Real-time acquisition, reconstruction and display of blood velocity distribution using nuclear magnetic resonance with an oscillating magnetic field gradient

Although magnetic resonance angiography has been an important development in medical diagnostic imaging, the images carry little or ambiguous information about blood velocity. To more fully characterize the performance of the circulatory system, the flow to an organ, or a particular vessel, more quantitative information is desirable. Described here is a multi-component system that has been developed to non-invasively measure the blood velocity distribution (BVD) in real time. This package consists of the MR pulse sequence and data reconstruction and display algorithms that take advantage of distributed computer resources for parallel processing. The significant feature of the BVD pulse sequence is that the data are acquired in the presence of an oscillating gradient, which serves to successively refocus the phase of the spins while allowing the phase of moving spins to evolve. By predicting the phase evolution of the signal from moving spins, the velocity distribution can be reconstructed. The spins are also localized in space along the direction of the gradient. The BVD method was evaluated with numerical simulations and phantom studies. To gain an initial appraisal of the clinical utility of the BVD method, two in vivo experiments were performed. In the first, observation of transient changes in the blood velocity were investigated by measuring the portal velocities after oral ingestion of a small amount of ethanol. These changes were compared to changes observed using pulsed Doppler ultrasound. In the second experiment, the velocity distribution in the portal vein was acquired using BVD and cine phase contrast from a population of patients with advanced liver disease who were also evaluated with Doppler. The range of velocities measured was used to correlate BVD, CPC and pulsed Doppler US. This study demonstrates that the general advantages of MR (easier and more reproducible prescription, arbitrary angulation of the direction of sensitivity, no depth of plane problem and the elimination of the need for access windows ) can be exploited for real-time non-invasive velocity measurement