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Whole Body Quadrature Birdcage RF Coil for 23Na MRI of Small Rodents at 9.4T and Its Application in Sodium Imaging of the Eye and Kidneys

Laura Boehmert¹, Helmar Waiczies², Daniel Wenz¹, Celal Oezerdem¹, Andre Kuehne², Paula Ramos Delgado³, Erdmann Seeliger⁴, Andreas Pohlmann¹, and Thoralf Niendorf^1,2

¹Berlin Ultrahigh Field Facility (B.U.F.F.), Max Delbrück Center for Molecular Medicine in the Helmholtz Association, Berlin, Germany, ²MRI.TOOLS GmbH, Berlin, Germany, ³Berlin Ultrahigh Field Facility (B.U.F.F.), Max Delbrück Center for Molecular Medicine in the Helmholtz, Berlin, Germany, ⁴Institute of Vegetative Physiology, Charité - University Medicine Berlin, Berlin, Germany

Synopsis

Sodium ions play a major role in the physiology of the human organism. The kidneys and eyes both have a high sodium concentration, and we suggest that probing tissue sodium concentration in these organs using ²³Na MRI can add a very useful dimension to our understanding of renal and ocular disorders. Changes in the sodium concentration might be indicative of early pathophysiological changes in such diseases. Therefore we present a quadrature birdcage coil tailored for sodium imaging of small rodents at 9.4T, along with the RF coil design, EMF simulations and in vivo ²³Na MRI of the eye and kidney.

Introduction

Sodium ions (Na⁺) play an important role in the physiology of the human organism, with the sodium/potassium pumps being essential for the maintenance of homeostasis¹. High Na⁺ concentrations are found in the renal medulla and in certain compartments of the eye. In the kidney, tubular water resorption is driven by the osmotic gradient between the tubuli and the interstitium that largely relies on active transport of sodium². Active transport of Na⁺ also plays an important role in the human eye and its compartments, including the formation of the aqueous humor and the removal of the water and lactic acid from the retina³. Dysfunction in these processes could lead to changes in the sodium concentration and might be symptomatic of early pathophysiological changes in renal and ocular diseases^4,5. ²³Na MRI could add a very useful dimension to our understanding of renal, and ocular disorder but also for detailing differences between sodium deposition in muscle and skin in cardiac and environmental metabolic diseases. Recognizing the potential of whole body ²³Na MRI in animal models this work describes the design, construction and performance of a large volume ²³Na quadrature birdcage radiofrequency (RF) coil tailored for rodents at 9.4T. The applicability of the proposed whole body RF coil is demonstrated for in vivo ²³Na MRI of the rat eye and kidney.

Methods

The inner diameter of 62mm of the proposed low pass birdcage was given by the size of a typical loading (small rat) and the size of the MR scanner bore and contains 16 rungs (Figure 1A). Birdcage Builder⁶ was used in order to estimate the initial values of distributed capacitors. EMF simulations were carried out using CST Studio Suite 2016 and included: a rat-shaped 3D model (σ=0.91S/m, ε=65) (Figure 1B); and a cylindrical phantom (σ=0.72S/m, ε=63). RF coil bench measurements were performed on a saline phantom (V=200mL, [NaCl]=600mM, σ=0.72S/m, ε=63) using a network analyzer. To drive the birdcage (Figure 2A) in quadrature mode, an additional T/R switch and hybrid combiner was designed and built (Figure 2B). MR experiments were conducted on a 9.4T animal scanner (Biospec, Bruker, Germany). B₁⁺-field maps were acquired using double-angle-method⁷. For reference image a ¹H birdcage coil with same geometry as the ²³Na birdcage was used. The change to the quadrature driven ²³Na birdcage was done without moving the animal. ²³Na eye imaging required placing the rat’s head in the center of the birdcage coil, which resulted in a change in the coupling between the sample and the end ring of the coil. This change was compensated by adding a loading phantom.

Results

Figure 2C represents the S-parameter values measured on a homogeneous phantom. The reflection coefficients (S₁₁, S₂₂) were measured to be -49dB or better and the transmission coefficient (S₁₂, S₂₁) was -20dB. The transmit field (B₁⁺) simulation result was validated in a MR experiment. Figure 3 shows the simulated RF coil design in axial orientation and the corresponding B₁⁺-map for the virtual phantom and the measured B₁⁺-map for the cylindrical phantom for the same ROI. The B₁⁺-profile over the cross-section of the phantom ranged from 1.8-2.0µT/V for the measurement and 2.0-2.3µT/V for the simulation. Considering the uncertainty of losses in the RF-coil (copper, solder and capacitors), the magnitude of the measured and simulated B₁⁺-fields are in good agreement. Examples of in vivo ²³Na MRI of the rat kidneys and eyes are shown in Figure 4A-B using an overlay of ¹H and ²³Na MR images. For renal and ocular ²³Na MRI an in-plane spatial resolution as good as 780µm was achieved. This resolution and sensitivity facilitates the visualization of the renal corticomedullary sodium gradient. Our results also show that the sodium content in the lens is distinguishable from the sodium content in the aqueous and vitreous humor.

