Pulmonary flow (Qp) and pulmonary-to-systemic flow ratio (Qp/Qs) quantifications are essential in the management of pulmonary arterial hypertension associated with congenital heart disease (PAH-CHD), particularly for the evaluation of PAH severity and consideration for shunt correction [1]. The direct Fick method is the gold standard for estimating cardiac output in patients with PAH-CHD. It involves the direct measurement of oxygen consumption at the time of right heart catheterization (RHC) correction [1]. This relatively complex technique must be performed by skilled physicians’ specialist in CHD using dedicated equipment. In day-to-day clinical practice, less accurate methods like thermodilution and indirect Fick method, which estimates oxygen consumption, are inappropriately applied to patients with cardiac shunts [2], [3], [4]. Furthermore, these invasive methods expose to risks such as bleeding, hematomas, arrhythmias, and infection [5].

Therefore, there has been a significant push towards the development of non-invasive methods over the past few decades. Cardiac magnetic resonance imaging (MRI) using two-dimensional phase-contrast (2D PC) imaging is a reliable non-invasive tool for estimating the shunt fraction, showing a good correlation with traditional invasive assessment in patients with CHD [6], [7], [8]. However, a significant challenge in 2D PC lies in the complex plane prescription specific to each vessel, during the examination time. This complexity is amplified by the intricate blood flow patterns inherent to congenital heart diseases, which may contribute to potential misinterpretations of flow data [9].

Recent advancements have given rise to four-dimensional (4D) flow MRI, a technique that is accurate and reproducible [10]. This innovative method provides a rich set of quantitative and qualitative data on the flow patterns, with possibility of retrospectively place accurate planes within a 3D volume to optimally assess flow data. Notably, it has evolved to be compatible with a brief, free-breathing acquisition period during a MRI examination, enhancing its practicality and patient comfort [11]. Previous studies have shown the usefulness of 4D flow MRI as a promising diagnostic and prognostic tool in patients with pH [12], [13], [14] and those with CHD [15], [16], [17], [18]. Despite its potential, there are limited data comparing 4D flow MRI with direct Fick method in PAH associated with CHD.

The purpose of this study was to evaluate the accuracy 4D flow MRI compared to RHC, in measuring Qp, Qs and the pulmonary-to-systemic flow ratio (Qp/Qs) in patients with PAH-CHD.