Spectral Analysis
Aliasing
Sample Volume
Spectral Analysis

Breaks up the frequency components of a
complex wave or signal and spreading
them out in order of increasing frequency.

Presents Doppler shift frequencies in
frequency order
Spectral Analysis
1.
Allows visualization of the quantitative
data in the Doppler signal in the form of a
Doppler frequency spectrum so that
Doppler shift can be evaluated

The Doppler shift voltage from the detector
does not go directly to the display, but
undergoes further processing
Spectral Analysis
2.
3.
4.
Vertical axis represents frequency shift or
velocity; the horizontal axis represents time
Allows measurements of peak, mean, and
minimum flow velocities to be made
Permits evaluation of Doppler waveform
characteristics such as flow direction and
frequency range present (spectral broadening)
Components of the Spectrum
1.
2.
Spectral broadening
Window
Spectral broadening
- vertical thickening of the spectral trace

The spectral trace would be a thin line if all the
cells are moving at the same speed
Spectral broadening

With disturbed or turbulent flow, a greater
variation in velocities of various portions of the
flowing blood produces a greater range of
Doppler shift frequencies - resulting in a
broadened spectrum.
Note: Spectral broadening does not necessarily
mean flow that is turbulent or disturbed, but may
be the result of abnormal vessel geometry like a
tortuous vessel
Spectral broadening

Indicative of disturbed or turbulent flow and can
be related to pathology, such as stenosis, at the
site of measurement or distal to it

Can be artificially produced by excessive Doppler
receiver gain

Some broadening are produced by beam
spreading, particularly with wide-aperture arrays
Window
-
refers to the dark anechoic area in the
lower portion of the spectral trace in systole
for normal flow.
-
The window is diminished or eliminated with
spectral broadening
Window
Flows properties that
influence the Spectrum
Narrow spectrum is expected near the
center of the vessel, where the cells are
moving the fastest
1.

Narrow spectra often seen in large vessels
Broad spectrum is expected near the walls,
where viscous drag is slowing the flow
2.
•
•
Broad spectra seen in small vessels
Intermediate spectra in medium-sized vessels
Flows properties that
influence the Spectrum
2.
Different flow conditions produce various
spectral presentations.

Vessel size, turns, & abnormalities, (plaque or
stenosis) can alter the flow characteristics. As the
ultrasound beam intersects this flow & echoes
are produced, many different Doppler shifts are
received from the vessel, even from a small
sample volume, and is referred to as the Doppler
frequency spectrum.
Flows properties that
influence the Spectrum
3.
4.
Various kinds of blood flow (plug, laminar,
parabolic, disturbed, and turbulent) produce
varying ranges of Doppler shift frequencies in
vessels
Impedance conditions downstream (distally)
give rise to very different spectral displays. Flow
reversal in early diastole and lack of flow in late
diastole indicate high resistance to flow
downstream (i.e. - due to vasoconstriction of
arterioles)
Distal Impedance


Changing the angle does not change the
relationship between peak systolic and
end-diastolic flows, changing the distal
resistance does
Pulsatility indexes are quantitative
indicators of distal impedance
Fast Fourier Transform (FFT

Technique that processes the Doppler shift
frequency information into the visual
Doppler spectrum (this process is also
referred to as Signal or Spectral Analysis)
FFT uses a complex mathematical technique
Fast Fourier Transform (FFT)
Math involved is accomplished by the pc


Returning echoes generated by each individual
red blood cell have many different frequencies
comprising the Doppler signal
FFT breaks down the complex Doppler signals
into a format displaying each individual
frequency
Fast Fourier Transform



Doppler shift frequency is the vertical axis
Power (amplitude) of each frequency
component, demonstrated by the
brightness of the signal, is determined by
the number of RBCs moving at a certain
velocity & time
Horizontal axis represents time
Pulsatility & Impedance


The Doppler waveform defines a flow property
called pulsatility & has low, moderate, or high
features
Pulsatility index (PI) works because flow
impedance is not constant over the cardiac cycle
with distensible vessels and pulsatile pressures.
Recall that impedance includes flow resistance,
inertia of blood, and compliance of vessel walls
Pulsatility & Impedance

Impedance is greater at lower pressures
& smaller at higher pressures

Careful use of the wall filter must be
exercised because it can affect the
pulsatility index
Low pulsatility waveforms have broad
systolic peaks & forward flow
throughout diastole
High pulsatility waveforms have tall, narrow, sharp
systolic peaks & reversed or absent diastolic
Velocities

