Introduction

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.

Methods

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.

Results

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.

Discussion

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.

Conclusion

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.