Dual-phase imaging of cardiac metabolism using hyperpolarized pyruvate

Abstract

Purpose: Previous cardiac imaging studies using hyperpolarized (HP) [1-¹³ C]pyruvate were acquired at end-diastole (ED). Little is known about the interaction between cardiac cycle and metabolite content in the myocardium. In this study, we compared images of HP pyruvate and products at end-systole (ES) and ED.

Methods: A dual-phase ¹³ C MRI sequence was implemented to acquire two sequential HP images within a single cardiac cycle at ES and ED during successive R-R intervals in an interleaved manner. Each healthy volunteer (N = 3) received two injections of HP [1-¹³ C]pyruvate for the dual-phase imaging on the short-axis and the vertical long-axis planes. Spatial distribution of HP ¹³ C metabolites at each cardiac phase was correlated to multiphase ¹ H MRI to confirm the mechanical changes. Ratios of myocardial HP metabolites were compared between ES and ED. Segmental analysis was performed on the midcavity short-axis plane.

Results: In addition to mechanical changes, metabolic profiles of the heart detected by HP [1-¹³ C]pyruvate differed between ES and ED. The myocardial signal of [¹³ C]bicarbonate relative to [1-¹³ C]lactate was significantly smaller at ED than the ratio at ES (p < .05), particularly in mid-anterior and mid-inferoseptal segments. The distinct metabolic profiles in the myocardium likely reflect the technical aspects of the imaging approach such as the coronary flow in addition to the cyclical changes in metabolism.

Conclusion: The study demonstrates that metabolic profiles of the heart, measured by HP [1-¹³ C]pyruvate, are affected by the cardiac cycle in which that the data are acquired.

Keywords: cardiac; dual-phase; end-diastole; end-systole; hyperpolarized pyruvate.

Figure 1.. Dual-phase cardiac imaging scheme for hyperpolarized (HP) ¹³C-metabolites.

HP [¹³C]bicarbonate, [1-¹³C]lactate, [1-¹³C]alanine and [1-¹³C]pyruvate are imaged sequentially in an interleaved manner. For each metabolite, two images are acquired at end-systole (ES) and end-diastole (ED) within the same cardiac cycle. The acquisitions are repeated for in total 16 timepoints.

Figure 2.. Short-axis (SA) ¹³C images at end-systole (ES) and end-diastole (ED).

(A) ¹H cardiac multi-phase images on the SA plane from a representative subject are shown along the trigger delays. Images acquired at ES and ED are highlighted with red squares. (B) Images of hyperpolarized (HP) [¹³C]bicarbonate, [1-¹³C]lactate, [1-¹³C]alanine and [1-¹³C]pyruvate acquired at ES and ED are overlaid on the corresponding ¹H multi-phase images. The color scale of each image indicates the SNR.

Figure 3.. Vertical long-axis (LA) ¹³C images at end-systole (ES) and end-diastole (ED).

(A) ¹H cardiac multi-phase images on the vertical LA plane from a representative subject are shown along the trigger delays. Images acquired at ES and ED are highlighted with red squares. (B) Images of hyperpolarized (HP) [¹³C]bicarbonate, [1-¹³C]lactate, [1-¹³C]alanine and [1-¹³C]pyruvate acquired at ES and ED are overlaid on the corresponding ¹H multi-phase images. The color scale of each image indicates the SNR.

Figure 4.. Cross-sectional signal profiles from dual-phase short-axis (SA) ¹³C images.

Signal profiles along the reference lines (a), (b) and (c) for each metabolite at end-systole (ES) and end-diastole (ED) on the SA plane are shown in (A). Changes of diameters of the left ventricle along reference lines (a), (b) and (c) from ES to ED measured from HP [¹³C]bicarbonate and [1-¹³C]pyruvate images are summarized in (B).

Figure 5.. Cross-sectional signal profiles from dual-phase vertical long-axis (LA) ¹³C images.

Signal profiles along the reference lines (a), (b) and (c) for each metabolite at end-systole (ES) and end-diastole (ED) on the vertical LA plane are shown in (A). Changes of diameters of the left ventricle along reference lines (a) and (b) from ES to ED measured from HP [¹³C]bicarbonate and [1-¹³C]pyruvate images are summarized in (B).

Figure 6.. Dynamic dual-phase ¹³C imaging on the short-axis (SA) and the vertical long-axis (LA) planes.

Time-resolved images of hyperpolarized (HP) [¹³C]bicarbonate, [1-¹³C]lactate, [1-¹³C]alanine and [1-¹³C]pyruvate at end-systole (ES) and end-diastole (ED) on the SA plane (A) and on the vertical LA plane (B) from representative subjects are overlaid on the corresponding ¹H multi-phase images. Intravascular lactate and pyruvate are readily imaged in the ¹³C aortogram.

Figure 7.. Change in ¹³C metabolic profiles from end-systole (ES) to end-diastole (ED).

Time-averaged metabolic ratios measured at ES and ED on the short-axis (SA) (A) and the vertical long-axis (LA) (B) planes are overlaid on the corresponding ¹H multi-phase images. The changes in metabolic ratios within the myocardium from ES to ED are summarized in (C).