Shunt, don't block: A New Approach to Dual Nuclear Coil Design.
Matthew George Erickson1

1MBI, U of Florida, Gainesville, FL, United States


Development of dual nuclear coils with high efficiency on both 1H and X remains a challenge. The advent of hyperpolarized 13C imaging and hyperpolarized rare gas imaging has provided impetus for developing such a coil. In this work, we report prototype 1H/13C surface coil that makes use of tune/match circuits augmented by high performance shunt notch filters constructed from hybrid transmission line circuits. In this design, all conductive elements in the utility region are simultaneously resonant with high efficiency on both 1H and X. The design may be extended to both volume and array coils for most 1H/X pairs.


The advent of hyperpolarized 13C imaging and in vivo spectroscopy, along with hyperpolarized rare gas imaging, has provided impetus for developing highly efficient dual nuclear coils with “user friendly” attributes. Such coils facilitate both clinical imaging and clinical spectroscopy. Ideally, such a coil would have high efficiency on both 1H and X, along with high isolation between both 1H and X RF channels and an identical spatial profile on both 1H and X. A prototype 1H/13C surface coil meeting these design goals is described here, though the design concept may be applied to both volume and array coils for most 1H/X combinations.


A dual nuclear transmission line resonator was constructed for 1H/13C imaging and spectroscopy on a 4.7 T Agilent preclinical platform. The active loop in the resonator was simultaneously resonated on both 1H and 13C by terminating the loop with two hybrid transmission line elements connected in shunt with respect to one another. Each hybrid transmission line element was constructed from lengths of balanced, shielded transmission line constructed from Times Microwave HF-290 cable terminated with High Q piston capacitors. A 2 cm ID loop was constructed from copper clad Rexolite 1422 substrate. Copper traces were over 7 skin depths thick on the 13C frequency. The loop was resonated on both 1H and 13C by adjusting the termination capacitors while observing with a Measurement Instruments Model 59 grid dip meter. A 1H tune/match balanced L circuit and a 13C balanced L tune/match circuit were constructed from copper clad Rexolite 1422, and connected to the loop on the console end, again in shunt with respect to each other. Both tune/match circuits used Voltronics high Q piston capacitors. On 13C, piston capacitors were padded with fixed ATC chip capacitors. Balanced offset lines between the tune/match circuits and the loop were constructed from HF-290 coaxial cable. The 1H balanced L was shunted at the match plane with a 13C hybrid notch filter, whilst the 13C balanced L was shunted with a 1H hybrid notch filter. Notch filters were constructed from balanced, shielded 150 ohm line segments constructed from 75 ohm Times Microwave LMR-240 line, and terminated with high Q piston trimmer capacitors mounted on small copper clad Rexolite 1422 boards. Each tune/match circuit was connected to a high performance current balun via shielded, balanced lines. Baluns were constructed from Fair-Rite #31 Snap-It cores placed on lengths of 50 ohm LMR-240 coaxial cable. Balun common mode rejection of over -42 dB on both 1H and 13C was verified with a HP-8405A vector voltmeter augmented with a digital voltmeter.


Unloaded Q of 215 and 240, respectively, was for 1H and 13C using an Agilent VNA. 13C→1H isolation was -62 dB, and 1H→13C isolation was greater than -45 dB as measured by S12 and S21 measurements on an Agilent VNA. Both tune/match circuits adjusted smoothly, with virtually no interaction between the two. First, the coil was shimmed on 1H using a phantom of 13C enriched urea in ddH2O doped with a trace amount of OmniScan. A thermal 13C spectrum of urea was obtained at once, with no retuning or cable swapping. Next, thermal 13C spectra of natural abundance ethanol were acquired after 1H shimming. Finally, both conventional 1H decoupled and NOE enhanced 1H decoupled 13C spectra were acquired.


This design permits simultaneous resonance at high efficiency on all elements in the active region. No LC trap is used, thereby eliminating a major source of B1 inhomogeneity and loss. Instead, the tune/match circuits are augmented with low loss, shunt connected hybrid notch filters. This approach virtually eliminates RF leakage from the complementary channel, thus maintaining high Q whilst facilitating easy tuning.. The resonator has essentially the same spatial profile on each nucleus, which facilitates both spin decoupling and spin transfer experiments. Users may acquire 13C spectra and images immediately after obtaining 1H shim or 1H landmark images. We anticipated higher unloaded Q when the original 1H piston trimmer of marginal Q is replaced with a high Q VHF butterfly capacitor fabricated from copper clad Rexolite 1422, and when Rexolite 1422 stripline segments replace sections of coaxial cable.


A high efficiency dual nuclear transmission line resonator was successfully constructed and tested. The design lends itself to most 1H/X nuclear combinations. The construction of surface coils, volume coils, and multi-channel arrays all lie within the scope of this design. Outstanding isolation between 1H and X channels allows for virtually independent operation of 1H and X T/R channels. Scaling to human sized coils is straightforward.


Malathy Elumalai U of Florida/AMRIS

Eric Peterson SRI

Jacques Audet AMSAT

Todd Prox U of Florida Chemistry Machine Shop

Tom Mareci U of Florida/AMRIS

Huadong Zeng U of Florida/AMRIS

Joanna Long U of Florida/AMRIS

M W Maxwell AMSAT


Smith, P. H. (1969). Electronic applications of the Smith Chart in waveguide, component, and circuit analysis. New York, New York: McGraw Hill.

Mawell, Myron Walter. Helical Coaxial Resonator RF Filter. US Patent 3437959A 1969.


The prototype 1H/13C resonator with termination stubs and augmented T/R circuits.

Close up view of the surface coil loop. Loop resonates on both 1H and 13C at the midpoint.

13C (left) and 1H (right) tune/match circuits with notch filters.

Thermal 13C spectrum acquired immediately after 1H shim. No retuning was needed.

13C spectrum of natural abundance ethanol. From left: No 1H decoupling, decoupled, NOE enhanced.

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