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A Dedicated RF Coil System for 19F MRI of Myocardial Infarction at a 3 T Clinical MRI System

Ali Caglar Özen^1,2, Felix Spreter¹, Timo Heidt³, Constantin von zur Mühlen³, and Michael Bock¹
¹Deptartment of Radiology, Medical Physics, University Medical Center Freiburg, University of Freiburg, Freiburg, Germany, ²German Consortium for Translational Cancer Research Partner Site Freiburg, German Cancer Research Center (DKFZ), Heidelberg, Germany, ³Department of Cardiology and Angiology I, UHZ, University Medical Center and Faculty of Medicine, University of Freiburg, Freiburg, Germany

Synopsis

Fluorine magnetic resonance imaging is a non-invasive background-free MRI approach to image infiltrating monocytes. The transfer of¹⁹F MRI into large animals is needed for both method validation and the development of clinical applications. In this study, we have developed an 8 transmit/ 8 receive coil array tuned to 115.9 MHz (at 3 T) and developed a detachable 19F coil system for cardiac and thoracic MRI in pigs.

Introduction

¹⁹F MRI has been successfully implemented in small animals to study myeloid cell uptake and migration after myocardial infarction and in atherosclerosis [1-7], but only few reports are published where this imaging method has been used in human-size models such as pigs. The transfer of ¹⁹F MRI into large animals, however, is crucial for both method validation and the development of clinical applications. The purpose of this study is to develop an RF coil system to evaluate ¹⁹F cardiac MRI at 3 T in a pig model after myocardial infarction to assess early and late inflammatory responses. To this end, we have developed an 8 transmit/ 8 receive coil array tuned to 115.9 MHz and developed a detachable ¹⁹F coil system for cardiac and thoracic MRI in pigs.

Methods

The RF coil system consists of an 8 channel transmit system of 8 independent symmetrically distributed loop elements (size: 6x15 cm²) - 5 elements were fixed on a detachable upper part made of a polycarbonate plate. Tx array was driven in CP-mode by setting 45° phase increment between the consecutive elements. An 8 channel Rx coil array consists of a flexible 6-channel posterior (Fig1. B) and a two-channel planar anterior arrays (Fig1. C). EM-simulations were performed using the FDTD solver of Sim4Life 6.0 (ZMT, Zurich, Switzerland) with two pig models segmented from previously acquired 3D FLASH data sets, where of lung and heart tissue were modeled as uniform organs with isotropic dielectric properties [8] (Fig. 2 A,B).
To test the MRI performance of the system, a simplified version of the Tx array with two Tx coils was used together with the 6-element anterior part of the Rx array. The coil test was performed on a clinical 3T MRI system (PRISMA Fit, Siemens, Erlangen). For proper loading of the coil system, half-cylinder-shaped phantom (Fig. 2C) was filled with 3g/L NaCl and 1g/L CuSO₄ solution (ε_r = 78.2, σ = 0.46 S/m at 115.9 MHz). A 15-ml-test-tube filled with FC-84 liquid (3M, St. Paul, MN, USA) was measured using a 2D GRE sequence (TR =10ms, TE = 2ms, FA = 40°, BW = 330Hz/px, Base resolution = 128, FOV = 128mm)

Results

FDTD simulations of Tx coils show that a homogeneous B₁⁺ around the heart region can be obtained using 8, 7, or 5 loop elements with the maximum deviation from the mean || B₁⁺|| by 22%, 18%, and 25%, respectively (Fig. 3). As the RF power amplifier of the MRI system provides maximum peak RF power of 3.2kW at 115.9MHz, and a total input power of 2kW is assumed (ignoring losses in the 8-channel power splitter), a 180° pulse can be obtained with a 3ms-long rectangular RF pulse.
Test bench measurements of the anterior part of the Rx array were performed under phantom and in vivo loading inside the magnet room without changing the tuning and matching. In vitro and in vivo S_ij matrices are shown in Fig. 4. Coupling between the elements was always below -16 dB. Phantom images using the first prototype system are shown in Fig. 5. As the spectrum of FC-84 consist of several resonances that are assigned to the individual ¹⁹F atoms, chemical shift artefacts are observed along the readout direction. SNR of the spatially encoded different 19F components are 23, 45, and 11 (Fig. 5).

