Dynamic Dual Frequency Transmit and Receive Coil Pair for Development of a New Open MRI System
Charles Rogers III1, Gigi Galiana1, and Todd Constable1

1Radiology and Biomedical Imaging, Yale School of Medicine, New Haven, CT, United States


We present progress on development and testing of a two-coil array with each coil separately and dynamically tunable between dual frequencies. This test setup works toward the goal of developing a larger nine-element coil array for a new style of open MRI system utilizing the Bloch-Siegert shift at low Bo field. The array design is such that pairs of coils are selected for transmission at ~850kHz while allowing simultaneous receive at ~1MHz on the non transmitting coils. We discuss the construction and present measurements of RF scattering parameters of a two-coil test setup.


Developing a new type of low-cost, silent and open MRI scanner has the potential to benefit patient outcomes and reduce costs by allowing MRI to be more widely utilized with improved patient comfort. To achieve this, we are investigating the design of a scanner that uses a Bo that is time-varying, low-field (< 0.4 Tesla), and resistive. Our new design utilizes imaging techniques in non-linear fields with spatial encoding achieved by the Bloch-Siegert phase shift1 as an alternative to non-linear gradients2-4.

Previous work with RF for spatial encoding has mostly focused on generating linear phasors5, but RF is naturally nonlinear and thus nonlinear spatial encoding with Bloch-Siegert shift is not difficult to perform. The phasors in the new open MRI design will be created by a nine-element coil array with unique pairs of transmit coils selected. The combination of coil pairs should provide enough variability in the phasors to be sufficient to utilize previously demonstrated nonlinear imaging methods2-4. The seven non-transmitting coils can then be dynamically set for each phasor to act in a parallel receive configuration. The goal in this work is to use Bloch-Siegert encoding much like a readout gradient such that we simultaneously apply RF spatial encoding while reading out the MR signal. Simultaneous Tx/Rx has been previously reported6 for cases where both Tx and Rx are at the same frequency. To use Bloch-Siegert RF as a readout field during receive we require coil elements tuned to different frequencies.

To test the feasibility and performance of the RF coil array, we constructed a pair of coils shown in Fig. 1 with resonant frequencies controlled by a network of PIN diodes and capacitors for frequency tuning and matching as summarized in Fig. 2. The resonant frequency of each coil can be either 1 MHz (our receiver frequency), or 865 kHz (our off-resonant frequency for the Bloch-Siegert shift). The frequency is set by using forward or reversed bias in a PIN diode network that puts a specific coil element either in Bloch-Siegert transmit mode or in receive mode. Preliminary measurements of the test coil pair shown in Fig. 1, suggest that we can achieve S11 between the coils of greater than 30 dB when the spacing between coil centers is close to the expected 0.75 fraction of the coil diameter7. The preliminary S21 measurement indicates that the off resonant coupling (one coil set to Tx frequency and the other set to Rx) between the coils is reduced by 40 dB, as shown in Fig. 3.


This two element array demonstrates the feasibility of performing simultaneous Bloch-Siegert encoding analogous to a readout gradient while simultaneously using other coil elements for receive. At low field this nonlinear approach to spatial encoding can eliminate the need for spatial encoding gradient fields and their associated hardware.


No acknowledgement found.


1. Kartäusch, R., Driessle, T., Kampf, T., Basse-Lüsebrink, T.C., Hoelscher, U.C., Jakob, P.M., Fidler, F. and Helluy, X., 2014. Spatial phase encoding exploiting the Bloch–Siegert shift effect. Magnetic Resonance Materials in Physics, Biology and Medicine, 27(5), pp.363-371.

2. Constable, R.T. and Galiana, G., Yale University, 2016. Single-echo imaging with nonlinear magnetic gradients. U.S. Patent Application 15/037,867.

3. Wang, H., Tam, L.K., Constable, R.T. and Galiana, G., 2016. Fast rotary nonlinear spatial acquisition (FRONSAC) imaging. Magnetic resonance in medicine, 75(3), pp.1154-1165.

4. Gallichan, D., Cocosco, C.A., Dewdney, A., Schultz, G., Welz, A., Hennig, J. and Zaitsev, M., 2011. Simultaneously driven linear and nonlinear spatial encoding fields in MRI. Magnetic resonance in medicine, 65(3), pp.702-714.

5. Sharp, J.C. and King, S.B., 2010. MRI using radiofrequency magnetic field phase gradients. Magnetic resonance in medicine, 63(1), pp.151-161.

6. Sohn, S.M., Vaughan, J.T., Lagore, R.L., Garwood, M. and Idiyatullin, D., 2016. In vivo MR imaging with simultaneous RF transmission and reception. Magnetic resonance in medicine, 76(6), pp.1932-1938.

7. Roemer, P.B., Edelstein, W.A., Hayes, C.E., Souza, S.P. and Mueller, O.M., 1990. The NMR phased array. Magnetic resonance in medicine, 16(2), pp.192-225.


Figure 1: Photo of the two test coils used for measuring S11 and S21 at the desired Tx and Rx frequencies to utilize the Bloch-Siegert shift. The coils are each approximately 15cm in diameter and consist of 4 turns in each of square magnet wire with a single-strand cross sectional area of about 3 mm2.

Figure 2: Circuit diagram of one of the coils used to select between two frequencies using PIN diodes and tuning and matching capacitors. If the PIN CTRL is set from a PIN driver (not shown) to forward bias D1-D3, the circuit will operate in Tx mode at 865 kHz. When reversed biased, the coil will be tuned to the Rx frequency at 1 MHz. The inductance of the coil is shown as L6 with the associated effective series resistance given by R4.

Figure 3: Vector network analyzer measurements S11 and S21 of the dynamically tunable coil pair. Marker 1 is at the Tx frequency of 865 kHz and shows the RF input matching S11 of -38 dB. At the 1 MHz Rx frequency shown at marker 2, the S21 is -41 dB.

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