Specialized computational methods for denoising, B1 correction, and kinetic modeling in hyperpolarized 13 C MR EPSI studies of liver tumors

Abstract

Purpose: To develop a novel post-processing pipeline for hyperpolarized (HP) ¹³ C MRSI that integrates tensor denoising and $B_1^{+}$ correction to measure pyruvate-to-lactate conversion rates (k_PL ) in patients with liver tumors.

Methods: Seven HP ¹³ C MR scans of progressing liver tumors were acquired using a custom ¹³ C surface transmit/receive coil and the echo-planar spectroscopic imaging (EPSI) data analysis included B₀ correction, tensor rank truncation, and zero- and first-order phase corrections to recover metabolite signals that would otherwise be obscured by spectral noise as well as a correction for inhomogeneous transmit ( $B_1^{+}$ ) using a $B_1^{+}$ map aligned to the coil position for each patient scan. Processed HP data and corrected flip angles were analyzed with an inputless two-site exchange model to calculate k_PL .

Results: Denoising averages SNR increases of pyruvate, lactate, and alanine were 37.4-, 34.0-, and 20.1-fold, respectively, with lactate and alanine dynamics most noticeably recovered and better defined. In agreement with Monte Carlo simulations, over-flipped regions underestimated k_PL and under-flipped regions overestimated k_PL . $B_1^{+}$ correction addressed this issue.

Conclusion: The new HP ¹³ C EPSI post-processing pipeline integrated tensor denoising and $B_1^{+}$ correction to measure k_PL in patients with liver tumors. These technical developments not only recovered metabolite signals in voxels that did not receive the prescribed flip angle, but also increased the extent and accuracy of k_PL estimations throughout the tumor and adjacent regions including normal-appearing tissue and additional lesions.

Keywords: B1 correction; analysis; hyperpolarized 13C-pyruvate MR.