Discussion and Conclusion

This work reports on the design, construction and feasibility of a whole body ²³Na quadrature birdcage RF coil tailored for rats at 9.4T. Equipped with this ²³Na RF technology, we will commence animal studies to elucidate the intertwined roles of renal tissue hypoperfusion, hypoxia, energy metabolism and sodium metabolism en route to developing novel treatment options and effective prophylactic regimens for acute kidney injury and chronic kidney disease. Likewise, the proposed ²³Na MRI approach will allow the assessment of the sodium content in the eye and its compartments in vivo as part of our translational research in humans and in animal models⁸. The broad roles of this sodium in processes related to eye physiology suggest a wide range of questions for in vivo ophthalmological investigations afforded by ²³Na MRI.

Acknowledgements

This work was supported in part by the Bundesministerium für Bildung und Forschung (BMBF, German Federal Ministry for Education and Research; grants VIP+ 03P00081, VIP+ 03P00082).

References

1. Murphy, E. and D.A. Eisner, Regulation of intracellular and mitochondrial sodium in health and disease. Circ Res, 2009. 104(3): p. 292-303.

2. Douglas C. Eaton, J.P.P., Vander's Renal Physiology. 7th ed2009: Mc Graw Hill LANGE.

3. Kaufman PL, A.F., Levin LA, Alm A, Adler's Physiology of the Eye 2011: Elsevier Health Sciences.

4. Garner, W.H., et al., Sodium-23 magnetic resonance imaging of the eye and lens. Proc Natl Acad Sci U S A, 1986. 83(6): p. 1901-5.

5. Maril, N., et al., Detection of evolving acute tubular necrosis with renal 23Na MRI: studies in rats. Kidney Int, 2006. 69(4): p. 765-8.

6. Chin, C.L., et al., BirdcageBuilder: Design of Specified-Geometry Birdcage Coils with Desired Current Pattern and Resonant Frequency. Concepts Magn Reson, 2002. 15(2): p. 156-163.

7. Akoka, S., et al., Radiofrequency map of an NMR coil by imaging. Magn Reson Imaging, 1993. 11(3): p. 437-41.

8. Wenz, D., et al., Millimeter spatial resolution in vivo sodium MRI of the human eye at 7 T using a dedicated radiofrequency transceiver array. Magn Reson Med, 2018. 80(2): p. 672-684.

Figures

Figure 1: A) Circuit diagram of the birdcage coil showing the rungs and capacitors. B) Three dimensional design of the ²³Na birdcage probe holding a rat model simulated using finite integration technique (CST Studio Suite) and the casing of the birdcage. The coil contains 16 rungs, each manufactured from 35 µm copper on a 0.1 mm thick FR-4 substrate.

Figure 2: A) Photograph of the open birdcage with the cable traps, capacitors and trimmer capacitors and (up) the final version RF coil illustrated with a rat placed inside (down). B) Photograph of the box with T/R switch and hybrid combiner and C) S-parameter matrix measured using a homogeneous phantom as a load at 105.8 MHz.

Figure 3: Schematic view of the coil design in axial orientation with the corresponding simulated B₁⁺-map for the quadrature mode in a phantom compared with the measured B₁⁺-map. B₁⁺-maps were acquired using double-angle-method⁷ with nominal excitation flip angles of 60° and 120°.

Figure 4: A) shows ¹H- and ²³Na image of the rat kidneys (axial slice of a 2D FLASH for ¹H (grey): TR: 100ms, TE: 4.2ms, Matrix: 256x256, FOV: (50x50)mm² and a 2D FLASH for ²³Na: TR: 60ms, TE: 1.5ms, Matrix: 64x64, FOV: (50x50)mm², slice thickness: 10mm, TA: 19min) and B) a ¹H- and ²³Na image of the rat eyes (coronal slice, 2D FLASH for²³Na: TR: 60ms, TE: 1.5ms, Matrix: 64x64, FOV: (50x50)mm², slice thickness: 7mm, TA: 29min).

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

1923