Conclusion

As seen in FDTD simulations, a homogeneous B₁⁺ distribution can be achieved over the heart of an adult pig with an array of local Tx loop coils. In combination with an optimized Rx coil array which conforms to the special geometry of a pig supine position high SNR can be achieved. S_ij parameters do not deviate significantly between the phantom and in vivo loading conditions.

Acknowledgements

Grant support by the German Science Foundation (DFG) under CRC 1425 (Project P15) is gratefully acknowledged.

References

[1] Jacoby, C., Temme, S., Mayenfels, F., Benoit, N., Krafft, M.P., Schubert, R., Schrader, J. and Flögel, U. (2014), Probing different perfluorocarbons for in vivo inflammation imaging by 19F MRI: image reconstruction, biological half‐lives and sensitivity. NMR Biomed., 27: 261-271. https://doi.org/10.1002/nbm.3059

[2] Ahrens ET, Flores R, Xu H, Morel PA. In vivo imaging platform for tracking immunotherapeutic cells. Nat. Biotechnol. 2005; 23: 983– 987.

[3] Flögel U, Ding Z, Hardung H, Jander S, Reichmann G, Jacoby C, Schubert R, Schrader J. In vivo monitoring of inflammation after cardiac and cerebral ischemia by fluorine magnetic resonance imaging. Circulation 2008; 118: 140– 148.

[4] Nahrendorf M, Pittet MJ, Swirski FK. Monocytes: protagonists of infarct inflammation and repair after myocardial infarction. Circulation. 2010 Jun 8;121(22):2437-45. doi: 10.1161/CIRCULATIONAHA.109.916346. PMID: 20530020; PMCID: PMC2892474.

[5] Nahrendorf M, Swirski FK. Monocyte and macrophage heterogeneity in the heart. Circ Res. 2013 Jun 7;112(12):1624-33. doi: 10.1161/CIRCRESAHA.113.300890. PMID: 23743228; PMCID: PMC3753681.

[6] Temme, S., Jacoby, C., Ding, Z., Bönner, F., Borg, N., Schrader, J. and Flögel, U. (2014), Technical Advance: Monitoring the trafficking of neutrophil granulocytes and monocytes during the course of tissue inflammation by noninvasive 19F MRI. Journal of Leukocyte Biology, 95: 689-697. https://doi.org/10.1189/jlb.0113032

[7] Bönner F, Merx MW, Klingel K, Begovatz P, Flögel U, Sager M, Temme S, Jacoby C, Salehi Ravesh M, Grapentin C, Schubert R, Bunke J, Roden M, Kelm M, Schrader J. Monocyte imaging after myocardial infarction with 19F MRI at 3 T: a pilot study in explanted porcine hearts. Eur Heart J Cardiovasc Imaging. 2015 Jun;16(6):612-20. doi: 10.1093/ehjci/jev008. Epub 2015 Mar 1. PMID: 25733209.

[8] Hasgall PA, Di Gennaro F, Baumgartner C, Neufeld E, Lloyd B, Gosselin MC, Payne D, Klingenböck A, Kuster N, “IT’IS Database for thermal and electromagnetic parameters of biological tissues,” Version 4.0, May 15, 2018, DOI: 10.13099/VIP21000-04-0.

Figures

Fig. 1: Schematic and layout of the dedicated ¹⁹F MRI RF coil system. Printed circuit boards of the Tx and Rx coil elements are shown on the left hand side. S_ij curves, circuit diagrams and the component values can be found in https://github.com/alibaz/F19Coils

Fig. 2: Representative model of the half-cylinder phantom placed on a rectangular phantom as in the test bench measurements (left). Small and large pig models generated by segmenting 3D FLASH image set (center, right).

Fig. 3: 8 (left), 7 (center) and 5 (right) channels of the pTx ¹⁹F coil system for the segmented large pig. White circle encloses the heart. Reducing the number of elements does not degrade the performance of Tx coil system significantly.

Fig. 4: S matrix of the 6 channel anterior part of the 19F MRI coil system measured with phantom and in vivo loading. In the phantom S_ii<-20dB and S_ij<-16 dB, and in the animal an S_ii<-14dB and S_ij<-18 dB was achieved.

Fig. 5: A transverse slice from the GRE image of the FC-84 test tube. As the spectrum of FC-84 three resonances corresponding to the individual 19F atoms, chemical shift artefacts are observed along the readout direction.

Proc. Intl. Soc. Mag. Reson. Med. 29 (2021)

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