Biomedical Engineering Reference
In-Depth Information
the ultrasound probe is non-invasive, it is also possible to investigate the changes in embo-
li formation months to years after implantation. A good example of this application is a
Transcranial Doppler Ultrasound (TCD) system, which is often used to record high-inten-
sity transient Doppler signals (HITS) in human patients or large animals (such as sheep).
HITS are usually induced by microemboli, which can generate a high-pitched sound, dif-
ferent from the sounds that the blood flow normally makes and different from the normal
particle matter within the blood. The duration of a HITS is usually less than 50 ms, and
the amplitude of a HITS is usually much higher than the background blood flow signal.
Based on the velocity and duration of a HITS, the nature of the microemboli, for example,
particle emboli (more dangerous to patients) or gaseous emboli, can be distinguished.
Another major use for ultrasound technology is to obtain vascular architectures. The
advantage of ultrasound technology is that it has a low cost, is not harmful, and has
the ability to carry out real-time imaging and the ability to obtain flow information at the
same time as geometry information. This capability makes it possible to form a feedback
loop for calculating the flow through real geometries and validating the flow calculations
with the ultrasound recordings. The disadvantage of this technique is that most ultra-
sound probes have a limited scanning depth (at larger depths, the resolution decreases)
which restricts the usage of this technique to superficial blood vessels, such as the carotid
arteries, which are critical blood vessels or capillaries in the skin, which may not be critical
blood vessels. However, if an ultrasound probe can be inserted into the body, deep blood
vessels or tissue can be assessed. A good example is the transesophageal echocardiogra-
phy. With this technique, an ultrasound probe is inserted into an animal (or human
patient) through the mouth and passed into the esophagus. This places the probe in close
proximity to the aortic root and importantly, the aortic valve. Short-axis and long-axis
views of the aortic valve and the left ventricle can be obtained and therefore the diameter
of the aortic root can be determined. Meanwhile, the pressure gradient across the aortic
valve, blood flow velocity, and other fluid properties can be calculated. By adjusting the
location and angle of the probe head, information about the left ventricle and right atrium
can be obtained as well. Actually, transesophageal echocardiography is often used in
patients before a heart valve replacement surgery, to determine the size of the original
heart valve and local blood flow conditions.
Commonly, ultrasound measurements are carried out under the Brightness mode
(B-mode), which renders a two-dimensional image of the blood vessel, where the intensity
could be correlated to the blood velocity. Based on the Brightness mode imaging, three-
dimensional ultrasound images can also be achieved, as the ultrasound probe is normally
small and handheld. Three-dimensional images can be obtained by moving the probe in
relation to the imaging location. This set of two-dimensional Brightness mode images can
then be reconstructed into a three-dimensional image. A few groups have investigated the
use of this technique to obtain information about blood vessel architecture and the flow
through these blood vessels, and they all found that it was necessary to either have a pre-
cise computer-controlled mechanism to maneuver the probe or an accurate monitoring
system for the probe location in relation to the imaging area. This has proved to be rela-
tively difficult to carry out, but the information gained from obtaining ultrasound images
from different orientations has proved to be beneficial. Also, the amount of noise associ-
ated with the image is reduced by overlaying and reconstructing images from multiple
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