Dynamic T 2 * relaxometry of hyperpolarized [1-13 C]pyruvate MRI in the human brain and kidneys

Abstract

Purpose: This study aimed to quantify $T_2^{*}$ for hyperpolarized [1-¹³ C]pyruvate and metabolites in the healthy human brain and renal cell carcinoma (RCC) patients at 3 T.

Methods: Dynamic $T_2^{*}$ values were measured with a metabolite-specific multi-echo spiral sequence. The dynamic $T_2^{*}$ of [1-¹³ C]pyruvate, [1-¹³ C]lactate, and ¹³ C-bicarbonate was estimated in regions of interest in the whole brain, sinus vein, gray matter, and white matter in healthy volunteers, as well as in kidney tumors and the contralateral healthy kidneys in a separate group of RCC patients. $T_2^{*}$ was fit using a mono-exponential function; and metabolism was quantified using pyruvate-to-lactate conversion rate maps and lactate-to-pyruvate ratio maps, which were compared with and without an estimated $T_2^{*}$ correction.

Results: The $T_2^{*}$ of pyruvate was shown to vary during the acquisition, whereas the $T_2^{*}$ of lactate and bicarbonate were relatively constant through time and across the organs studied. The $T_2^{*}$ of lactate was similar in gray matter (29.75 ± 1.04 ms), white matter (32.89 ± 0.9 ms), healthy kidney (34.61 ± 4.07 ms), and kidney tumor (33.01 ± 2.31 ms); and the $T_2^{*}$ of bicarbonate was different between whole-brain (108.17 ± 14.05 ms) and healthy kidney (58.45 ± 6.63 ms). The $T_2^{*}$ of pyruvate had similar trends in both brain and RCC studies, reducing from 75.56 ± 2.23 ms to 22.24 ± 1.24 ms in the brain and reducing from 122.72 ± 9.86 ms to 57.38 ± 7.65 ms in the kidneys.

Conclusion: Multi-echo dynamic imaging can quantify $T_2^{*}$ and metabolism in a single integrated acquisition. Clear differences were observed in the $T_2^{*}$ of metabolites and in their behavior throughout the timecourse.

Keywords: dynamic transverse relaxometry; hyperpolarized 13C; kinetic model; metabolic imaging; multi-echo gradient echo sequence.