Synopsis

This talk will cover the basic theory and design of a RF coil, characterization of the RF coil on the bench including tuning, matching and quality factor with a vector network analyzer, on the scanner with B₁ maps and signal-to-noise ratio measurements and electromagnetic field simulations. Various types of RF coils as well as the recent RF coil concepts will be addressed.

Objectives

This lecture aims at:

• Providing basic RF coil design concepts such as tuning, matching, Q-factor and losses.
• Providing theory of building and characterizing of a RF coil.
• Clarifying RF coil characterization concepts on the bench and on the MRI scanner.
• Providing a brief overview of various RF coil designs.
• Reviewing the most recent developments in RF coil types.

Theory of RF coils

Matching and tuning

Radiofrequency (RF) coils play significant role in MR image quality. They are used for transmitting and receiving the RF signal on MRI scanners. To maximize the power transmission from the RF coil to the patient, it is crucial to match the impedance of the coil with the impedance of the MR scanner electronics and cabling while in the ‘patient’ loaded case. Also, the coil circuity needs to be tuned at the Larmor frequency which depends on the type of the nucleus and the strength of the main magnetic field (e.g., 128 MHz for ¹H at 3T). As the power transmission to the patient is maximized, the difference between the forward and reflected power will be minimized, therefore the transmit/receive switch circuitry as well as the RF amplifiers will be damage-protected from the excessive reflected power. As the coil tuned to the Larmor frequency and matched to the 50Ω (Ohm), tipping the magnetization from its equilibrium position during the appωlied RF pulse will be effective, e.g. the difference between the nominal and actual flip angle is minimized. Tuning and matching of the RF coil can be measured by using the vector network analyzer and quantified as:

S_ii=20log((Z_i-Z₀)/(Z_i+Z₀)

where Z₀ is the desired impedance (50Ω in this case) and Z_i is the RF coil impedance. Impedance is a complex quantity including resistance (R) and reactance (X) in alternating-current theory. During the tuning and matching process on the bench with the vector network analyzer (VNA), the aim is nulling the admittance component of the impedance and bring the resistance value to 50Ω by changing the lumped element values (mainly capacitor (C) values in pF (pico Farad) or inductor (L) values in nH (nano Henri) by changing the coil conductor length/geometry).

Z=R+iX=R+i(ωL-(1/ωC))

matched at R=50Ω and resonance at the Larmor frequency ω=1/√(LC)

Losses, quality factor, RF field and signal-to-noise ratio

The resistance R is also the power lost in the coil. These losses consist of coil circuitry losses (R_coil), resistive losses (R_R) and patient’s load losses (R_load). Specifically, R_coil mainly depends on the material, length and cross-section including capacity effects and skin effects at high frequencies; R_R is related to radiation losses, which relates to the scanner’s frequency, size of the conductor and the electromagnetic properties of the surroundings; and R_coil is related to the r$$elative power absorbed due to eddy-currents and electric fields in conductive tissue. These losses have direct impact on signal-to-noise ratio (SNR) of the MR image and can be visible at the bench with VNA.

The sharpness of the resonance peak on the VNA is described by the quality factor (Q-factor) which is defined as the resonance frequency divided by the full-width-half-maximum bandwidth. Without loading, the Q-factor of the coil is mainly determined by the Larmor frequency, the inductance L and the Rcoilas Q=L/Rcoil. Upon loading of the RF coil with a patient, the Q-factor should drop. If the ratio of the unloaded to loaded Q-factor is 6, the power loss in the patient (R_load) is 5-fold greater than the losses in the coil components (R_coil). A low Q-factor may indicate high loss or a large frequency bandwidth [Q.1]. The SNR of the image is strongly related with the following two parameters:

Signal≈B₁^-sin(B₁⁺)

Noise=√(4kTRΔf)

where k is the Boltzmann constant, T is the temperature of the coil and the patient in Kelvin, R is the resistance and Δf is the receiver bandwidth. While signal is linearly proportional to the receive field, B₁^-, i.e. magnetic field induced in the receive coil, and is sinusoidally proportional to the transmit field, B₁⁺, i.e. magnetic field generated by the transmit coil.

