Editorial for “Quantitative Assessment of Regional Pulmonary Transit Times in Pulmonary Hypertension”

Editorial for “Quantitative Assessment of Regional Pulmonary Transit Times in Pulmonary Hypertension” Clinical assessment of patients with suspected pulmonary hypertension (PH) is commonly focused on confirming the diagnosis, identifying the etiology, and determining the severity of disease. Echocardiography is the best initial noninvasive imaging modality to start the PH workup; however, right heart catheterization (RHC) is required to confirm the diagnosis. A mean pulmonary artery pressure of ≥25 mm Hg at rest on direct measurement of the pulmonary artery pressure is required to confirm the presence of PH. Additional uses of RHC in these patients include identifying the type of PH (precapillary, postcapillary, or combined) and risk stratification by providing measurements like pulmonary vascular resistance index (PVRI), mixed venous oxygen saturation (mVO2), and cardiac index (CI) to guide treatment decisions. Although RHC is considered a low-risk, minimally invasive procedure, complications related to venous access and catheterization can still occur. As such, there is an impending need to find a reliable and noninvasive imaging modality to confirm the diagnosis of PH. Pulmonary transit time (PTT) is a measure used in CT and MRI studies to quantify pulmonary hemodynamics and is defined as the time for a contrast bolus to travel from the right to the left ventricle. PTT on CTA has been shown to be correlated with cardiac function and pulmonary hemodynamics while performing well in detecting PH. A PTT value ≥14 seconds has been shown to best detect PH with 91% sensitivity and 88% specificity (area under the receiver operating characteristic curve: 0.95). However, CTA is associated with radiation and inferior temporal resolution. Recently, there has been increasing interest in the use of magnetic resonance imaging (MRI) to evaluate PH as it offers a noninvasive structural and functional evaluation of the cardiovascular system in a single examination without radiation exposure. Multiple studies explored the use of 3D contrast-enhanced magnetic resonance angiography (CE-MRA) in the diagnosis and workup of PH; however, it does not provide dynamic information about the pulmonary circulation and can suffer from overlaying of venous and arterial signals. Therefore, conventional CE-MRA has fallen out of favor for evaluation of PH. Time-resolved CE-MRA is now more commonly used as it allows for intravenous contrast bolus tracking through the cardiopulmonary circulation in three dimensions over time. Studies have shown significant correlations between time-resolved CE-MRA findings and prognostic indicators obtained from RHC. Traditionally, PTT on time-resolved CE-MRA images of the cardiopulmonary system is calculated based on signal enhancement vs. time curves, which were generated by the placing regions of interest (ROI) at the pulmonary artery and left atrium. In this issue, Moore, et. al, for the first time, have taken advantage of regional contrast agent dynamics to quantitatively measure regional PTTs (rPTTs) in eight ROIs, including arterial (truncus anterior, left anterior segmental, and left and right basal trunk) and venous branches (left and right superior and inferior pulmonary veins) on time-resolved CE-MRA in 43 patients with a resting mPAP ≥ 25 mmHg or >30 mmHg during exercise on RHC and compared with 24 healthy controls. Mean arterial and venous rPTTs were calculated by averaging all four arterial and venous ROIs, respectively. To avoid bias, they normalized the average arrival times within a vessel ROI in relation to the main pulmonary artery ROI. This study is the first investigation to evaluate the regional PTT (rPTT) and compare it across different WHO PH subgroups (pulmonary arterial hypertension [PAH], PH due to left heart disease [PH-LHD], PH due to chronic lung disease [PH-CLD], and chronic thromboembolic hypertension [CTEPH]). They found elevated rPTT in all vascular regions of patients with PH compared to healthy controls, with an average rPTT increase in arterial and venous branches of 0.85 0.15 seconds (47.7%) and 1.0 0.18 seconds (16.9%), respectively. All PH subgroups had significantly prolonged arterial rPTTs compared to healthy controls, while only PH-LHD, and CTEPH had significantly increased venous rPTTs. These findings could potentially be used to differentiate the one PH group from another. The introduction of rPTT and even new break down to arterial and venous rPTT will open the door for more studies on time-resolved CE-MRA. Future studies could investigate