PURPOSE:

A multinuclear surface coil was built for studying metabolism from hyperpolarized carbon (¹³C) MR spectroscopy, and energetics from Phosphorus (³¹P) MRS in a single setting (1). To visualize low concentration metabolites such as bicarbonate, alanine, and endogenous phosphorus metabolites, high signal-to-noise ratio is critical. In this work, we evaluate the relative performance of the multinuclear coil as compared to single tuned surface coils by measuring transmit efficiency, SNR, flip angle maps, and Quality factor ratio. Assessing these performance metrics would allow for identifying potential design modifications to the existing multinuclear coil.

METHODS:

All experiments were carried out on a 3T animal MRI scanner (Biospec, Bruker, Billerica MA). The ¹³C/³¹P coil (Fig 1A) was constructed such that an inner loop with 3 cm diameter was tuned to 51.65 MHz (³¹P frequency) and a concentric outer loop with 5 cm diameter was tuned to 32.09 MHz (¹³C frequency). An LCC trap circuit (2) was included on the ¹³C channel to decouple the two channels (1). Separate single tuned coils with diameters matching the size of the ¹³C and ³¹P loops on the multinuclear coil were constructed (Fig 1B). Phantom localization was carried out using a volume proton (¹H) coil (72 mm diameter, Bruker, MA).

The Quality (Q) factor ratio (Qunloaded/Qloaded) was measured for all coils and for the case without LCC trap for the ¹³C channel (LCC trap replaced by effective capacitance). The SNR efficiency (SNRefficiency = Sqrt(1-Qloaded/Qunloaded)) was calculated from the Q values (3).

The optimal system reference power, i.e. transmit efficiency, was calculated using the small spherical phantom (6.65 M urea, 3.03 M diethyl(2-oxopropyl) phosphonate, doped with Gadolinium (Magnevist)) positioned at the center of the coil. The multinuclear coil was loaded with the saline phantom for all measurements. The input power required to obtain a 90 degree flip angle was calculated by varying the power for a series of single pulse acquisitions.

To characterize the performance of the coils spatially, SNR and flip angle maps were measured. The coils were loaded with a phantom consisting of ¹³C-urea and sodium phosphate (55mL: 0.92 M Sodium phosphate, 1.39 M 13C-Urea, 60 uL Gadolinium (Magnevist)). A small phantom filled with H₂O was placed at the center of the coil for positioning the coil at the isocenter of the scanner. A 2D CSI sequence (flip angle = 90 degrees, slice thickness = 10 mm, matrix size = 16x16, FOV = 64 x 64, TR = 1s, averages = 2) was used to obtain spectroscopic images. The SNR was calculated voxel-by-voxel by dividing the peak absolute signal by the standard deviation of the real part of the first voxel, corresponding to a region outside of the imaging object. The same 2D CSI sequence was used for calculating flip angle maps using a double angle method (4) acquired with 90 and 45 degree flip angles. Thresholds were kept constant for each nucleus and applied to avoid calculating flip angle values in regions of low SNR. Based on the background proton images, masks were applied to both the SNR and flip angle maps. Data analysis was done in MATLAB (Mathworks, 2019).

RESULTS:

The carbon channel of the multinuclear coil required 81% higher transmit power, compared to the single tuned coil (Multinuclear coil ¹³C channel: 0.95 W, Single tuned ¹³C coil: 0.525 W), to achieve a 90 degree flip angle at the center of the coil. No difference in power was observed for the phosphorus channel (Multinuclear coil ³¹P channel: 0.07 W, Single tuned ³¹P coil: 0.07 W). The Q factor of the ¹³C channel of the multinuclear coil showed a decrease as compared to the single tuned coil with a reduction in SNR efficiency of 27.3%. On the other hand, the percent difference in SNR efficiency was 4.18% for the ¹³C channel without LCC trap, and 6.53% for the ³¹P channel of the multinuclear coil (Table). The SNR maps showed a reduced mean SNR for the multinuclear coil (Fig. 2) with a larger decrease in SNR for the carbon channel. Flip angle maps (Fig. 3) showed that the expected nominal flip angle of 45 degrees was achievable within the phantom when the system power was calibrated using the urea/diethyl(2-oxopropyl) phosphonate phantom.

DISCUSSION and CONCLUSIONS:

This work evaluated the performance of a ¹³C/³¹P coil in comparison with individually tuned surface coils. A coplanar design of the multinuclear surface coil with a central aperture was originally designed for imaging the rodent brain (1). Our results demonstrated that the performance of the ¹³C channel of the multinuclear coil was impacted by the LCC trap circuit, which was included to decouple the two channels and improve coil sensitivity (2). Coil losses, introduced by the additional lumped elements on the LCC trap circuit, likely contributed to the lower transmit efficiency, Q ratio, and SNR of the carbon channel. As coil losses are dominant in coil SNR at low operating frequencies and for small imaging samples (5), modifications to the coil could include designs without the LCC trap circuit. For example, orthogonal coil elements that do not require a LCC trap circuit (6), or cryo-coils with low coil losses (7).

Acknowledgements

This work was supported by NIH Training Grant T32CA151022, American Cancer Society Research Scholar Grant 18-005-01-CCE, NIH R01CA172845, NIH R01CA197254, NIH grant P41 EB013598 and P41 EB017183.