B₁⁺=(B_x+iB_y)/2

B₁^-=(B_x-iB_y)/2

The transmit (B₁⁺) and the receive (B₁^-) component of the RF field depends on the x and y component of the magnetic field.

Specific Absorption Rate (SAR)

The time-varying magnetic field generates electric field (E-field) which needs to be in consideration in terms of tissue heating. Tissue heating is characterized by specific absorption value (SAR) in W/kg given by:

SAR=(σE²)/(2ρ)

is a function of load size, geometry of coil, Larmor frequency (ω), RF pulse duration (τ), RF pulse repetition time (TR), tissue conductivity (σ), tissue dielectric constant (ε_r) and tissue density (ρ).

While the magnetic field generated by the RF coil can be characterized by the MR scanner and electromagnetic field (EM) simulations, the SAR value can only be calculated by the EM simulations. Therefore, to characterize the RF coil, the realistic EM simulation is necessary with the bench and MRI measurements. As a result, matching, tuning, losses, transmit and receive field, SAR and SNR are required parameters to verify the RF coil design parameters before proceeding to obtain MR images. All these aspects are summarized in references [R.1-4] and recommended books.

Transmit receive coil switch

The transmit/receive (T/R) switch is required for the coils for transmitting and receiving action. During the applied RF pulse for duration of micro-to milliseconds, the coil receives high power input typically in the range of kW. During transmit, all receive chain is blocked by the T/R switch to protect preamplifiers from high power input. It is controlled by the DC line of the scanner using PIN (positive intrinsic negative) diodes or MEMS (micro-electromechanical systems) [MEMS.1] on the coil circuitry. The receive signal is in the range of millivolts, therefore it is amplified by several stages of preamplifiers before digitized to be an image.

Volume Coils

Surface coils

Recommended books

1. Jin J. Electromagnetic analysis and design in magnetic resonance imaging: CRC Press; 1998.
2. Mispelter J.,Lupu L., Briguet A., NMR Probeheads for Biophysical and Biomedical Experiments. London: Imperial College Press, 2006.
3. Vaughan J.T., Griffith J.R. edt. RF coils for MRI. John Wiley Sons, UK 2012.
4. Webb, A.G. edt. Magnetic Resonance Technology: Hardware and System Component Design, The Royal Society of Chemistry 2016.
5. Collins C.M. Electromagnetics in Magnetic Resonance Imaging. IOP, 2016.
6. Taflove A., Hagness S.C. Computational electrodynamics: the finite difference time domain method. Artech House, Boston, 2000.

References

RF coil reviews:

[R.1] Ipek Ö. Radio-frequency coils for ultra-high field magnetic resonance. Analytical biochemistry 2017; 529, 10-16.

[R.2] Collins C.M., Wang Z. Calculation of radiofrequency electromagnetic field and their effects in MRI of human subjects. Magn Reson Med. 2011; 65, 1470-1482.

[R.3] Fujita, H. New horizons in MR technology: RF coil designs anfd trends, Magn. Reson. Med. Sci. 2017; 6 (1), 29-42.

[R.4] Doty F.D., Entzminger G. Kulkarno J., Pamarthy K. Staab J.P. Radio frequency coil technology for small-animal MRI. NMR Biomed. 2007; 20, 304-325.

Q-factor:

[Q.1] Kumar A., Edelstein W.A., Bottomley P.A., Noise figure limits for circular loop MR coils, Magn. Reson. Med. 2009; 61, 1201-1209.

MEMS:

[MEMS.1] Maunder A. Rao M. Robb F. Comparison of MEMS switches and PIN diofes for switched dual tuned RF coils. Magn Reson Med. 2018; 1746-1753.

Volume coils:

[V.1] Clement J.D., Magill A., Lei H., Ipek O., Gruetter R. Slotted-tube-resonator design for whole body MR imaging at 14T. Proc. Intl. Mag. Reson. Med., 2016.

