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Chapter 7 - System Operation
turn, the image quality depends on system design and skills of operators as
well as their understandings of system features.
The ultrasound imaging algorithms are based on the sound traveling
1540m/s in tissue. However, in different tissue types, velocity of sound may
be different. In soft tissue, the error is usually by 2%, and sometimes even
reaches 5%, particularly when a lot of fat tissue occurs in the measured area.
In order to ensure the best measurement precision, align the probe carefully
and keep the sound beam aligned with the direction of flow.
Note: The calculation precision of blood flow parameters depends on: System
design and signal processing algorithms used in the system;
Quality of reflection signals, including signal-to-noise ratio and Doppler
change.
Skills of the users, whether they can correctly adjust the probe in line with
the direction of the exam item and select the system parameters.
All of these above will affect the quality of reflection signals and the suitabil-
ity of processing algorithms.
Due to the above and other factors, the aggregate error of such an absolute
measurement like flow velocity sometimes even reaches ± 50% or higher.
Even in ideal laboratory environment, it is very difficult to achieve an error
within ± (5-10)%.
Therefore, absolute measurement should be avoided, but give priority to
the relative measurements such as resistance index and pulsatility index,
etc, of which the errors are around 10%.
For the absolute measurement, angle between beam axis and flow direction
is very important. If the angle exceeds 60°, the measurement error will in-
crease dramatically.
Even if the angle is less than 60°, the operator should be aware that error also
depends on the parallel positional accuracy between flow direction line and
actual flow direction.
The calculation precision of the average velocity will be greatly affected by
the relative position of sampling volume and blood vessel. The sampling
volume must be large enough to be able to "ensonify" the entire sample vol-
ume. The sampling volume must be aligned with the blood vessel axis.
Users must receive trainings from skilled experts to ensure proper use of the
Portable Ultrasound Diagnostic Scanner.
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Portable Ultrasound Diagnostic Scanner User Manual
Measurement
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