Magnetic resonance angiography-derived flow parameters to assess thoracic aortic disease risk

In their recent study, Korpela et al. [1] investigated magnetic resonance angiography (MRA)-derived flow displacement and wall shear stress within dilated ascending aortas of 30 individuals without surgical intervention, including a primary assessment, as well as a follow-up study. Their study’s objective was to test if the flow displacement and amount of local wall shear stress are linked to the aortic expansion rate. A minority (20%) of patients (n = 6) showed a clinically and statistically significant aortic expansion rate (1.5–2.5 mm per year). The authors found an association between decreased global WSS, the quantity of flow displacement and the aortic expansion rates. Seventeen years ago, Markl et al. [2] began publishing a series of reports from Freiburg, Germany, on thoracic aortic flow patterns investigated by 4D-MRA—work then expanded by the Lübeck group [3] and continued in Chicago, USA. 4D-MRA has been applied by these and other groups to characterize flow patterns in numerous cardiac and aortic, acquired and congenital, pathologies, including but not limited to coarctation [4], BAV [4], TAVI [5], and Marfan [6], but also ventricular assist devices, tetralogy of Fallot and Fontan physiology. Hope et al. from Stanford University shed light on BAV haemodynamics using this technique, enhancing the evidence with DC Miller to patients after valve-sparing root replacement. Even extremely rare situations like iatrogenic neonatal Type B aortic dissection are part of the long list of case reports and small case series reporting that (i) the technique can be used in the respective pathological setting; (ii) there are distinct disturbances of local flow patterns and abnormal distribution patterns of mechanical load; and (iii) we need more research clarifying the actual implications of these MRA findings for the natural clinical history of diseases and patient prognosis. The list of reports on novel flow findings applying 4D-MRA methodology is as long as the list of possible cardiovascular diagnoses and their sequelae. There is, however, an extreme paucity of long-term follow-up studies in homogeneous patient groups focusing on the question of clinical implications and prognostic value. Against this background, the recent study by Korpela et al. goes a step beyond describing findings, trying to clarify the clinical usefulness of 4D flow MRA in predicting risk. However, it describes follow-up intervals that are rather short, their patient cohort is small and there are too few events of interest (pathological expansion rates). In this respect, it is an underpowered study, albeit with a very important objective. It contrasts the many reports mentioned especially considering their years of publication. A high number of patients suffering from ascending aortic aneurysm or dilation probably underwent their initial scans many years ago at various specialized international institutions. The potential to study long-term associations between the initial 4D flow MRA quantifications, aortic expansion rates and clinical courses probably exists. Instead of adding to the long list of single-centre reports describing no or limited follow-up, multicentre efforts should be made to investigate patients who had their initial scan several years ago, as this would help tremendously in understanding 4D flow and its clinical implications. Clinical specialists need to cooperate closely with experts in medical physics and with specialized radiologists to conduct such important and doable analyses. There are 3 quantities we should routinely examine in patients presenting proximal thoracic aortic ectasia: (i) flow displacement, (ii) local heterogeneities of elasticity and (iii) wall shear stress. Boczar et al. [7] recently showed that aortic stiffness and the pulsatile load are linked to expansion rates; moreover, pulse wave velocity can be quantified via MRI [8]. Thirty-four years ago, Klipstein et al. from London concluded in their work on MRA-derived flow patterns in the ascending aorta that ‘Magnetic resonance velocity mapping is a very powerful tool for the study of cardiovascular physiology. Its non-invasiveness, its quantitative two-dimensional data, its accuracy, and its high spatial resolution make it suitable for clinical use’ [9]. Now, over 3 decades later, let us finally implement and exploit it. While all diagnoses we as cardiac and aortic specialists treat imply pathological flow patterns, correcting flow is our goal in almost all the lesions we treat. We must acknowledge that flow is still a blind spot in clinicians’ routine care of patients even at specialized institutions. As Richter and Edelman [10] put it: ‘Flow is not a standard part of our [cardiological/cardiac surgical] clinical lexicon’. Yet this is exactly what it should become.