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Traditional Doppler methods measure only the axial component of the velocity vector. The lack of information on the beam-flow angle creates an ambiguity that can lead to large errors in velocity magnitude estimates. Different triangulation techniques so far have been proposed, which basically perform multiple measurements of the Doppler frequency shift originating from the same region. In this work, an original approach is introduced, in which two ultrasound beams with known relative orientation are directed toward the same vessel, but only one of them is committed to perform a Doppler measurement; the second (reference) beam has the specific task of detecting the beam-flow angle. The latter goal is obtained by accurately identifying the achievement of the target 90 reference-beam-to-flow angle through the inspection of the backscattered Doppler signal spectrum. In transverse flow conditions, in fact, such spectrum is expected to be centered on the zero frequency, and even small deviations from the desired 90 orientation cause noticeable losses of spectral symmetry. Validation of the new method has been performed through experimental tests, which show that the beam-flow angle can be estimated with high accuracy (rms errors lower than 1 ), and repeatable velocity magnitude measurements are possible. A procedure for automatically tracking the desired orientation by the reference beam is also introduced and shown suitable for implementation in steerable linear array transducers.

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A velocity profile is the distribution of the velocity of fluid particles flowing through a cross section of a vessel 1 . Typical shapes of velocity profiles are observed in physiologic and pathologic situations such as the entrance of the aorta, the aortic arch, downstream bifurcation or stenosis and turbulent flow 2 , and they have also recently been reported in the umbilical cord 3 . The conventional Doppler waveform shape, in real time, roughly indicates the type of flow within the vessel, but until now only semiquantitative or qualitative methods have been described secondary to technical limitations. A preliminary study using computerized analysis of pulsed Doppler-generated flow-velocity waveforms demonstrated how different profiles of the fetal aorta correlated with pregnancies complicated by pregnancy-induced hypertension, using limited technology and not in real time 4 .Over the past 7 years investigators in vascular science and electronic engineering have been studying a new system implementation using a software and hardware configuration to accurately analyze the signals from Doppler detection 5 -9 . The essential differences between the previous studies and ours lie in the numbers and positioning of the gates and the real-time process. Our system employs 128 range cells in a parallel array, while in the other studies they are arbitrarily targeted. Our arrangement of range cells permits us, using the graphic and analytic software, to asses the blood velocity profile during multiple consecutive systolic-diastolic cycles adaptable to the dimensions of the interrogated vessel. The ability to record and store velocity profiles for post processing analysis permits the subsequent review for an indefinite period, which will be essential in characterizing the changes in normal and pathologic pregnancies. The objective of this pilot study was to evaluate the applicability of Doppler processing to a fetal waveform using novel system hardware and a software package called 'Global Acquisition and Signal Processing' (GASP) for Doppler multigate signals. We elected to interrogate the fetal descending aorta during the third trimester because of its laminar blood flow and its potential clinical utility. Increase in the resistance to flow in the placental vascular bed can profoundly influence the flow in the descending aorta 10 . Informed consent was obtained from the participants in the trial. All growth parameters and morphology were confirmed to be normal at birth. Our experimental ultrasound system consisted of an ultrasound machine (Esaote AU3, Florence, Italy) and a PC. This configuration provides the ability to analyze all data detected by the transducer and then, in real time, to display a velocity profile, without interfering with the routine performance of the ultrasound equipment.As described in previous articles, the system has been carefully tested in vitro and in vivo 5,6 . The GASP software allows the PC to manage data both for real-time display and storage for subsequent analysis. While the colorcode...

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