B-K Medical Pro Focus 2202, User Manual

Page: 225 / 356

General Pro Package 225
Accuracy in Vascular Calculations
Input
parameter
The accuracy of the Doppler frequency is 5 % of the maximum blood flow velocity
(
θ
= 55°), which is tabulated in the General Transducer Information (BB0555) for
each transducer capable of Doppler imaging.
The accuracy of time measurement is related to the digital resolution of the screen
and is 0.2 % of the full time scale of the image field.
Accuracy of
equations
The accuracy of the Doppler equations is calculated using the accuracy for the
Doppler frequency and the time resolution.
Rounding
errors
The finite resolution on the floating point numbers introduces an error into the
calculations. The amount of this error is, however, insignificant compared to that
introduced by the other factors listed above. Therefore, it is not taken into
consideration.
Noise
reduction
Selecting too high a noise reduction level introduces an unnecessary error: part of the
spectrum itself is removed along with the noise. The amount of this error depends on
the shape of the spectrum.
Curve
definition and
Doppler
spectrum
The generation of automatic curves includes a “smoothing” which contributes to the
accuracy of the curve.The amount depends on the shape of the spectrum, but it is
typically less than 5%. To keep the amount low, it is essential to display only a few
heart-rate cycles.
Here are some examples to guide you in evaluating the total expected accuracy of the
calculations. In the examples, A = accuracy.
Accuracy of FACC and ACC Calculations
A F
max
( )
A F
min
( )
5% = =
A time
( )
0.2% of the full time range =
Assuming F
max
t
PS
( )
F
max
t
SS
( )
» then
A F
acc
( )
A F
max
( )
0.002 T
×
t
PS
t
SS
–
( )
-----------------------------
Example: T 4 t
PS
t
SS
–
( ) ×
A F
acc
( )
6% =
⇒
=
+ =