Jelena Bock

Functional information regarding cardiac performance, pressure gradients, and local flow derangement are available from blood acceleration fields. Thus, this study examines a 2D and 3D phase contrast sequence optimized to efficiently encode three‐directional, time‐resolved acceleration in vitro and in vivo. Stenosis phantom acceleration measurements were compared to acceleration derived from standard velocity encoded phase contrast‐magnetic resonance imaging (i.e., “velocity‐derived acceleration”). For in vivo analysis, three‐directional 2D acceleration maps were compared to velocity‐derived acceleration using regions proximal and distal to the aortic valve in six healthy volunteers at 1.5 and 3.0 T (voxel size = 1.4 × 2.1 × 8 mm, temporal resolution = 16–20 ms). In addition, a 4D acceleration sequence was evaluated for feasibility in a healthy volunteer and postrepair biscuspid aortic valve patient with an ascending aortic aneurysm. The phantom magnetic resonance acceleration measurements were more accurate (nonturbulent root mean square error = 2.2 vs. 5.1 m/s² for phase contrast‐magnetic resonance imaging) and 10 times less noisy (nonturbulent σ = 0.9 vs. 13.6 m/s² for phase contrast‐magnetic resonance imaging) than velocity‐derived acceleration. Acceleration mapping of the left ventricular outflow tract and aortic arch exhibited signal voids colocated with complex flow events such as vortex formation and high order motion. 4D acceleration data, visualized in combination with the velocity data, may provide new insight into complex flow phenomena. Magn Reson Med 67:50–61, 2012.

In this work, we present a systematic phantom comparison and clinical application of noninvasive pressure difference mapping in the human aorta based on time‐resolved 3D phase contrast data. Relative pressure differences were calculated based on integration and iterative refinement of pressure gradients derived from MR‐based three‐directional velocity vector fields (flow‐sensitive 4D MRI with spatial/temporal resolution ∼ 2.1 mm³/40 ms) using the Navier‐Stokes equation. After in vitro study using a stenosis phantom, time‐resolved 3D pressure gradients were systematically evaluated in the thoracic aorta in a group of 12 healthy subjects and 6 patients after repair for aortic coarctation. Results from the phantom study showed good agreement with expected values and standard methods (Bernoulli). Data of healthy subjects showed good intersubject consistency and good agreement with the literature. In patients, pressure waveforms showed elevated peak values. Pressure gradients across the stenosis were compared with reference measurements from Doppler ultrasound. The MRI findings demonstrated a significant correlation (r = 0.96, P < 0.05) but moderate underestimation (14.7% ± 15.5%) compared with ultrasound when the maximum pressure difference for all possible paths connecting proximal and distal locations of the stenosis were used. This study demonstrates the potential of the applied approach to derive additional quantitative information such as pressure gradients from time‐resolved 3D phase contrast MRI. Magn Reson Med, 2011.

Time‐resolved phase contrast (PC) MRI with velocity encoding in three directions (flow‐sensitive four‐dimensional MRI) can be employed to assess three‐dimensional blood flow in the entire aortic lumen within a single measurement. These data can be used not only for the visualization of blood flow but also to derive additional information on vascular geometry with three‐dimensional PC MR angiography (MRA). As PC‐MRA is sensitive to available signal‐to‐noise ratio, standard and novel blood pool contrast agents may help to enhance PC‐MRA image quality. In a group of 30 healthy volunteers, the influence of different contrast agents on vascular signal‐to‐noise ratio, PC‐MRA quality, and subsequent three‐dimensional stream‐line visualization in the thoracic aorta was determined. Flow‐sensitive four‐dimensional MRI data acquired with contrast agent provided significantly improved signal‐to‐noise ratio in magnitude data and noise reduction in velocity data compared to measurements without contrast media. The agreement of three‐dimensional PC‐MRA with reference standard contrast‐enhanced MRA was good for both contrast agents, with improved PC‐MRA performance for blood pool contrast agent, particularly for the smaller supra‐aortic branches. For three‐dimensional flow visualization, a trend toward improved results for the data with contrast agent was observed. Magn Reson Med, 2010.