ABSTRACT
Conventionally, two different approaches have been used for ultrasonic Doppler flow measurements: continuous wave (CW) and pulsed wave (PW) Doppler.
A CW system is shown in Figure 5.2. A probe consisting of two piezoelectric elements, one for transmitting the ultrasound signal and one for receiving echoes returned from blood, is excited by an oscillator. The Doppler-shifted echoes are amplified, demodulated, and band-pass filtered to remove the carrier frequency and other spurious signals. Suppose that the ultrasound signal generated by the oscillator is given by
A
cos(
ω
t
), where
A
denotes signal amplitude and
ω
, the angular frequency,
=
π
f. The demodulated signal would be
f v c
fd = 2 cosθ
g A t B t ABd d d( , ) cos( ) cos[( ) ] {cos[(ω ω ω ω ω ω= + = 1 2
2 + +ω ωd dt t) ] cos( )}
where the echoes are represented by
B
cos[(
ω
+
ω
)
t
] and
ω
=
π
f
. The magnitude of constant
B
is determined by the scattering strength of blood. Much work has been done to better understand the relationship between the
Doppler power generated by blood and hematological and hemodynamic factors (Shung et al., 1992; Mo and Cobbold, 1993). Doppler power from blood has been
found to be related to flow disturbance, hematocrit, and the degree of red blood aggregation; this is in turn affected by the concentration of plasma proteins such as fibrinogen and local shear rate. The output of the demodulator contains the ultrasound carrier frequency and the Doppler shift, as illustrated in Figure 5.3, where the signals in the time and frequency domains are shown on the left and right, respectively. The carrier signal can be readily removed by band-pass filtering by setting the cut-off frequency of the band-pass filter at the high end to be much lower than the carrier frequency.
