A 13C/31P surface coil to visualize metabolism and energetics in the rodent brain at 3 Tesla

Abstract

Purpose: We constructed a ¹³C/³¹P surface coil at 3 T for studying cancer metabolism and bioenergetics. In a single scan session, hyperpolarized ¹³C-pyruvate MRS and ³¹P MRS was carried out for a healthy rat brain.

Methods: All experiments were carried out at 3 Tesla. The multinuclear surface coil was designed as two coplanar loops each tuned to either the ¹³C or ³¹P operating frequency with an LCC trap on the ¹³C loop. A commercial volume proton coil was used for anatomical localization and B₀ shimming. Single tuned coils operating at either the ¹³C or ³¹P frequency were built to evaluate the relative performance of the multinuclear coil. Coil performance metrics consisted of measuring Q factor ratio, calculating system input power using a single-pulse acquisition, and acquiring SNR and flip angle maps using 2D CSI sequences. To observe in vivo spectra, a bolus of hyperpolarized [1-¹³C] pyruvate was administered via tail vein. In vivo¹³C and endogenous ³¹P spectra were obtained in a single scan session using 1D slice selective acquisitions.

Results: When compared with single tuned surface coils, the multinuclear coil performance showed a decrease in Q factor ratio, SNR, and transmit efficiency. Flip angle maps showed adequate flip angles within the phantom when the transmit voltage was set using an external phantom. Results show good detection of ¹³C labeled lactate, alanine, and bicarbonate in addition to ATP from ³¹P MRS.

Conclusions: The coil enables obtaining complementary information within a scan session, thus reducing the number of trials and minimizing biological variability for studies of metabolism and bioenergetics.

Keywords: Hyperpolarized 13C MRS; Multinuclear RF coils; Phosphorus MRS.

Figure 1:

Surface coils and in vivo set up. Multinuclear coil designed for metabolic and bioenergetics imaging for a rat brain at 3T is shown in A. The inner loop (3 cm diameter) was tuned to the ³¹P frequency, and the outer loop (5 cm diameter) was tuned to the ¹³C operating frequency. An LCC trap was included on the ¹³C loop, and a phantom was positioned at the center of the coil for center frequency adjustment and power calibration. Single tuned coils (B) were constructed to match the diameter of either the ¹³C loop or ³¹P loop in the multinuclear coil (A) for evaluating coil performance. The multinuclear coil was positioned on the top of a rat head (C).

Figure 2:

Signal-to-noise ratio maps for surface coils. The Carbon channel of the multinuclear coil showed a 36.27 % and 38.03 % mean decrease in SNR in the axial and sagittal plane measurements respectively. In the SNR maps, the phosphorus channel of the multinuclear coil showed a 31.44 % and 17.83 % mean decrease in SNR in the axial and sagittal plane respectively. Mean values were calculated over the entire slice. Masks based on the background FLASH images were applied.

Figure 3:

Flip angle maps for surface coils. Maps calculated using the double angle method with an expected flip angle of 45-degrees. A constant threshold was applied for either carbon or phosphorus coil flip angle maps. Masks based on the background FLASH image were applied to all maps.

Figure 4:

Proton images in the presence of surface coils. A signal drop in the phantom is visible near the coil elements, which indicates an interaction between the ¹H volume coil and the surface coils.

Figure 5:

In vivo carbon and phosphorus spectra. Slice positioning for both carbon (orange) and phosphorus (yellow) acquisitions on a sagittal slice of the head, obtained using a RARE sequence, is shown in A. Phosphorus spectrum (B) shows Phosphomonoesters (PME), inorganic phosphate (Pi), Phosphodiesters (PDE), Phosphocreatine (PCr) and ATP peaks. Carbon spectrum (C) obtained by summing spectra from 18 s to 1 min demonstrate downstream metabolic products of pyruvate namely lactate, alanine, and bicarbonate. A line broadening of 3 Hz was applied for both carbon and phosphorus spectra.

Figure 6:

Dynamic 13C spectra for first 20 time frames (TR = 3s). Lactate and pyruvate hydrate peaks are observed in the initial time frames while alanine appears in later frames.