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a Chapter 4 Spectral Doppler Sonography: Waveform Analysis and Hemodynamic Interpretation 39
Fig. 4.5. Spectral broadening.
Top: Flow-guided placement of
the Doppler sample volume in
the uterine artery. Note the
spectral broadening of the
waveform
Principle of Doppler Flowmetry
The Doppler flowmetric technique has been used to
measure umbilical venous flow [6], descending aortic
flow [7], and fetal right and left ventricular outputs
[8]. The theoretical basis of Doppler flow quantification
is as follows (Fig. 4.6). The volumetric flow in a
given instant (Q) is the product of the spatial mean
velocity across the vascular lumen at that instant
…v…t†† multiplied by the vascular cross-sectional area
at that instant …A…t††:
Q…t† ˆv…t†A…t†
…1†
The mean velocity is estimated from the mean Doppler
frequency shift (f d ) by Doppler velocimetry provided
the angle of insonation (h) is also known:
v…t† ˆf d …t†c=2f t cos h
…2†
where f t is the transducer frequency and c is the
speed of sound in blood. The equation (1) can now
be rewritten as follows:
Q…t† ˆ…f d …t†c=2f t cos h†A…t†
…3†
Fig. 4.6. Doppler sonographic volumetric quantification.
Graphic depiction of the principle of even insonation technique.
Q volumetric flow in a given instant, vÅ (t) spatial
mean velocity across the vascular lumen at that instant,
A(t) vascular cross-sectional area at that instant
Arterial circulation is pulsatile. Therefore the volumetric
flow passing through the arterial tree fluctuates
through the cardiac cycle. The temporal average
flow over the cardiac cycle is obtained from the temporal
average product of the instantaneous mean velocity
(mean frequency shift) and the vascular crosssectional
area.