Detailing Local Multi-Channel RF Surface Coil versus Body RF Coil Transmission for Cardiac MRI at 3 Tesla: Which Configuration is Winning the Game?
Oliver Weinberger1,2, Lukas Winter1, Matthias A Dieringer1, Antje Els1, Celal Oezerdem1, Antonino Cassara3, Harald Pfeiffer3, and Thoralf Niendorf1,2

1Berlin Ultrahigh Field Facility (BUFF), Max Delbrueck Center for Molecular Medicine (MDC), Berlin, Germany, 2Experimental and Clinical Research Center (ECRC), Charité Medical Faculty, Berlin, Germany, 3Physikalisch Technische Bundesanstalt (PTB), Berlin, Germany

Synopsis

In this work a local four-channel transmit/receive RF coil dedicated for cardiac MR at 3T is compared to a conventional built-in body RF coil in conjunction with a four-channel receive-only RF coil. SAR and B1+ simulations of both configurations are shown. The invivo efficiency performance of both coils in respect to B1+/sqrt(SAR) is demonstrated in 12 healthy subjects. The efficiency surplus of the local RF coil was used to increase the applicable flip angle FASSFP of a standard high resolution 2D SSFP protocol or to shorten the used repetition time TRSSFP by 54%.

Introduction

In current clinical cardiac MR (CMR) integrated body RF coils are commonly used for 1H excitation, which deposit the RF energy/SAR in a large volume of the patient/subject. Progress in ultrahigh field MR (B0≥7T), where body RF coils are not provided, demonstrated that local transceiver (TX/RX) RF coil arrays are suitable for RF excitation of deep lying regions such as the heart(1,2). Recognizing this opportunity, this work demonstrates the feasibility of CMR at 3T with a local four-channel TX/RX RF coil array(3,4). This setup is benchmarked against the clinical standard of a body RF coil for excitation in conjunction with a four-channel RX-only RF coil. Our assessment includes electromagnetic field (EMF) simulations to detail SAR and B1+-homogeneity and -efficiency for the local and body RF coil configuration and their in vivo performance for high spatial resolution 2D SSFP-CINE imaging of the heart.

Methods

12 volunteers underwent CMR at a 3T whole-body MR system (MAGNETOM Verio, Siemens Healthcare, Erlangen, Germany). A four-channel transceiver RF coil (4TX/4RX) was constructed with 4 rectangular loop elements (H-F=180mm, L-R=120mm) (Fig 1). Basic B1+-phase shimming was facilitated by phase-shifting cables. The TX phase setting Φ was derived from EMF simulations (CST AG, Darmstadt, Germany) using voxel model Duke. Φ minimizes

$$f(\phi)=\frac{std(B_1^+(\phi))}{mean(B_1^+(\phi))}-\frac{MOS(B_1^+(\phi))}{SOM(B_1^+(\phi))}\frac{1}{\sqrt{\max(SAR_{10g}(\phi))}}$$

so simultaneously optimizes B1+-homogeneity (1st term), -efficiency (2nd term) and local SAR10g (3rd term). |B1+| denotes the magnetization within Duke’s heart, MOS/SOM is the magnitude of sum/sum of magnitude. Rapid SAR10g computations were conducted with pre-calculated 4x4-SAR-matrices (SimOpTx, Vienna, Austria). For all simulations maximum SAR10g was calculated and used to determine the maximum RF input power according to IEC guidelines (in normal operating mode)(5). The 4TX/4RX RF coil was benchmarked against the built-in body RF coil (BC) together with a home-built four-channel receive-only RF coil (4RX), resembling the 4TX/4RX coil geometry. Protocol parameters of invivo 2D Bloch-Siegert-B1+-mapping: spatial resolution=5.3x5.3x6mm³, 4.5ms Fermi pulse, off-center frequency: 4kHz, TE/TR=7.3/80ms, TA=15s. Based on the B1+-maps flip angle maps were calculated. Protocol parameters of invivo 2D SSFP CINE technique: spatial resolution=1.8x1.8x6mm³, 30 cardiac phases, TA=15s (within one breath hold), TE/TR=1.4/3.2ms, FA maximal to reach SAR limit. For the body RF coil an additional SSFP protocol with a reduced flip angle FA’ was acquired, which assumes local instead of whole-body SAR limits:

$$FA'=\sqrt{\frac{local SAR limit}{whole-bodySARlimit}}*\sqrt{\frac{whole-body SAR_{Simulation}}{\max(localSAR_{Simulation})}}*FA\approx0.7*FA$$

To increase FAROI TRSSFP was changed by varying the receiver bandwidth BWRX, while keeping all other imaging parameters constant.

Results

The SAR simulation of the body coil showed that the local SAR10g limit is by a factor of 40% more restrictive than the whole-body SAR limit (Fig 2). Therefore the local SAR limit was additionally used for the body coil. The transmit efficiency $$$B_1^+@SARlimit=B_1^+*\sqrt{\frac{SARlimit}{SAR_{Simulation}}}\sim FA$$$ was chosen to compare the transmission regimes: within Duke’s heart B1+@SAR-limit=130/92µT for BC@whole-body/local SAR limit, and 97µT for 4TX/4RX@local SAR limit (Fig 3). The simulated transmit homogeneity std(B1+)/mean(B1+) was 15% for BC/4RX and 30% for 4TX/4RX. The quality of the in vivo images of all transmission regimes was clinically acceptable and not affected by signal voids. The transmit efficiency advantage of 4TX/4RX manifests in a higher mean FAROI (Fig 4 top row), while the BC is more B1+-homogenous. For an apical SAX this efficiency of the 4TX/4RX setup (TRmin=3.8ms) reduced TRmin by 29%/54% compared to the body RF coil transmission at whole-body/local SAR limit (TRmin=4.7/8.3ms). The TRmin shortening of 4TX/4RX helped to reduce SSFP banding artifacts and relaxed the constraints on the fidelity of volume-selective B0-shimming.

