The B-flowmode is not a Doppler technique based on the processing of the Doppler shift but a B-mode scanning technique that compares gray-scale scans over time to identify changes in the spatial positions of reflectors (blood cells) by the successive emission of coded pulse packets. The echo signals are subtracted from one another, and the brightness is determined by the number of reflectors and partly also by their velocity. The conventional B-scan generated from the stationary echoes is display around the flow information. Since the successive pulses are emitted at defined intervals and in digitally encoded form, it is possible to eliminate interfering echoes and use only the encoded echoes in the subtraction procedure. Hence, only the amplitude signal reflected by moving particles is processed in the interval between two pulses. As the signal strength increases not only with the number of reflecting particles (volume flow) but also with flow velocity, a jet within a stenosis is depicted with a higher signal intensity. The advantage of this technique lies in the simultaneous display in a single image of hemodynamic parameters (comparable to angiography) and morphologic details of the vessel wall with a high resolution while showing no or only little angle dependence. Disadvantages of B-flow imaging are the occurrence of artifacts in highly pulsatile, atherosclerotic vessels, the susceptibility to artifacts caused by wall motion, and the still limited scanning depth. With further technical advancement, B-flow imaging will, in principle, enable morphologic quantification of stenoses and differentiation of the vessel wall from the patent lumen even if only slow flow is present (e.g. in ulceration). The B-mode provides a high-resolution display of the vessel wall contour with separate representation of blood flow in the B-flow mode. There is no superimposition of blood flow information as in color duplex scanning.
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