[V.2] Ibrahim, T.S., Hue Y-K. Boada F.E., Gilbert R. Tic Tac Toe: Highly-Coupled, Load Insensitive Tx/Rx Array and a Quadrature Coil Without Lumped Capacitors. in Intl. Soc. Mag. Reson. Med. 2008. 136.

Birdcage:

[B.1] Hayes C.E., Axel L. An efficient, highly homogeneous radiofrequency coil for whole-body NMR imaging at 1.5T. J Magn Reson 1985;63:622–628.

[B.2] de Zwart J.A., Ledden P.J., van Gelderen P. Bodurka J., Chu R., Duyn J.H. Signal-to-noise ratio and parallel imaging performance of a 16-channel receive-only brain coil array at 3.0 Tesla. Magn Reson Med. 2003; 51,22-26.

[B.3] Alagappan V., Nistler J., Adalsteinsson E., Setsompop K., Fontius U., Zelinski A., Vester M., Wiggins G.C., Hebrank F., Renz W. Degenerate mode band‐pass birdcage coil for accelerated parallel excitation. Magn Reson Med 2007; 57(6):1148- 1158.

[B.4] Sadeghi-Tarakameh A., Kazemivalipour E., Demir T., Gundogdu U., Atalar E. Design of a Degenerate Birdcage Radiofrequency Transmit Array Coil for the Magnetic Resonance Imaging Using Equivalent Circuit Model. Proceeding of the 34th Annual Scientific Meeting of ESMRMB, Barcelona, Spain 2017:300-301.

TEM:

[TEM.1] Ibrahim T.S., Lee R., Baertlein B.A., Abduljalil A.M., Zhu H., Robitaille P.L., Effect of RF coil excitation on field inhomogeneity at ultra high fields: a field optimized TEM resonator. Magn. Reson. Imaging2001;19,1339-1347.

[TEM.2] Vaughan J.T., Adriany G., Garwood M., Yacoub E., Duong T., DelaBarre L., Andersen P., Ugurbil K. Detunable transverse electromagnetic (TEM) volume coil forhigh-field NMR. Magn Reson Med.2002:47(5):990–1000.

[TEM.3] Wiggins G.C., Potthast A., Triantafyllou C., Wiggins C.J. and Wald L.L.Eight‐channel phased array coil and detunable TEM volume coil for 7 T brain imaging. Magn Reson Med. 2005; 54, 235-240.

Microstrip:

[M.1] Zhang, X.; Ugurbil, K., and Chen, W. Microstrip RF surface coil design for extremely high-field mri and spectroscopy. Magn Reson Med, 46 (3):443–50, 2001.

[M.2] Orzada S., Kraff O., Oehmigen N., Gratz M., Johst S., Volker M.N., Rietsch S.H.G., Floser H., Fiedler T., Shooshtary S., Solbach K., Wuich H.H., Ladd M.E., A 32-channel integrated body coil for 7 Tesla whole-body imaging, Proc. Intl. Mag. Reson. Med., 2016.

Loops:

[L.1] Boskamp EB. Improved surface coil imaging in MR: decoupling of the excitation and receiver coils. Radiology 1985;157:449–452.

[L.2] Vaidya M.V., Collins C.M., Sodickson D.K., Brown R., Wiggins G.C, Lattanzi R. Dependence of B1+ and B1- field patterns of surface coil on the electrical properties of the sample and the MR operating frequency. Concepts Magn Reson Part B Magn Reson Eng. 2016;46, 25-40.

[L.3] Webb A.G., Collins C.M. Verluis M.J., Kan H.E. Smith N.B. MRI and localized proton spectroscopy in human leg muscle at 7 Tesla using longitudinal travelling waves. Magn Reson Med. 2010;63(2), 297-302.

