In vivo Biofluid Mechanics - Biofluidmechanics: An Introduction to Fluid Mechanics Macrocirculation and Microcirculation - page 371

FIGURE 15.2 Ultrasound pulse Doppler ultrasound

wave intersecting with a blood vessel.
Based on the apparent shift of the
ultrasound frequency, one can obtain
information about the blood velocity.
This shift occurs due to the reflection
of sound waves after coming into con-
tact with blood particles.

Velocity

θ

Blood vessel

great deal of information about the flow of blood through vascular networks that are rela-
tively close to the skin. Doppler ultrasound technology takes advantage of the apparent
change in the ultrasound frequency caused by the velocity of blood and the incident angle
of the sound wave in relationship to the blood vessel ( Figure 15.2 ). The apparent fre-
quency shift can be calculated from

2 vf 0

c

f D 5 cos ðθÞ ð 15 1 Þ

:

where f D is the recorded Doppler frequency, v is the blood velocity, c is the speed of

sound, f 0 is the

emitted frequency from the probe. Because the blood velocity is variable based on the
radial position within the blood vessel and the pulsatile nature of blood, the recorded fre-
quency is variable. Therefore, a velocity distribution can be obtained if the receiving
equipment is sensitive enough to differentiate between these small changes in frequency
within a small blood vessel. An additional software component to the general Doppler
ultrasound technique is the use of the color flow imaging module. This add-on color codes
the blood velocity, and therefore, one can obtain a real-time depiction of the relative blood
flow speed across the radial direction of a blood vessel as well as the direction of the blood
flow. Color Doppler scans are normally obtained in conjunction with the spectral Doppler
mode (also known as the Duplex Doppler mode). Using the spectral Doppler mode,
images can obtain a time-dependent mean velocity curve over a small section of the blood
vessel. Therefore, one can determine if there are temporal shifts within the blood flow of
that vessel. The usefulness of this technique is that the blood velocity can be compared
between vessels of similar sizes. This would help to determine if regions are not obtaining
enough blood, potentially due to a partially blocked vessel upstream of the imaging loca-
tion or other existing pathologies. Lastly, Doppler ultrasound is useful to quantify the fre-
quency of emboli or other large particulate matter by recording high-magnitude transient
changes to the velocity profile.
Doppler ultrasound has proved to be a good research tool to characterize the potential
for new cardiovascular devices to induce thromboemboli within the cardiovascular sys-
tem. These new devices can be implanted into an animal, and after post-operative recov-
ery the generation of high-intensity transient events can be compared to prior
implantation in order to determine if this device induces new emboli formation. Because

θ is the angle between the ultrasound probe and the blood vessel, and