dual-tuned 1H/23Na coil designs, and simulated proton and sodium B1 efficiency for human head and knee imaging at 7T. Based on the simulation results, neither the traveling-wave coil nor the local dipole array for proton imaging affects sodium B1 efficiency. Note that the local dipole array requires additional splitters and phase shifters (Figure 1b) to connect to the quadrature interface box and complicated cabling and trap/balun circuits, which may in practice degrade sodium B1 efficiency. Compared to a local dipole array, the traveling-wave coil is easier to implement. However, it has notable B1 efficiency decrease for proton imaging due to shielding of the sodium coil, especially in human head imaging. By moving the traveling-wave coil nearer to the human head and using its near-field rather than far-field, the contribution of this shielding effect may be reduced. Practical experiments are needed to verify whether the proton MR signal from a traveling-wave coil is high enough for anatomical localization and B0 shimming, to work in concert with an optimal single-tuned sodium coil for clinical applications.

Acknowledgements

We thank Drs. Ping Wang (University of Utah) and Ed Mojahed (Philips Healthcare) for helpful discussions. Dr. Rachelle Crescenzi's work was partially supported by Lipedema Foundation Grant #12 and American Heart Association 18CDA34110297.

References

1. Kopp C, Linz P, Dahlmann A, Hammon M, Jantsch J, Muller DN, Schmieder RE, Cavallaro A, Eckardt KU, Uder M, Luft FC and Titze J. 23Na magnetic resonance imaging-determined tissue sodium in healthy subjects and hypertensive patients. Hypertension. 2013;61:635-40.

2. Kopp C, Beyer C, Linz P, Dahlmann A, Hammon M, Jantsch J, Neubert P, Rosenhauer D, Muller DN, Cavallaro A, Eckardt KU, Schett G, Luft FC, Uder M, Distler JH and Titze J. Na+ deposition in the fibrotic skin of systemic sclerosis patients detected by 23Na-magnetic resonance imaging. Rheumatology (Oxford). 2016.

3. Crescenzi R, Marton A, Donahue PMC, Mahany HB, Lants SK, Wang P, Beckman JA, Donahue MJ and Titze J. Tissue Sodium Content is Elevated in the Skin and Subcutaneous Adipose Tissue in Women with Lipedema. Obesity (Silver Spring). 2018;26:310-317.

4. Deger SM, Wang P, Fissell R, Ellis CD, Booker C, Sha F, Morse JL, Stewart TG, Gore JC, Siew ED, Titze J and Ikizler TA. Tissue sodium accumulation and peripheral insulin sensitivity in maintenance hemodialysis patients. J Cachexia Sarcopenia Muscle. 2017.

5. Schneider MP, Raff U, Kopp C, Scheppach JB, Toncar S, Wanner C, Schlieper G, Saritas T, Floege J, Schmid M, Birukov A, Dahlmann A, Linz P, Janka R, Uder M, Schmieder RE, Titze JM and Eckardt KU. Skin Sodium Concentration Correlates with Left Ventricular Hypertrophy in CKD. J Am Soc Nephrol. 2017.

6. Shen GX, Boada FE, Thulborn KR. Dual-frequency, dual-quadrature, birdcage RF coil design with identical b1 pattern for sodium and proton imaging of the human brain at 1.5 T. Magn Reson Med 1997;38: 717–725.

7. Brunner DO, De Zanche N, Frohlich J, Paska J, Pruessmann KP. Trav-elling wave nuclear magnetic resonance. Nature 2009;475:994–998.

8. Wiggins GC, Brown R, Fleysher L, Zhang B, Stoeckel B, Inglese M, Sodickson DK. A nested dual frequency birdcage/stripline coil for sodium/proton brain imaging at 7T. In Proceedings of the 18th Annual Meeting of ISMRM, Stockholm, Sweden, 2010. Abstract 1500.

9. Shajan, Gunamony, et al. "Three‐layered radio frequency coil arrangement for sodium MRI of the human brain at 9.4 Tesla." Magnetic resonance in medicine 75.2 (2016): 906-916.

10. Yan X, Xue R, and Zhang X. "A monopole/loop dual-tuned RF coil for ultrahigh field MRI." Quantitative imaging in medicine and surgery 4.4 (2014): 225.

11. Raaijmakers, Alexander JE, et al. "The fractionated dipole antenna: A new antenna for body imaging at 7 T esla." Magnetic resonance in medicine 75.3 (2016): 1366-1374.

12. Chin, Chih‐Liang, et al. "BirdcageBuilder: design of specified‐geometry birdcage coils with desired current pattern and resonant frequency." Concepts in Magnetic Resonance: An Educational Journal 15.2 (2002): 156-163.

Figures

Figure 1 a) Simulation model and RF interface using the traveling-wave coil and local birdcage coil for proton and sodium imaging. b) Simulation model and RF interface using local dipole array and local birdcage coil for proton and sodium imaging. To connect the 8-ch dipole array to proton quadrature ports, a set of power splitters and 180-degree phase shifters were used.

Figure 2 a) Simulated B1 efficiency of the traveling-wave coil with and without the local birdcage sodium coil for human head imaging. The sodium coil exhibits a shielding effect which blocks the RF field in the human head. It is found this shielding effect is mainly caused by the birdcage coil’s rings rather than rungs. b) Simulated B1 efficiency of the local birdcage coil with and without the local traveling-wave proton coil for human head imaging. Since the traveling-wave coil is positioned quite far away from the imaging area, it does not affect the B1 efficiency at the sodium frequency.

Figure 3 a) Simulated B1 efficiency of the traveling-wave coil with and without the local birdcage sodium coil for human knee imaging. Similar to the head imaging case, the local birdcage coil exhibits a shielding effect and reduces the B1 efficiency of the traveling-wave coil. b) Simulated B1 efficiency of the local birdcage coil with and without the local traveling-wave proton coil for human knee imaging. Similar to the head imaging case, the traveling-wave coil does not affect the B1 efficiency at the sodium frequency.

Figure 4 a) Simulated B1 efficiency of the local dipole array with and without the local birdcage sodium coil for human head imaging. The sodium coil has little effect on the B1 field of a dipole array (mainly a longitudinal current). b) Simulated B1 efficiency of the local birdcage coil with and without the local dipole array. The local dipole antenna will not affect the sodium coil's efficiency. However, the dipole antenna needs to be fed at the center and thus the cabling and trap/balun circuits have to cross the birdcage coil, which may reduce the birdcage coil's B1 efficiency.

Figure 5 a) Simulated B1 efficiency of the local dipole array (segmented by meander lines) with and without the local birdcage sodium coil for human knee imaging. b) Simulated B1 efficiency of the local birdcage coil with and without the local dipole array.

Proc. Intl. Soc. Mag. Reson. Med. 27 (2019)
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