[L.4] Brown R., Storey P., Geppert C., McGorty K., Leite A.P.K., Babb J., Sodickson D.K., Wiggins G.C., Moy L., Breast MRI at 7 Tesla with a bilateral coil and T1-weighted acquisition with robust fat suppression: image evaluation and comparison with 3 Tesla, European Radiology2013;23, 2969-2978.

[L.5] van de Bank, B.L., Voogt I.J., Italiaander M., Stehouwer B.L., Boer V.O. Luijten P.R., Klomp D.W.J. Ultra high spatial and temporal resolution breast imaging at 7T. NMR in Biomedicine, 2013. 26(4): p. 367-375.

[L.6] McDougall, M.P., et al., Quadrature transmit coil for breast imaging at 7 tesla using forced current excitation for improved homogeneity. Journal of Magnetic Resonance Imaging, 2014. 40(5): p. 1165-1173.

[L.7] Avdievich N.I., Giapitzakis I.A., Pfrommer A., Borbath T., Henning A. Combination of surface and ‘vertical’ loop elements improves receive performance of a human head transceiver array at 9.4 T. NMR in Biomed 2018;31(2):e3878.

[L.8] Wiggins G.C., Triantafyllou C., Potthast A., Reykowski A., Nittka M., Wald L.L.32-channel 3 Tesla receive-only phased-array head coil with soccer-ball element geometry.Magn Reson Med.2016;56(1):216–223.

Dipole:

[D.1] Ipek O., Raaijmakers A.J., Klomp D.W., Lagendijk J.J., Luijten P.R., van den Berg C.A. Characterization of transceive surface element designs for 7 tesla magnetic resonance imaging of the prostate: radiative antenna and microstrip, Physics in medicine and biology, 2012; 57 343-355.

[D.2] Raaijmakers A.J.E., Ipek O., Klomp D.W.J., Possanzini C., Harvey P.R., Lagendijk J.J., van den Berg C.A.T. Design of a radiative surface coil array element at 7 T: The single-side adapted dipole antenna. Magn Reson Med. 2011; 66(5):1488–1497.

[D.3] Ipek O., Raaijmakers A., Lagendijk J., Luijten P., van den Berg C. Optimization of the radiative antenna for 7-T magnetic resonance body imaging. Concepts in Magnetic Resonance Part B: Magnetic Resonance Engineering, 2013;43B(1):1–10.

[D.4] Gang C., Cloos M., Wiggins G.C. An interleaved opposing monopole transmit-receive array for 7T brain imaging. Proc 22nd Annual Meeting ISMRM, Milan; 2014.

[D.5] Lee W., Cloos M.A., Sodickson D.K., Wiggins G.C., Parallel transceiver array design using the modified folded dipole for 7T body applications. Proc 21st Annual Meeting ISMRM, Salt Lake City; 2013.

[D.6] Lakshmanan K., Cloos M., Lattanzi R., Sodickson D.K. The circular dipole. Proc 22nd Annual Meeting ISMRM, Milan; 2014.

[D.7] Oezerdem C., Winter L., Graessl A., Paul K., Els A., Weinberger O., Rieger J.,Kuehne A., Dieringer M., Hezel F. ,Voit D., Frahm J., Niendorf T. 16-channel bow tie antenna transceiver array for cardiac MR at 7.0 tesla. Mag Reson Med. 2016; 75(6):2553–2565.

[D.8] Ipek O., Gruetter R. Feasibility of crossed-dipole antenna to excite a circularly-polarizedfield for human brain imaging at 7T, a design study. Proc. Intl. Soc. Mag. Reson. Med. 24(2016), 3539.

[D.9] Raaijmakers A.J.E., Italiaander M., Voogt I.J., Luijten P.R., Hoogduin J.M., Klomp D.W.J., van den Berg C.A.T. The fractionated dipole antenna: A new antenna for body imaging at 7 Tesla. Magn Reson Med 2016; 75(3):1366–1374.

[D.10] Zivkovic I., O’Reilly T., Brimk W., Webb A. Evaluation of Egyptian axe dipole antenna as an array element for head imaging at 7T MRI. Proc 26th Annual Meeting ISMRM, Paris; 2018.

