Cardiac T2∗ measurement of hyperpolarized 13 C metabolites using metabolite-selective multi-echo spiral imaging

Abstract

Purpose: Noninvasive imaging with hyperpolarized (HP) pyruvate can capture in vivo cardiac metabolism. For proper quantification of the metabolites and optimization of imaging parameters, understanding MR characteristics such as $T_2^{\ast }$ s of the HP signals is critical. This study is to measure in vivo cardiac $T_2^{\ast }$ s of HP [1-¹³ C]pyruvate and the products in rodents and humans.

Methods: A dynamic ¹³ C multi-echo spiral imaging sequence that acquires [¹³ C]bicarbonate, [1-¹³ C]lactate, and [1-¹³ C]pyruvate images in an interleaved manner was implemented for a clinical 3 Tesla system. $T_2^{\ast }$ of each metabolite was calculated from the multi-echo images by fitting the signal decay of each region of interest mono-exponentially. The performance of measuring $T_2^{\ast }$ using the sequence was first validated using a ¹³ C phantom and then with rodents following a bolus injection of HP [1-¹³ C]pyruvate. In humans, $T_2^{\ast }$ of each metabolite was calculated for left ventricle, right ventricle, and myocardium.

Results: Cardiac $T_2^{\ast }$ s of HP [1-¹³ C]pyruvate, [1-¹³ C]lactate, and [¹³ C]bicarbonate in rodents were measured as 24.9 ± 5.0, 16.4 ± 4.7, and 16.9 ± 3.4 ms, respectively. In humans, $T_2^{\ast }$ of [1-¹³ C]pyruvate was 108.7 ± 22.6 ms in left ventricle and 129.4 ± 8.9 ms in right ventricle. $T_2^{\ast }$ of [1-¹³ C]lactate was 40.9 ± 8.3, 44.2 ± 5.5, and 43.7 ± 9.0 ms in left ventricle, right ventricle, and myocardium, respectively. $T_2^{\ast }$ of [¹³ C]bicarbonate in myocardium was 64.4 ± 2.5 ms. The measurements were reproducible and consistent over time after the pyruvate injection.

Conclusion: The proposed metabolite-selective multi-echo spiral imaging sequence reliably measures in vivo cardiac $T_2^{\ast }$ s of HP [1-¹³ C]pyruvate and products.

Keywords: ${\mathrm{T}}_2^{\ast }$; dynamic nuclear polarization; heart; hyperpolarized pyruvate; multi-echo spiral imaging.