60 DEGREES:
Is It the Gold Standard?
R. Eugene Zierler, M.D.
The D. E. Strandness, Jr. Vascular Laboratory
University of Washington Medical Center
Division of Vascular Surgery
University of Washington, School of Medicine
DISCLOSURE INFORMATION
R. Eugene Zierler, M.D.
No relevant
financial or commercial
relationships
to declare
WHY DOES DOPPLER ANGLE MATTER?
Key Points
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Measurement of blood flow velocities using Doppler
ultrasound is the primary method for classifying the
severity of arterial stenosis
This approach is based on the Doppler equation and
some assumptions about the nature of blood flow
Interpretation of velocities requires an appreciation of
the possible errors in Doppler velocity measurements
Accuracy and Minimizing Variability are both important
DOPPLER VELOCITY MEASUREMENT
What Does Doppler Really measure?

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Measures the Doppler frequency shift (not velocity)
Calculates a velocity based on the Doppler equation
Measured
V=
Calculated
∆f C
2 f cos θ
What is this
and why do
we need it?
DOPPLER VELOCITY MEASUREMENT
What Do We Mean by “Doppler angle”?
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The Doppler angle “θ” is the angle between the ultrasound
beam and the direction of blood flow
The convention is to use the angle between the line of the
ultrasound beam and the long axis of the vessel (vessel wall)
This assumes that flow is axial or parallel to the vessel wall
DOPPLER VELOCITY MEASUREMENT
Why Is the Doppler Angle So important?
Calculated
Measured
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For axial flow, V1 is the true
velocity vector
Doppler measures V
(projection of V1 on the beam)
Calculates V1 using cos θ

For non-axial flow (V1,V3,V4),
use of the Doppler equation and
cos θ results in velocity
measurement errors – because:

The true velocity vector is not
known (and may not be axial)!
DOPPLER VELOCITY MEASUREMENT
What Happens When the Angle Changes?
The cos varies from 0 to 1: cos 0° = 1 and cos 90° = 0

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Same arterial site
3 different angles
∆f =
θ = 35°
θ = 55°
θ = 85°
2 f V cos θ
C
Doppler shift decreases as the angle increases
DOPPLER VELOCITY MEASUREMENT
What Happens When the Angle Changes?
V=
64 cm/s
107 cm/s
θ = 40°
θ = 50°
∆f C
2 f cos θ
116 cm/s
θ = 60°
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144 cm/s
θ = 70°
Same arterial site
4 different angles
Calculated velocity increases as the angle increases
DOPPLER VELOCITY MEASUREMENT
So What is the “Best” Doppler Angle?
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Large and small angles are both “bad”
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Since the true direction of flow is not exactly known,
the Doppler angle can only be estimated

Large Angles (approaching 90°): Errors and variability
in Doppler velocity measurements increase

Small Angles (less than 30°): Refraction, reflection,
and the “aperture effect” can produce errors in
Doppler velocity measurements
DOPPLER VELOCITY MEASUREMENT
Why Large Angles Are Bad

As the estimated Doppler angle approaches 90°
 Frequency shift becomes very small
and system sensitivity is reduced
 Cosine changes more rapidly, resulting in
larger errors in calculated velocity
Doppler
shift
decreases
with
increasing
angle
in
Calculated
velocity
error vs.
angle error
for various
Doppler
angles
Courtesy of F. Kremkau
DOPPLER VELOCITY MEASUREMENT
Why Small Angles Are Bad
Incident
Ultrasound
Beam
Both of these
effects
increase at
angles
approaching 0°
Refraction and Reflection
Reflected
Ultrasound
Refracted
Ultrasound
Courtesy of F. Kremkau
DOPPLER VELOCITY MEASUREMENT
Why Small Angles Are Bad
The Aperture Effect
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θ edge is smaller than θ mid
Wide aperture beam forming results
in angle errors at smaller angles
Focused beam crosses the vessel
at a range of angles
Smallest angle (θ edge) should be
used to calculate velocity
Scanners use θ mid to calculate
velocity
Leads to overestimation of peak
velocity
Courtesy of F. Kremkau
DOPPLER VELOCITY MEASUREMENT
The “60 Degree Angle”
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The original duplex scanner
had no velocity scale
Spectral waveforms showed
Doppler shift in KHz
A protractor was used to
measure the Doppler angle
Velocities obtained using a
programmable HP calculator
and the Doppler equation
∆f C
V=
2 f cos θ
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60 degrees was a “convenient”
angle for carotid duplex
Cosine of 60 degrees = 0.5
DOPPLER VELOCITY MEASUREMENT
How Can we Improve Accuracy and Minimize Variability?
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Both large angles (≈90°) and small angles (<30°) can
result in errors
Optimal angle is in the range of 30° to 60°
The true velocity vector (direction of flow) is not
known, but the arterial wall is always visualized
The color-flow “jet” does not help (2D vs. 3D)
Using a consistent and easily obtained angle is best
DOPPLER VELOCITY MEASUREMENT
Is 60 Degrees the “Gold Standard”
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Original selection of 60° was mostly for convenience
First (and subsequent) carotid velocity criteria were
based on a 60° angle
Other velocity criteria (peripheral arterial, renal)
have also been validated using a 60° angle
The 60° angle has become the standard through
long-term and widespread use
The “60° or less” rule still applies
60° and parallel to the vessel wall
is a good “compromise” between theory and practicality