The effect of transmit B₁ inhomogeneity on hyperpolarized [1-¹³C]-pyruvate metabolic MR imaging biomarkers

Purpose: A specialized Helmholtz-style ¹³C volume transmit “clamshell” coil is currently being utilized for ¹³C excitation in pre-clinical and clinical hyperpolarized ¹³C MRI studies aimed at probing the metabolic activity of tumors in various target anatomy. Due to the widespread use of this ¹³C clamshell coil design, it is important that the effects of the ¹³C clamshell coil B₁⁺ profile on HP signal evolution and quantification are well understood. The goal of this study was to characterize the B₁⁺ field of the ¹³C clamshell coil and assess the impact of inhomogeneities on semi-quantitative and quantitative hyperpolarized MR imaging biomarkers of metabolism. Methods: The B₁⁺ field of the ¹³C clamshell coil was mapped by hand using a network analyzer equipped with an S-parameter test set. Pharmacokinetic models were used to simulate signal evolution as a function of position-dependent local excitation angles, for various nominal excitation angles, which were assumed to be accurately calibrated at the isocenter. These signals were then quantified according to the normalized lactate ratio (nLac) and the apparent rate constant for the conversion of pyruvate to lactate (k_PL). The percent difference between these metabolic imaging biomarker maps and the reference value observed at the isocenter of the clamshell coil was calculated to estimate the potential for error due to position within the clamshell coil. Finally, regions were identified within the clamshell coil where deviations in B₁⁺ field inhomogeneity or imaging biomarker errors imparted by the B₁⁺ field were within ±10% of the value at the isocenter. Results: The B₁⁺ field maps show that a limited volume encompassed by a region measuring approximately 12.9 × 11.5 × 13.4 cm (X-direction, Y-direction, Z-direction) centered in the ¹³C clamshell coil will produce deviations in the B₁⁺ field within ±10% of that at the isocenter. For the metabolic imaging biomarkers that we evaluated, the case when the pyruvate excitation angle (θ_P) and lactate excitation angle (θ_L) were equal to 10° produced the largest volumetric region with deviations within ±10% of the value at the isocenter. Higher excitation angles yielded higher signal and SNR, but the size of the region in which uniform measurements could be collected near the isocenter of the coil was reduced at higher excitation angles. The tradeoff between the size of the homogenous region at the isocenter and signal intensity must be weighed carefully depending on the particular imaging application. Conclusion: This work identifies regions and optimal excitation angles (θ_P and θ_L) within the ¹³C clamshell coil where deviations in B₁⁺ field inhomogeneity or imaging biomarker errors imparted by the B₁⁺ field were within ±10% of the respective value at the isocenter, and thus where excitation angles are reproducible and well-calibrated. Semi-quantitative and quantitative metabolic imaging biomarkers can vary with position in the clamshell coil as a result of B₁⁺ field inhomogeneity, necessitating care in patient positioning and the selection of an excitation angle set that balances reproducibility and SNR performance over the target imaging volume.