[D.11] Eryaman Y, Geurin B, Kosior R, Adalsteinsson E, Wald LL. Combined Loop dipole arrays for 7T brain imaging. In: Proc 21st Annual Meeting ISMRM, Salt Lake City; 2013.

[D.12] Ertürk M.A., Raaijmakers A.R., Adriany G., Ugurbil K., Metzger G.J. A 16-channel combined loop-dipole transceiver array for 7 Tesla body MRI. Magn Reson Med. 2017; 77(2):884–894.

[D.13] Clement J.D., Gruetter R., Ipek O. A human cerebral and cerebellar 8-channel transceive RF dipole coil array at 7T. Magn Reson Med. 2019; 81(2), 1447-1458.

[D.14] Clement J, Gruetter R., Ipek O. 31-channel receive coil array combined with an 8-channel whole-brain dipole transmit array. Proc. Intl. Soc. Mag. Reson. Med. 26(2018), 0146.

[D.15] Steensma B.R., Voogt I.J., Leiner T., Luijten P.R., Habets J., Klomp D.W.J., van den Berg C.A.T., Raaijmakers A.J.R. An 8-channel Tx/Rx dipole array combined with 16 Rx loops for high-resolution functional cardiac imaging at 7 T. Magn Reson Mat in Phys, Biol Med. 2018; 31(1):7–18.

[D.16] Paska J., Cloos M.A., Wiggins G.C. A rigid stand-off hybrid dipole, and birdcage coil array for 7T imaging. Mag. Reson. Med. 2018;80,822-832.

Flexible coils:

[F.1] Corea J.R., Flynn A.M., Lechene B., Scott G., Reed G.D., Shin P.J., Lustig M., Arias A.C. Screen-printed flexible MRI receive coils, Nat Commun. 2016;7.

[F.2] Mager D., Peter A., Tin L.D., Fischer E., Smith P.J., Hennig J., Korvink J.G. An MRI Receiver Coil Produced by Inkjet Printing Directly on to a Flexible Substrate, IEEE Transactions on Medical Imaging 2010;29,482-487.

[F.3] Nordmeyer-Massner J.A., De Zanche N., Pruessmann K.P. Stretchable coil arrays application to knew imaging under varying flexion angles. Magn Reson Med. 2012; 67, 872-879.

Wireless Transmit:

[W.1] Zeng X., Xu S., Cao C., Wang J. Qian C. Wireless amplified NMR detector for improved visibility of image contrast in heterogeneous lesions. NMR Biomed. 2018; 31:e3963.

[W.2] Qian C., Murphy-Boesch J., Dodd S. Koretsky A. Sensitivity enhancement of remotely coupled NMR detectors using wirelessly powered parametric amplification. Magn Reson Med. 2012; 68(3), 989-996.

Wireless Receive:

[W.3] Schnall M.D., Barlow C., Subramanian V.H., Leigh J.S. Wireless implanted magnetic‐resonance probes for in vivo NMR. J Magn Reson. 1986;68(1):161‐167.

[W.4] Quick H.H., Zenge M.O., Kuehl H., Kaiser G. Aker S., Massing S., Bosk S. Ladd M.E. Interventional magnetic resonance angiography with no strings attached: wireless active catheter visualization. Magn Reson Med. 2005;53(2): 446-455.

Microcoils:

[Mic.1] Olson D.L., Peck T.L., Webb A.G., Magin R.L., Sweedler J.V. High‐resolution microcoil 1H‐NMR for mass‐limited, nanoliter‐volume samples. Science. 1995;270(5244):1967‐1970.

[Mic.2] Atalar E., Bottomley P.A., Ocali O., Correia L.C., Kelemen M.D., Lima J.A. Zerhouni E.A. High resolution intravascular MRI and MRS by using a catheter receiver coil. Magn Reson Med. 1996;36(4):596‐605.

Acknowledgements

References