Time Domain Colour Flow Imaging: Methods and Benefits Compared to Doppler

Abstract

The Frequency is the preferential parameter to describe the Doppler effect but it is possible to choose its dual parameter: the time. This different perspective allows to understand that pulse Doppler techniques don’t measure a frequency shift on the transmitted ultrasound frequency but detect the Doppler effect on the pulse rate frequency. In this case, the successive echoes coming from moving targets are seen as time shifted due to the displacement of the medium. The direct measurement of this time shift can be performed thanks to time domain correlation techniques. These latter have been studied at LEP during the five last years, and have lead to a commercial system called CVI for Color Velocity Imaging and producted by Philips Ultrasound International at Santa Ana,CA. This paper describes and explains the advantages of this method compared to pure Doppler techniques in the particular case of flow mapping. Accuracy, frame rate, sensitivity are indeed strongly increased. In comparison,the autocorrelation algorithm working on the so called “Doppler signal” and used in most of the systems suffers from a large variance mainly due to the random behavior of the local frequency. Moreover it will be demonstrated that thanks to this time processing, new modalities are anticipated to make a truly quantitative tool for the characterization of blood flows.