Peak velocity is the maximum velocity at
any given time

Mean velocity is the average of available
signals

Minimum velocity refers to the lowest
diastolic velocity
•Velocity Ratios
2 commonly used measurements derived
from these velocities are:
1.
2.
pulsatility index (PI)
resistivity index (RI)
Pulsatility Index
= (maximum velocity - minimum velocity)
mean velocity
Resistivity Index
(Resistivity Index of Pourcelot)
= (maximum velocity- minimum velocity)
maximum velocity
Time Domain Correlation




another technique that processes the Doppler
shift frequency data into visual Doppler
spectrum
Recently developed
Determines the velocity of a moving reflector by
measuring the change in spatial location during
a known time interval
Unique features include the ability to identify a
particular echo signal waveform in multiple echo
wavetrains consisting of many echoes & track
the individual echo signals in time
Autocorrelation techniques




Used to rapidly obtain Doppler shift frequencies
used in color flow Doppler
Faster process than Fast Fourier Transform
technique
Less accurate than spectral Doppler because it
represents the mean Doppler shift
Compares measurements acquired from the
same reflector
Autocorrelation techniques

Autocorrelation detector does not depict
spectral analysis of the Doppler signal from
each sample volume

Based on pulsed sampling technique & is
subject to aliasing
(Mis)interpretation of
spectral trace information
1.
Largest Doppler shift does not necessarily
represent the fastest blood flow
- undisturbed laminar flow is necessary
for the largest spectral Doppler shift to
correspond to the largest flow speed
(Mis)interpretation of
spectral trace information
2.
Highest Doppler shifts correspond to the
fastest flow & the lowest to the slowest


Is true only for undisturbed & non-turbulent flow
in which all portions of fluid are moving parallel
to each other with a common Doppler angle
If flow is disturbed, turbulent, or helical, or the
vessel is tortuous, this assumption is not valid
(Mis)interpretation of
spectral trace information
3.
Peak Doppler shifts may not represent the
fastest flow but, possibly, slower flow that
is moving directly toward or away from the
transducer (small Doppler angle, high
Doppler shift)
(Mis)interpretation of
spectral trace information
4.
Lowest Doppler shifts do not necessarily
represent the slowest flow but, possibly,
faster flow that is moving nearly
perpendicular to the beam (large Doppler
angle, low Doppler shift)
(Mis)interpretation of
spectral trace information
5.
Spectral traces that have vertical axes
calibrated in speed units (m/s or cm/s) are
correctly calibrated presentations only if
the Doppler angle incorporation is proper
& straight, parallel, laminar, undisturbed
flow is being measured
(Mis)interpretation of
spectral trace information
6.
Doppler instruments detect a Doppler shift
that is proportional to speed of flow along
the sound beam direction

If flow is not straight, large Doppler shifts do not
necessarily correspond to the fastest flow &
small ones do not necessarily correspond to
the slowest flow
Basic Principles of
Color Flow Imaging

1.
Color-Doppler instruments present anatomic
information in:
Conventional gray-scale form

2.
Echoes returning from stationary tissues are detected &
presented in gray scale in appropriate locations along
scan lines
Doppler-shift frequencies at locations along each
scan line, are presented as color at locations in
the image
Color flow mapping (CFM) or color flow imaging (CFI)
Depending on
whether the motion
is toward or away
from the transducer,
the Doppler shift is
either positive or
negative. At
locations along
scan lines where
Doppler shifts are
detected,
appropriate colors
are assigned to the
display
Color Acquisition

Color information is obtained by positioning
many pulsed Doppler lines & gates over the
area of interest called a packet
Packets are composed of between 3-30
pulses per line of color
Large packet size require/provide:




More accurate velocity measurement &
color sensitivity
More time required to collect data
More pulses to make single frame
Lower frame rate: ⇩ temporal resolution
Doppler Shift Calculations


Mean frequency is calculated by
autocorrelation technique and assigned a
color depending on flow direction & velocity
Color-Doppler instruments require multiple
pulses per scan line (same idea as a
multiple focus scan line) for the
autocorrelation process
Instruments
Color-Doppler instrument has the same block
diagram as the gray-scale sonographic
instrument. The beam former, signal processor,
image processor, and display perform the same
functions as they do in the sonographic
instrument. The major differences are that the
signal processor must include the ability to detect
Doppler shifts and the display must be capable of
presenting echoes in color
Color maps
- are used to aid the examiner interpreting
the study in determining flow direction
Two different modes of maps
1. Velocity mode
2. Variance mode
Velocity mode

all measured velocities for each gate are
averaged then the colors are ranged up &
down
Variance mode




