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Classic Doppler equation can only provide the axial velocity of blood flow. To acquire the complete flow vector,
estimation of the non-axial flow velocity is essential. For Doppler-bandwidth-based transverse estimation, however,
accuracy is limited because of the complex dependence of the Doppler bandwidth on the geometry and the location of
the sample volume in the vessel. Specifically, the Doppler bandwidth tends to be overestimated because it is
conventionally decided from the difference between maximum Doppler frequency and Doppler shift frequency. The
maximum Doppler frequency only depends on the peak flow velocity within the vessel and can be used as a stable
parameter in flow estimation. However, the Doppler shift frequency is susceptible to the position of the sample volume
and it decreases when the sample volume is not centered within the vessel. The distance between the center of the
sample volume and the central line of the vessel is referred to as the position offset of the sample volume. Based on the
stable nature of maximum Doppler frequency, a novel method utilizing the differential maximum Doppler frequencies
from two parallel beams with different beam widths is proposed to improve the accuracy of transverse estimation. In vitro
experiments were performed to validate the proposed method and results were compared with the conventional
method. In this study, a steady flow condition was considered and two 5-MHz pistons were used to generate the two
beams with different widths. For the conventional method, it is demonstrated that the Doppler bandwidth is severely
overestimated when the position offset is present. For the proposed method, however, the differential maximum Doppler
frequency is relatively stable even in the presence of the position offset as long as the sample volume is sufficient in
length. Hence, both accuracy and stability of the transverse estimation can be significantly improved by taking
advantage of the differential maximum Doppler frequency.
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