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As
high as 35 percent of the applications for accreditation received
by the ICAVL demonstrate improper angle correction techniques,
making angle correction issues one of the most common causes
for delayed decisions. The purpose of this article is to review
proper angle correction techniques, so that your laboratory
or department can understand how the reviewers evaluate your
images with respect to angle correction. There are different
thoughts about how to angle correct, for example, parallel to
the vessel wall versus parallel to blood flow. It is not the
purpose of this article to dictate which method to employ. However,
all members of a laboratory should use the same technique following
the laboratory’s written protocol. In addition, all submitted
cases should demonstrate uniform techniques that follow the
guidelines established by the authors of the diagnostic criteria.
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Comparison
of (A) correct and (B) incorrect angle adjustment.
Click
here for additional images.
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The
physical principles revolve around the fact that the vast majority
of ultrasound machines detect and calculate the variables of
the Doppler equation, except for the angle between the sound
beam and blood flow that is cos q.
The vascular technologist/sonographer must input this information
into the ultrasound unit via the angle correction control. When
used correctly, the velocity of the blood flow can then be measured.
Usually, the cursor is placed parallel to the walls of the vessel
being investigated to determine angle q.
The ultrasound system software then uses the value of cos q
in the Doppler equation to determine the velocity scale used
to measure the Doppler spectral signals. Since we are dealing
with the cosine of an angle, errors of varying severity are
introduced when an incorrect value is used. When angle correction
is not properly applied, the calculated velocity measurements
may overestimate or underestimate the severity of disease. Utilizing
erroneous velocity measurements to arrive at a final diagnosis
may place the disease severity in a higher or lower category
and thus adversely impact the care the patient receives.
Errors
are amplified when angles of greater than 60 degrees are used
in the determination of velocities. At Doppler angles of less
then 60 degrees, an error in the angle correction will result
in a very small error in the velocity measurement. As the angle
increases between 60 to 90 degrees, the cosine of q
(and therefore the denominator of the Doppler equation) rapidly
approaches zero; therefore, small errors in the measurement
of q result in increasingly
large errors in the calculated velocity. It is imperative that
a Doppler angle of 60 degrees or less be utilized whenever possible
for Doppler velocity measurements.
The
method of angle correction used must be based on the diagnostic
criteria utilized by the laboratory. It is important to use
the same techniques as described in the criteria and the same
range of angles.
A
survey of ICAVL accredited laboratories was undertaken and reported
by Beatrice Madrazo, MD, RVT, of William Beaumont Hospital in
Michigan in 1998. Based on the survey, Dr. Madrazo reported
that approximately 72 percent of accredited laboratories were
utilizing the diagnostic criteria published by the Eugene Strandness
et al from the University of Washington. The guidelines established
by the University of Washington for those specific criteria
include using a consistent angle of 60 degrees and with the
Doppler adjusted parallel to the vessel wall.
For
all duplex examinations, regardless of the criteria being used,
the Doppler angle must be 60 degrees or less. When angles of
greater than 60 degrees are obtained, care must be taken to
either reposition the transducer or the patient so that the
Doppler angle is 60 degrees or less. Occasionally, due to the
patient's anatomy, this may not be possible. Whenever angles
greater than 60 degrees are used, this must be noted and care
must be taken in using these velocity measurements in determining
the severity of disease and in reaching a diagnostic conclusion.
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