228
Dynamics
of
the
Vascular System
Typical diagnostic ultrasound utilizes frequencies in the range of
1
MHz
to 15 MHz.
Thus, the corresponding wavelengths are 0.78 mm and
0.156 mm, respectively.
The transit-time ultrasound measurement of blood velocity utilizes
two crystal transducers placed at two different locations, serving as
transmitter and receiver. With known ultrasound velocity, c, and the
transit time At
=
tl
-
t2, we have:
2vD
At
=
c2
case
(8.1.15)
where D/cose is the distance between the transceiver and
8
is the angle
between the axial blood velocity and the transceiver.
A
more common approach is the ultrasound Doppler technique, based
on the back scattering of ultrasound by red blood cells.
Turbulence,
therefore,
increases the scattering. Two commonly used types are
continuous wave Doppler (CWD) and pulsed wave Doppler (PWD). In
the CW mode, the Doppler shifted frequency,fd, of the back scattered
ultrasound
is:
(8.1.16)
where v is the blood velocity,
0
is
the angle between the ultrasound beam
and the centerline, and& is the transmitted ultrasound frequency. In the
PW mode, a velocity profile across the vessel can be obtained. Signal
from cells scatters in a range at
a
depth
of:
ctd
z=-
2
(8.1.1 7)
By pulsing the ultrasound beam, one can obtain range resolution along
the beam. Generally,
a
short burst of ultrasound is transmitted with a
repetition frequency
f.
The backscattered signal is received and sampled
after a time delay td.
Velocity profiles can also be obtained by the use of thermal-
convection velocity sensors, such as hot-wire anemometers. Thermistors