A measure of the spread around the mean
an indicator of the extent of turbulence or
flow disturbance
average velocity is calculated then the
colors are side-to-side (used primarily in
cardiac imaging to show turbulence)
shown as a change in hue from left to right
across the map
Components of Color
Various combinations of:
1. Hue
2. Saturation
3. Luminance
are used to indicate the sign, mean
magnitude & power (variance) of the
Doppler shifts found at each location
Hue

perceived color; one or any combination
of primary colors, red, yellow, green, blue,
that is detected by the human eye
Faster velocities are represented
by lighter colors or hues
Saturation

degree to which the original color is diluted
with white



Red mixed with white, becomes pink
The paler the pink the color is (more white), the
less saturated it is
The purer the color is (less white),
the greater the saturation
Less color saturation represents faster flow
Saturation


The deeper (purer) the color is, represents
slower flow
Less color saturation of flow represents
faster flow
Luminance

the brightness of the hue & saturation
presented representing echo intensity
Brighter represents stronger amplitude
from the returning echo
Color Controls

Color window
Baseline shift

Gain
Color velocity range -PRF

Steering angle
control
Color map selection
Variance

Color inversion

Wall filter
Smoothing

Priority
Ensemble length
Color window

Provides for the location, width, & depth of
region of interest
- Wider color windows (viewing areas) reduce
frame rates because more scan lines are
required for each frame
Color Window
Gain
Steering angle control

permits avoidance of 90° angles
Steering angle
Color inversion
- alternates the color assignments on
either side of the baseline on the color
map (if red was on top and blue on the
bottom, the inversion control would place
blue on top and red on the bottom)
Inverted Map
Wall filter

allows elimination of clutter caused by
tissue & wall motion
Setting the wall filter too high will
remove slower blood-flow signals
Wall Filter
Color Wall Filter
Color Wall Filter
Priority

selects the gray-scale echo strength below
which color will be shown instead of gray
level at each pixel location
Priority
Baseline shift

allows shifting the baseline up or down to
eliminate aliasing
Baseline
Color velocity range (PRF)

sets the PRF & the limit at the color bar
extremes
Decreasing the value permits observation
of slower flows (smaller Doppler shifts) but
increases the likelihood of aliasing for
faster flows
Color map selection

Allows the operator to select from a variety
of map choices a color map that best
demonstrates the diagnostic information
provided in the area of interest
Color Maps
Smoothing
Smoothing (also called persistence) provides frame-to-frame averaging to
reduce noise
Ensemble length


# of pulses used for each color scan line
minimum is 3, with 10 to 20 being common
Greater ensemble lengths provide more:
1.
2.
3.
4.
accurate estimates of mean Doppler shift
improved detection of slow flows
complete representation of flow within a vessel
lower frame rates
Line Density
Incorrect Angle
Angle Correction
Sample volume size
Color Doppler Limitations


Advantages
Disadvantages
Advantages



CDI can visualize vessels too small to be seen on
the gray scale image as well as aid in placement
of the Doppler sample gate
Detects the direction, mean, variance, &
amplitude of the Doppler spectrum at each of the
hundreds of sample volume locations in the
interrogated anatomy
Provides a quick visual guide for selective
Doppler interrogation
Disadvantages




angle dependence
lower frame rates
lack of detailed spectral information
Lack of color information is open for
interpretation (lack of color in a vessel
could be caused by lack of flow or flow
viewed with a 90° Doppler angle)
Disadvantages



Absence of color can indicate no flow in the vessel or that the
vessel is perpendicular to the beam
Linear-array presentation of color-Doppler information is
sometimes inadequate when the vessel runs parallel to the skin
surface. Phasing is used to steer each emitted pulse from the
array in a given direction away from perpendicular. All the color
pulses and color scan lines are steered at the same angle,
resulting in a parallelogram presentation of color-Doppler
information on the display
Angle is important with any Doppler technique. In convex and
linear arrays, pulses and scan lines travel in different directions
away from the transducer. They would have different Doppler
angles to the flow in a straight vessel.
Color Power Mode


has many names - color power Doppler,
ultrasound angio, color Doppler energy,
and color power angio
presents 2D Doppler information by colorencoding the strength of the Doppler shifts
power of the Doppler shifts is determined
by the concentration of moving scatterers
producing the Doppler shifts
Advantages






independent of Doppler shift frequency or angle
does not rely on velocities of reflectors
relies mainly on the amplitude strength of the
Doppler signal
is more sensitive to slow flow and flow in small
or deep vessels
is free of aliasing thus lower PRFs can be used
to detect slow flows
can determine the presence or absence of flow
Disadvantages

takes more time for image build up

greater susceptibility to motion artifacts flash artifacts

loss of information in direction, speed, &
flow character