4D flow imaging of the thoracic aorta: is there an added clinical value?

Review Article on Advanced Imaging in The Diagnosis of Cardiovascular Diseases 4D flow imaging of the thoracic aorta: is there an added clinical value? Federica Catapano1, Giacomo Pambianchi1, Giulia Cundari1, João Rebelo2, Francesco Cilia1, Iacopo Carbone1, Carlo Catalano1, Marco Francone1, Nicola Galea1,3 1 Department of Radiological, Oncological and Pathological Sciences, Sapienza University of Rome, Rome, Italy; 2Department of Radiology, Centro Hospitalar São João, Alameda Prof. Hernâni Monteiro, Porto, Portugal; 3Department of Experimental Medicine, Sapienza University of Rome, Rome, Italy Contributions: (I) Conception and design: N Galea, M Francone; (II) Administrative support: I Carbone, C Catalano; (III) Provision of study materials or patients: F Catapano, G Pambianchi, G Cundari, F Cilia, N Galea, I Carbone, M Francone; (IV) Collection and assembly of data: F Catapano, G Pambianchi, G Cundari, F Cilia, N Galea; (V) Data analysis and interpretation: F Catapano, G Pambianchi, G Cundari, F Cilia, N Galea; (VI) Manuscript writing: All authors; (VII) Final approval of manuscript: All authors. Correspondence to: Dr. Nicola Galea, MD PhD. Department of Experimental Medicine, “Sapienza” University of Rome, viale Regina Elena 324, 00161 Rome, Italy. Email: . Abstract: Four-dimensional (4D) flow MRI has emerged as a powerful non-invasive technique in cardiovascular imaging, enabling to analyse in vivo complex flow dynamics models by quantifying flow parameters and derived features. Deep knowledge of aortic flow dynamics is fundamental to better understand how abnormal flow patterns may promote or worsen vascular diseases. In the perspective of an increasingly personalized and preventive medicine, growing interest is focused on identifying those quantitative functional features which are early predictive markers of pathological evolution. The thoracic aorta and its spectrum of diseases, as the first area of application and development of 4D flow MRI and supported by an extensive experimental validation, represents the ideal model to introduce this technique into daily clinical practice. The purpose of this review is to describe the impact of 4D flow MRI in the assessment of the thoracic aorta and its most common affecting diseases, providing an overview of the actual clinical applications and describing the potential role of derived advanced hemodynamic measures in tailoring follow-up and treatment. Keywords: Four-dimensional flow imaging (4D flow imaging); phase contrast sequence; cardiovascular magnetic resonance; thoracic aorta; fluid dynamics Submitted Apr 24, 2020. Accepted for publication Jun 10, 2020. doi: 10.21037/cdt-20-452 View this article at: http://dx.doi.org/10.21037/cdt-20-452 Introduction More than two decades after its introduction, fourdimensional flow magnetic resonance imaging (4D flow MRI) has been established as an unrivaled method for the non-invasive in vivo assessment of fluid dynamics. The reference technique of 4D flow MRI is a threedimensional (3D) phase contrast sequence with threedirectional velocity encoding, which has been widely implemented over the years and nowadays offers high © Cardiovascular Diagnosis and Therapy. All rights reserved. reproducibility and robustness. Although the development of this technique required a long time due to the heterogenity of technical strategies proposed, intrinsic difficulties in reproducibility and of the complex processing of such a large amount of data, today 4D flow MRI is widely available and many analysis softwares are commercially available for clinical validation. A number of studies (and clinical trials) have been conducted over the years and are still ongoing to explore the application of 4D flow MRI in many different areas of Cardiovasc Diagn Ther 2020;10(4):1068-1089 | http://dx.doi.org/10.21037/cdt-20-452 Cardiovascular Diagnosis and Therapy, Vol 10, No 4 August 2020 1069 cardiovascular system and to define its actual additional value. In particular, research interests are mainly oriented in extrapolating quantitative flow data and related energy parameters that may emerge as clinical biomarkers to improve early diagnostics and prognostic prediction. This review aims to shed light on state of the art of 4D flow MRI in the clinical assessment of a large spectrum of pathological conditions affecting the thoracic aorta and to give useful elements for its immediate introduction in daily clinical routine. imaging, is directly proportional to the Venc preset value, but inversely related to the signal-to-noise ratio (SNR) in the corresponding magnitude images (i.e., vnoise is proportional to Venc/SNR) (5). It should be kept in mind that an increase in the Venc values is needed to avoid aliasing, but at the same time, keeping the Venc as low as possible will reduce velocity noise and improve image quality (5,6). The best compromise should be found in each patient. Therefore, it is advisable that the set VENC value should be approximately 10% higher than the registered maximum velocity (7). When the maximum velocity of aortic flow is unknown or not conceivable, a solution may be to acquire one or more bidimentional phase contrast sequences (2D PC MRI) in the regions of stenosis identified on the MR angiography or cine images. Another promising approach is the use of a multi-velocity encoding (multi-VENC) that additionally improves pathline tracking and streamline estimation (8). A structured workflow is suggested as shown below. Technique Patient preparation The 3D Phase Contrast sequence, also known as ‘flowsensitive’ or ‘velocity mapping’, is the technique typically used in 4D-flow MRI consisting in a short TE (5–7 ms) and TR (2–4 ms) radiofrequency-spoiled gradient-echo. It allows for a rapid imaging of the vessels flow in absence of contrast agent administration; however a recent study concluded that the echoplanar imaging (EPI) sequence produces higher data quality (1,2). Even if the contrast agent administration is not necessary in 4D flow MRI, the sequence acquisition after gadolinium improves the contrast between the blood and the surrounding tissues, resulting in a better signal-to-noise ratio and velocity-to-noise ratio (1). In order to depict even the subtle changes in flow a high spatial resolution (isotropic voxel size: 1.5–3.0 mm for the aorta) is fundamental, whereas the highest possible temporal resolution, (generally <40 ms; preferably ≥20 phases) enables to catch even the rapid flow phenomena (3,4). The maximum velocity encoding (VENC) value to set before starting the 4D flow MRI sequence acquisition is typically around 150 ms (Table 1), even though this value should be increased in the presence of flow accelerations (e.g., valve stenosis, coarctation) to avoid aliasing artifacts. As for the 2D-PC MRI, the quality of velocity images can be degraded by noise. The velocity noise, in this kind of Cardiovascular MRI exams are usually time-consuming (40–50 min) and (...truncated)