4D flow MRI: impact of region of interest size, angulation and spatial resolution on aortic flow assessment

Objectives. In cardiovascular magnetic resonance, the 3D time-resolved phase-contrast technique, also known as 4D flow, is gaining increasing attention due to applications that exploit three-directional velocity encoding throughout the cardiac cycle. Blood flow volume assessment usually requires an expert to draw regions of interest (ROI) around the vessel cross section, whereas the errors involved in this estimation have not been thoroughly investigated. Our objective is to quantify the influence of ROI sizing, angulation and spatial resolution of the reconstructed plane employed in blood flow measurements using 4D flow. Approach. Three circular ROIs were drawn around the ascending, arch and descending aorta of healthy volunteers (n = 27) and patients with a dilated ascending aorta or bicuspid valve (n = 37). We applied systematic changes of ROI diameter (up to +/- 10%), tilt angle (up to +/- 25 degrees) and spatial resolution (from 0.25 to 2 mm) of the reconstructed oblique planes, calculating the effects on net, forward and backward blood flow volumes. Main results. Patients had a larger ascending aorta than healthy volunteers with similar ages and male sex proportion (60 +/- 15 y.o. vs 58 +/- 16 y.o. and 84% vs 70%, respectively). Higher forward and backward flow volumes were observed in the ascending aorta and the aortic arch of the patients with respect to controls (p < 0.001), whereas net volumes were similar: 74.0 +/- 20.8 ml versus 75.7 +/- 21.8 ml (p = 0.37), respectively. The ascending aorta was the most sensitive to ROI modifications. Changes of +/- 10% in the ROI diameter and +/- 25 degrees in tilt angles produced flow volume differences of up to 9 ml (10%) and 18 ml (15%) in controls and patients, respectively. Modifying the reconstructed planes spatial resolution produced flow volume changes below 2 ml. Significance. Since the setting of the ROI size and plane angle could produce errors that represent up to 20% of the forward and/or backward aortic flow volume, a good standardization for vessel segmentation and plane positioning is desirable.