· Standard 2D phase contrast magnetic resonance imaging (PC – MRI) is tipically used to quantify blood flow in the heart, through heart valves and in great vessels.

· 4D flow MRI has the potential to provide a comprehensive retrospective evaluation of vascular hemodynamics with full volumetric coverage, becoming a promising tool for cardiac and vascular diagnosis.

· PC MRI requires prior knowledge (approximation) of the maximum velocities that are expected in the vessel of interest to avoid aliasing.

Those with interest in non-invasive quantification of blood flow using phase contrast 2D and 4D MRI.

· To review standard 2D PC MRI methods for quantification of blood flow in clinical applications.

· To review advances in 4D flow MRI for flow quantification in different vascular territories.

· To review potential limitations and potential sources of errors for velocity mapping and parameters derived from those measurements.

The purpose of this presentation will be to review the data acquisition, post processing, and blood flow quantification using standard 2D PC and 4D Flow MRI. Sources of error for the blood flow measurements will also be analyzed.

Cardiovascular MRI has undergone substantial developments over the last decades and offers capabilities for evaluating cardiac anatomy and function including cardiac viability, perfusion, blood flow, and vascular anatomy. 2D phase contrast MRI is widely used clinically for the noninvasive assessment of flow volumes and peak velocities in a single plane with one-directional velocity encoding [1]. Improvements in MR hardware, sequence design, and image reconstruction have facilitated accelerated cardiovascular imaging, permitting to capture volumetric velocity fields with three-directional velocity encoding over a 3D volume throughout the cardiac cycle in clinically feasible scan times [2]. Data sets obtained from such examinations can provide information on the anatomy, vascular lumen, and hemodynamic information from a single acquisition. This is all inherently co-registered and obtained in 5-20 minute acquisitions, depending on cardiac and respiratory gating needs, spatial and temporal resolution, and volume coverage [3].

PC MRI requires the user to define an upper velocity limit given by the velocity sensitivity encoding parameter (Venc). Venc is defined as the (positive or negative) maximum velocity that can be detected without error. For velocities exceeding the Venc, velocity aliasing occurs. In this case, the acquisition needs to be repeated with an increased Venc or antialiasing correction needs to be performed to achieve data that can be properly quantified. PC MRI thus requires prior knowledge (approximation) of the maximum velocities that are expected in the vessel of interest [4].

Several sources of error, such as background phase contributions from eddy-currents, or method-specific problems, such as velocity aliasing and noise, can compromise MR velocity maps and need to be addressed prior to data quantification or visualization. Software for blood flow quantification from 2D PC MRI is widely available and can be used for images acquired in all different platforms. In contrast, extensive work is being done to standardize a way to quantify flow from 4D flow MRI datasets.

Blood flow can be quantified by standard 2D CINE PC techniques that can easily be applied during a single breath hold, which may be sufficient for several clinical applications. On the other hand, 4D flow MRI offers the ability of retrospective quantitative evaluation of blood flow at any location of interest within the 3D volume (Fig1). Despite relative long scan times, on the order of 5–20 minutes (depending on heart rate, breathing compensation efficiency, and applied sequence), 4D flow MRI inherently provides an easy scan prescription (positioning of a single 3D volume). This may be especially advantageous in cases where multiple breath-held 2D CINE PC MRI scans are needed such as in congenital heart disease. Particularly in young pediatric patients, where breath-held scans are often not feasible, the free breathing 4D flow MRI acquisition with volumetric coverage may provide an advantage over standard MR techniques [5, 6]. The methods discussed here provide blood flow information for a wide range of cardiovascular pathologies. 4D flow MRI has the potential to significantly change the way flow imaging is clinically conducted, as well as expand the indications for velocity sensitive imaging by providing unique insights into the velocity fields and additional functional parameters. These non-invasive measures can possibly enhance diagnosis, therapy planning, and therapy monitoring in a wide range of cardiovascular imaging, including all major vascular territories and the heart.