[Color-Doppler instruments: their design principles and modes of use]

The use of color-Doppler US equipment allows the clinician to evaluate, at the same time and within the same image, both shape and structure of the examined organs and the flow patterns in the vessels within them. Such information as direction and velocity of blood flow are displayed with a variety of intravascular colors, according to a conventional code. Moreover, this diagnostic method can provide physiological and anatomical pieces of information in the same image and is therefore widely used in medical imaging. However, the images obtained by means of color-Doppler units are not always easy to understand and diagnostic misinterpretations may occur. In this paper we will summarize the physical and technological principles on which such units are based. In the first half of paper, the problems relative to acquisition and construction of the images are presented, together with the different technical approaches used to obtain flow velocity information. Moreover, the relationships between the "black-and-white" and the "color" parts of the images are described. The principles according to which color is assigned to vessels are reported, together with the patterns exhibited by a vessel according to the type of transducer used to examine it. The non-linear correlation between the color scale and flow velocities is also discussed. The second half of the paper deals with technical parameters of color-Doppler scanning, discussing the criteria of choice of transducers, the determination of Doppler angle, the setting of both wall filters and scanning depth. The problem of pulse repetition frequency (PRF) setting is also addressed; this parameter is especially critical in color-Doppler studies. The authors believe that careful setting of all examination parameters and the good knowledge of the physical and technological factors underlying color-Doppler images are the basis for a correct and useful approach to this fascinating technique.