Добавил:
Sekretar
kiopkiopkiop18@yandex.ru
t.me/Prokururor I Вовсе не секретарь, но почту проверяю
Опубликованный материал нарушает ваши авторские права? Сообщите нам.
Вуз:
Предмет:
Файл:Ординатура / Хирургия / Библиотека им академика М.И. Перельмана / Книга_5772_Библиотеки_им_академика_М_И_Перельмана.pdf
Physical and Technical Principles of Color Doppler Sonography
The function g′(t) is the complex conjugate of g(t). Both functions are superimposed and multiplied by each other, and their
product is integrated from –⬁ to +⬁.
This mathematical folding process yields the value of the
autocorrelation at time t. One of the first practical applications
of autocorrelation was in electrical engineering to filter out
very weak signals from noise and measure them. In ultrasound
likewise, autocorrelation can determine the mean frequency
shifts of extremely weak blood-flow echoes, whose amplitudes
are up to 40 dB fainter than B-mode echoes, against a background of noise. To increase sensitivity at low signal-to-noise
ratios, the measurement is repeated several times per scan
line. The resulting correlation vectors are added to give the
amplitude-weighted temporal mean value of the phase difference, from which the mean velocity can be computed. As a result, the time needed to generate a color image line is several
times that needed to produce a gray-scale line. When there is a
reasonably high signal-to-noise ratio as in cardiological studies, the number of samplings can be reduced to achieve a
higher frame rate. The velocities calculated by autocorrelation
are color-encoded and superimposed on the B-mode image
(Fig. 1.
13).
1
Other color-encoding techniques are autoregression, the
maximum entropy method (MEM), and the time-domain or
high-frequency cross-correlation technique mentioned earlier.
Autoregression. Autoregression (AR) techniques are a spin-off
of radar technology. Unlike autocorrelation, which analyzes
successive pulses at designated time intervals, the time intervals are either varied or, in the case of MEM, randomly select-
7
ed
. In practice, the theoretically higher accuracy and resolution of autoregression techniques have been offset by an unfavorable signal-to-noise ratio.
Cross-correlation. The cross-correlation technique compares
the high-frequency signal of a pair of echo wavetrains to determine the velocity-related position change of characteristic
echoes. An advantage of cross-correlation is its ability to detect
even high velocities
8
. The need to have a signal-to-noise ratio
higher than 6 dB has limited the routine abdominal use of this
method, which can be used only for superficial vessels and in
thin patients. Haerten
5
and Liu9have summarized the advan-
tages and disadvantages of the techniques.
Color encoding. It is customary to use a red/blue color encoding format in which the color value indicates flow direction
and the color brightness indicates flow velocity. There are color
encoding maps that are particularly well suited for analyzing
slow flows and providing good color fill. In some color-flow
maps, green pixels are added to signify variance in flow velocities, indicating turbulence.
12
Fig. 1.13 Longitudinal color duplex scan of the common carotid
artery. The white frame outlines the color window in which the mean
flow velocity is analyzed for each pixel and superimposed on the Bmode image in color-encoded form. The color scale on the left indicates the direction of color encoding (red = flow toward the transducer, blue =flow away from the transducer). Areas devoid of flow are
shown in gray scale. The advantage of color encoding is evident when
compared with the pureB-mode image just to theleft of the color window: The hypoechoic wall thickening, which is clearly visible in the
color duplex image, contrasts poorly with the clear lumen in the Bmode image.
Relevant system settings:
B-mode center frequency: 7.2MHz
Doppler frequency: 5.14 MHz
Pulse repetition frequency: 868 Hz
Wall filter: F5 (corresponds to 100 Hz)
Motion detector. In some systems the scan data pass through a
motion detector prior to autocorrelation in order to separate
the essentially stationary tissue echoes from vascular echoes in
the B-mode and color images and to eliminate motion artifacts.
This filter can accurately discriminate between color-flow and
gray-scale information in the presence of slow flows.
Time averaging (ensemble size). The accuracy of flow-data
sampling in color Doppler can be increased by placing a number of transmitted pulses (4–20) at the same site for each color
line and performing autocorrelation on all of the sequentially
acquired data. Usually the sonographer can set the “time average” to determine the number of samples that are acquired.
Time averaging can increase color sensitivity (improved
signal-to-noise ratio for color Doppler), but the higher number
of transmitted pulses per color line prolongs the sampling time
and reduces the frame rate.
Power Doppler Imaging (or Energy Mode)
Whereas color Doppler involves the direction-dependent detection and display of blood flow, power Doppler supplies information on the overall quantity of blood flow. The quantities
of the flow components calculated by the correlator are detected without regard for direction. They are squared, integrated over time, and displayed in a color-encoded format. Integrating all the signals significantly improves sensitivity,
making it possible to image not just the flow in major vascular
trunks but also tissue perfusion (Fig. 1.
