Cardiovascular disease remains a leading global health concern, with a high prevalence and significant mortality, particularly due to ischemic heart disease (IHD) [1]. Non-invasive imaging modalities such as echocardiography, cardiac magnetic resonance (MR), and nuclear cardiology have been a mainstay in the diagnosis and risk stratification for IHD, and the assessment of left ventricular (LV) function including ejection fraction (LVEF), and regional wall motion are commonly used indicators [2]. However, LVEF, which is the fraction of the chamber volume between systole and diastole, provides a limited information of cardiac performance particularly in patients with preserved function or diastolic dysfunction. As demonstrated in numerous recent studies, LVEF alone is insufficient for accurate diagnosis or prognostic evaluation [3]. Thus, more sensitive markers of subclinical ventricular dysfunction have been investigated, such as myocardial wall strain [4]. In nuclear cardiology, the assessment of LV mechanical synchrony using phase analysis (PA) provides incremental diagnostic and prognostic value over conventional indicators [5]. PA has been originally reported as a useful method in determining the indication for cardiac resynchronization therapy, and is now widely used for IHD in clinical practice [6]. While the clinical utility of PA analysis has been established primarily with single photon emission computed tomography (SPECT), recent evidence demonstrates its increasing value in positron emission tomography (PET). One of the major advantages of PET-based PA compared to SPECT is its high temporal resolution, which allows for accurate quantification of synchrony parameters, enabling robust and reproducible evaluation of mechanical synchrony, facilitating both diagnostic interpretation and longitudinal assessment [7].

Recently, the blood flow analysis using four-dimensional (4D) flow CMR has become a promising imaging technique offering a clinical value for various cardiac disease. Novel hemodynamic markers such as the energy loss, wall share stress, and kinetic energy (KE) have been extensively investigated [8], [9]. Although, these markers have been initially employed to assess blood flow in the great vessels and structural cardiac disease, recent studies have focused on the visualization and quantification of intra-LV blood flow [10], [11]. The intra-LV KE has emerged as a novel marker refers to the quantification of the energy carried by moving blood within the cardiac chamber, and can be calculated from blood flow velocities in three spatial dimensions through cardiac cycle [8], [9]. Although LV mechanical synchrony is thought to be related to the efficiency of intra-LV blood flow, there has been no report to simultaneously examine.

Hybrid PET/MR has been recognized as an ideal modality in the evaluation of cardiac diseases, providing molecular and functional information in a single imaging session [12]. The aim of this study was to simultaneously investigate the association between LV mechanical synchrony and intra-LV blood flow energy using hybrid 13N-ammonia PET/MR in patients with IHD.