Blood flow and phase contrast CMR

Flow-sensitive cardiovascular magnetic resonance (CMR) is a widespread non-invasive imaging method for the clinical evaluation of blood flow in cardiovascular disease. The basic principle of phase contrast magnetic resonance imaging (MRI) is the use of bipolar gradients to encode blood velocity in the magnetic resonance (MR) signal phase. The most common type of flow-encoded scan two-dimensional (2D) cine phase contrast CMR with single-direction velocity encoding is clinically used to quantify cardiovascular flow and velocities. Trade-offs between resolution (temporal and spatial) and acquisition time are illustrated in the context of patient examination, considering high-velocity jet flow, patient breath-hold duration, respiratory motion artefacts, and patient comfort. In addition, the chapter describes how the velocity-to-noise ratio and aliasing behaviour of flow measurements are affected by the velocity-encoding sensitivity (VENC). An advantage of phase contrast MR is that flow encoding may be performed in all three spatial dimensions, improving peak velocity measurement accuracy. Several clinical applications (aortic stenosis, coarctation, and ventricular shunting) and best practices are explained in detail with illustrations. Analysis and post-processing of phase contrast data are summarized. The progressive development of advanced phase contrast techniques is discussed by adding incremental complexity, starting with 2D phase contrast (2D spatial and one-dimensional velocity) and ending with four-dimensional flow encoding (three-dimensional spatial and velocity). Methods to accelerate phase contrast, such as parallel imaging, are briefly discussed. Finally, the chapter concludes with a summary of emerging topics for accelerated scanning and special applications such as compressed sensing, real-time phase contrast, and ultra-short echo time imaging.