Discussion/Conclusion

Although whole-body SAR limits are used for the body RF coil in clinical cardiac MR, local SAR10g limits are more restrictive. When applying the maximum power to reach the whole-body SAR limit with the body RF coil the local SAR10g limit is already exceeded by 90%, which agrees with (6,7). The short distance to the human chest renders the local transmit RF coil more efficient than the body RF coil in terms of transmit efficiency $$$ B_1^+/\sqrt{P_{in}} $$$ as well as $$$B_1^+/\sqrt{localSAR_{10g}}$$$. High transmission efficiency is clinically relevant for CMR, where cardiac and respiratory motion makes rapid imaging techniques such as SSFP mandatory. Moreover, body RF coil transmission induces RF power deposition in a large volume including body regions far away from the ROI, which constitute an RF-heating related contraindication for patients with implants(8). Local transmit RF coils restrict the volume of power deposition and hence permit inclusion of these patients into CMR examinations. To conclude, pursuing local TX/RX RF coil arrays in CMR is a conceptually appealing alternative to body RF coil transmission and has high clinical potential and implications for future’s MRI.

Acknowledgements

Jan Rieger and Andre Kuehne from MRI.TOOLS GmbH, Berlin, Germany

References

(1) Snyder CJ, DelaBarre L, Metzger GJ, van de Moortele PF, Akgun C, Ugurbil K, Vaughan JT. Initial results of cardiac imaging at 7 Tesla. Magnetic resonance in medicine : official journal of the Society of Magnetic Resonance in Medicine / Society of Magnetic Resonance in Medicine 2009;61(3):517-524.

(2) Graessl A, Renz W, Hezel F, Dieringer MA, Winter L, Oezerdem C, Rieger J, Kellman P, Santoro D, Lindel TD, Frauenrath T, Pfeiffer H, Niendorf T. Modular 32-channel transceiver coil array for cardiac MRI at 7.0T. Magnetic resonance in medicine : official journal of the Society of Magnetic Resonance in Medicine / Society of Magnetic Resonance in Medicine 2014;72(1):276-290.

(3) Frauenrath T, Pfeiffer H, Hezel F, Dieringer MA, Winter L, Graessl A, Santoro D, Oezerdem C, Renz W, Greiser A, Niendorf T. Lessons Learned from Cardiac MRI at 7.0 T: LV Function Assessment at 3.0 T Using Local Multi-Channel Transceiver Coil Arrays. Proc ISMRM 2012:2803.

(4) Kraus O, Winter L, Dieringer MA, Graessl A, Rieger J, Oezerdem C, Hezel F, Kuehne A, Waxmann P, Pfeiffer H, Niendorf T. Local Coil versus Conventional Body Coil Transmission for Cardiac MR: B1+ Efficiency Improvements and Enhanced Blood Myocardium Contrast for 2D CINE SSFP Imaging at 3T. Proc ISMRM 2014:0947.

(5) IEC. Medical electrical equipment. Part 2-33: Particular requirements for the safety of magnetic resonance equipment for medical diagnosis. 2015;60601-2-33 Ed.3.2.

(6) Wang Z, Lin JC, Mao W, Liu W, Smith MB, Collins CM. SAR and temperature: simulations and comparison to regulatory limits for MRI. Journal of magnetic resonance imaging : JMRI 2007;26(2):437-441.

(7) Yeo DT, Wang Z, Loew W, Vogel MW, Hancu I. Local specific absorption rate in high-pass birdcage and transverse electromagnetic body coils for multiple human body models in clinical landmark positions at 3T. Journal of magnetic resonance imaging : JMRI 2011;33(5):1209-1217.

(8) Zilberti L, Bottauscio O, Chiampi M, Hand J, Lopez HS, Bruhl R, Crozier S. Numerical prediction of temperature elevation induced around metallic hip prostheses by traditional, split, and uniplanar gradient coils. Magn Reson Med 2015.

Figures

a) Photograph of local four-channel TX/RX RF coil array (4TX/4RX). b) EMF simulation setup of 4TX/4RX RF coil loaded with truncated voxel model Duke. Feeding ports are marked in red. c) EMF simulation setup of the body RF coil (BC) loaded with the voxel model Duke and a 4RX coil.

Maximum projection images of the simulated SAR10g-distributions for 1W RF input power. Formations of SAR hotspots can be seen for both transmission regimes, with the dominating SAR hotspots being marked by arrows (At the elbow twice the local SAR is allowed according to IEC). Note the different color scales.

B1+-fields of Duke derived from EMF simulations. Orthogonal slices and the borders of the heart are highlighted. 1st/2nd column: BC/4RX scaled to whole-body/local SAR limit. 3rd column: 4TX/4RX RF coil scaled to local SAR10g limit. Column 1 and 3 reflect the maximum achievable excitation fields governed by IEC guidelines.

1st row: SSFP-CINE images with minimal TR. The resulting FA (mean±std) to reach the denoted SAR limit is given. 2nd row: SSFP-CINE images with fixed FAROI=60° based on FA-map (3rd row). The resulting TRSSFP to reach the denoted SAR limit are given. Banding artifacts within the ROI are highlighted.



Proc. Intl. Soc. Mag. Reson. Med. 24 (2016)
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