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Saudi Board of Radiology: Physics Refresher Course
Doppler Ultrasound
Kostas Chantziantoniou, MSc2, DABR
Head, Imaging Physics Section
King Faisal Specialist Hospital & Research Centre
Biomedical Physics Department
Riyadh, Kingdom of Saudi Arabia
Introduction
The Doppler Effect refers to the change in frequency that results when either the
detector/observer or the sound source is moving with respect to each other.
Both source and detector
are stationary
Source moving with
respect to detector
• when sound source is moving towards the detector/observer (R), the sound
appears to have a higher frequency (shorter wavelength)
• when sound source is moving away from the detector/observer (L), the sound
appears to have a lower frequency (longer wavelength)
• if the sound source is moving perpendicular (90) to the detector/observer, there is
no change in frequency or in wavelength and thus no Doppler Effect is observed
• Doppler ultrasound is used primarily to identify and evaluate blood flow in vessels
• velocity and waveform information can be used to evaluate stenoses, resistance
and vessel patency
Doppler Frequency Shift (fD)
f
f0
doppler frequency shift (fD) = change in frequency
= (f0 - f)
(RBC)
where f0 is the original frequency and f is the frequency of the returning echo
(from RBC)
• maximum shift is obtained when  = 0°
• minimum shift is obtained when  = 90° (recall, imaging of strongest echoes)
• the shift is comparatively small and typically is between 0 - 15 kHz (audible sound)
• the shift is positive when the RBC is moving towards the transducer and is negative
when the RBC is moving away from the transducer
Flow Speed (v)
It can be shown, that
f
fD = (f0 - f) = 2 • v • f0 • cos 
c
f0
and re-arranging the above equation we have
v =
(RBC)
c • fD
2 • f0 • cos 
where f0 is the original frequency, f is the frequency of the returning echo (from RBC),
v is the speed of the interface (RBC), c is the speed of sound in soft tissue (1540 m/s),
fD is the Doppler shift and  is the angle between transducer and RBC flow direction
The velocity equation can be simplified to
v (cm/s) =
77 • fD(kHz)
2 • f0 (MHz) • cos 
ASIDE
The factor 77 is only valid when v, f0 and fD are given in the units shown above.
• the values of v calculated from the observed fD are only as accurate as the estimated
Doppler angle 
• one reason why sonography is combined with Doppler techniques is to estimate
the Doppler angle
• with a cross-sectional image of the vessel walls, the sonographer can adjust a flow
direction indicator which, when combined with an indication of the beam direction,
can yield the Doppler angle
• the instrument uses this value of angle to convert Doppler shift to flow speed
reflector speed  doppler shift 
operating frequency (f0)  doppler shift 
doppler angle  doppler shift 
NOTE
The moving reflector can be a tissue boundary (blood vessel wall or heart wall) or
blood cells in circulation
Operational Techniques
• high Q transducers are desirable for Doppler because they produce a narrow range
of ultrasound frequencies (f0)
• Doppler systems tend to run at lower frequencies than B-mode systems because
resolution is not as important and there is a need to minimize attenuation (blood is a
weak scatterer)
Flow Velocity Waveform (Spectrum)
• the range and frequency distribution
of flow velocities can be displayed
as a spectrum
• Spectral broadening is the result of
a mixture of velocities in the sample
and produces a shaded area below
the peak velocity value
Continuous Doppler
• in continuous Doppler, one transducer (high Q) continually transmits and another
transducer (low Q) continuously receives
• the frequency of the two signals are subtracted to give the Doppler shift, which is in
the audio range
• continuous wave Doppler is
inexpensive and does not suffer
from aliasing artifacts but lacks
depth resolution and provides
little spatial information
• continuous wave Doppler is
good for measuring fast flow and
deep lying vessels
• depth gain compensation is not
used in continuous wave Doppler
Pulsed Doppler
• pulsed Doppler allows both velocity and depth information (ranging) to be obtained
• the Doppler information is only provided for a specific area
• pulsed Doppler uses a longer pulse length than B-mode, typically up to 15 mm long
• Doppler information is displayed audibly and graphically as a waveform (spectrum)
• aliasing artifacts result in errors in estimating velocity
• the use of lower frequencies allow higher velocities to be measured without aliasing
• Dublex scanning involves
displaying Doppler data on real
time (B-mode) images and allows
velocity and position information
to be obtained simultaneously
• most of the time is spent in
Doppler mode, the B-mode image
being updated once a second
Pulsed Doppler
Continuous Doppler
Color Flow Doppler
• Color Doppler is a hybrid that combines anatomic information obtained using
B-mode system with flow information obtained using pulsed Doppler analysis
• pulse length in color Doppler is typically 2 mm
• colors (blue and red) are assigned dependent on motion (toward or away) from the
transducer
• turbulence (i.e.: variations in flow direction) can vary between green and yellow
• the depth of each color varies with the velocity of flow, stationary tissues appear gray
• information is provided over a large area and superimposed on a gray scale image
• color Doppler can detect flow in vessels too small to see by imaging alone
• spectral analysis may also be obtained using commercial color Doppler systems
• modern instruments incorporate both color Doppler and spectral Doppler
Power Doppler
• Power Doppler is a signal processing method that relies on the total strength of
the Doppler signals (amplitude) and ignores directional (phase) information
• the power (also known as energy) mode of signal acquisition is dependent on the
amplitude of all Doppler signals, regardless of the frequency shift
• this dramatically improves the sensitivity to motion (i.e.: slow blood flow) at the
expense of directional and quantitative flow information
• compared to conventional color flow imaging, power Doppler produces images
that have more sensitivity to motion and are not affected by the Doppler angle and
aliasing is not a problem as only strength of the frequency shifted signals are
analyzed
• greater sensitivity allows detection and interpretation of very subtle and slow
blood flow
• frames rates though tend to be slower and a significant amount of “flash artifacts”
occur, which are related to color signals arising from moving tissues, patient
motion or transducer motion
• Power Doppler does not mean more power to the patient, but in fact the power
level are typically the same as those in a standard color flow procedure
Circle of Willis
Submucosus fibroid, note small
vessels inside the tumor