1Center for fast Ultrasound Imaging, Technical University of Denmark, Lyngby, Denmark
Medical ultrasound has for more than 40 years been capable of estimating the blood velocity in the human circulatory system. This is of major diagnostic value for screening and investigating haemodynamic problems. Current commercial systems, however, only estimated the velocity along the ultrasound direction, which often is the least significant velocity component as the major vessels run parallel to the skin. Also at bifurcations, valves, and geometric changes the flow is not uni-directional and varies as a function of space and phase in the cardiac cycle.
A number of methods for solving this problem are presented in the paper. The transverse oscillation (TO) method can estimate the velocity transverse to the ultrasound beam by introducing a lateral oscillation in the received ultrasound field. The approach has been thoroughly investigated using both simulations, flow rig measurements, and in-vivo validation against MR scans. A range of other methods will also be presented. This includes synthetic aperture imaging using either spherical or plane waves with velocity estimation performed with directional beamforming or speckle tracking. The key advantages of these techniques are very fast imaging that can attain an order of magnitude higher precision than other methods.
The TO method obtains a relative accuracy of 10% for a fully transverse flow in both simulations and flow rig experiments. In-vivo studies performed on 11 healthy volunteers comparing the TO method with magnetic resonance phase contrast angiography (MRA) revealed a correlation between the stroke volume estimated by TO and MRA of 0.91 (p < 0.01) with an equation for the line of regression given as: MRA = 1.1•TO-0.4 ml. Several clinical examples of complex flow in e.g. bifurcations and around valves have been acquired using a commercial implementation of the method (BK Medical ProFocus Ultraview scanner). SA flow imaging was implemented on the experimental scanner RASMUS using an 8-emission spherical emission sequence and reception of 64 channels on a BK Medical 8804 transducer. This resulted in a relative standard deviation of 1.2% for a fully transverse flow. Plane wave imaging was also implemented on the RASMUS scanner and a 100 Hz frame rate was attained. Several vector velocity image sequences of complex flow was acquired and demonstrates the benefits of fast vector flow imaging.
The flow in the human body is pulsating, complex, and hardly ever laminar. The velocity vector changes over time and space, and current static angle compensation methods are inadequate for quantitative velocity estimation in everything but the simplest cases. The new vector velocity imaging schemes are capable of revealing a wealth of information regarding the human circulation in real time and without the use of contrast agents.