Power Doppler is also called transparent energy mode
(TEM). This term indicates that power information is added to
the B-mode image in such a way that, as in color duplex sonography, it does not mask the B-mode information in areas
devoid of flow and leaves that information unchanged. This allows for better anatomical orientation.
14).
Fig. 1.14 Comparison of B-mode and power Doppler,
illustrated for renal perfusion.
kidney in longitudinal section. There is normal differen-
tiation of the parenchyma and central echo complex.
the branch points of the segmental and interlobar arteries could be identified, but only power Doppler can
demonstrate parenchymal perfusion. There is no evidence of a complete or segmental infarction.
The advantages of the older one-dimensional techniques are
their relatively low cost and availability. On the negative side,
their use is time-consuming, especially in abdominal studies,
and it is not possible to evaluate the perfusion of tissues and
tumors.
plex sonography are Doppler techniques, the color information
is angle-dependent, much like the information in spectral
Doppler analysis. For a constant flow velocity in a vessel, the
color shading and intensity can vary depending on the angle
between the Doppler beam and the vessel axis. For example,
when a straight, uniformly perfused vascular segment is
scanned with a convex transducer array, the varying angles at
which the beams intercept the vessel lead to a color reversal
accompanied by a (narrow) black area at the site where the
beam is perpendicular to the vessel axis (Fig. 1.
15 a).
Duplex and Color Doppler Sonography
Although the power mode is also a Doppler technique and
therefore must be angle-dependent in principle, a signal can
still be acquired even when the beam–vessel angle is 90⬚
(Fig. 1.
15 b). This is because the power spectrum has sufficient
bandwidth to detect and integrate all nonzero spectral components. There are always transducer segments that insonate
the pixel volume at an angle other than 90⬚ and can thus con-
tribute to the power Doppler image (see Fig. 1.
hand, color Doppler information represents only the mean
frequency, and so a 90⬚ incident beam does not yield a colorflow signal. As a result, power Doppler is largely independent
of the beam–vessel angle and is not affected by aliasing, since
the direction of the velocity does not enter into the calculation.
viewers, when the spokes in the wheels of a passing stagecoach
appear to rotate backward. Pulsed Doppler, with its pulse repetition frequency, is analogous to the frames of a movie camera:
With a time delay of T between frames, the viewer cannot tell
19). On the other
Fig. 1.15 Comparison of color duplex sonography and power Doppler: angle dependence.
color Doppler mode shows typical angle dependence of color encoding: blood on the left side is flowing away from the transducer and is
encoded red, while blood on the right side is flowing toward the transducer and is encoded in the “opposite” color, blue. The blood on the
far right appears light blue because the vessel is scanned at a small
Doppler angle relative to the transducer (“pseudojet”). Since the
beam-vessel angle is 90⬚ at the center of the vessel, no flow is de-
tectable and that area appears black (“pseudo-occlusion”).
13
entire vessel is uniformly filled with color.
Physical and Technical Principles of Color Doppler Sonography
whether the wheel has turned, say, 225⬚ forward or –135⬚
backward (Fig. 1.
16). In any sequence of images, the human
brain always interprets the motion as minimal. Thus it will interpret rotation as occurring in reverse if the amount of rotation between two images is greater than one-half the period of
the rotation, i.e., 180⬚. Similarly, when the FFT processor in an
ultrasound system calculates the Doppler frequencies from the
sampled values of the demodulated Doppler signals at the beat
frequency T = 1/PRF, it does so under the assumption of minimal frequency values. For example, if the actual frequency is
3
/2 PRF, it will be interpreted and displayed as an aliasing
frequency of
1
/2 PRF.
Nyquist limit. The highest frequency that can be detected
without aliasing, called the Nyquist limit, is equal to one-half
the PRF in both flow directions in directional Doppler
tion 9).
∆f
ⱕ1/2 PRF (Nyquist limit) (9)
max
1
(equa-
Baseline shift. When aliasing occurs in a Doppler spectrum,
positive frequencies above the Nyquist limit are displayed as
negative frequencies at the bottom of the spectral trace
(Fig. 1.
17). In color Doppler, this “wrap-around” effect appears
as a zone of bright color reversal. This can be corrected in superficial vessels by increasing the PRF (Fig. 1.
17, bottom) and in
deeper vessels by shifting the baseline downward or upward
(Fig. 1.
18). This can double the range of detectable velocities
both in the Doppler spectrum and in the color-flow image, provided the flow direction is not a concern. The maximum measurable velocity can be determined from the considerations on
one-dimensional techniques and from the Doppler formula.
The Doppler formula yields information on the maximum flow
velocity that can be detected without aliasing. This limiting
frequency is equal to the pulse repetition frequency PRF. The
14
a
b
1.5
1.0
0.5
0
–0.5
–1.0
–1.5
Fig. 1.16 Principle of aliasing.
a PW Doppler and the individual images in color duplex sonography
are analogous to the individual frames in a movie camera. If the sampling time (T) is too long, the viewer cannot tell whether the wheel has
rotated 245⬚ forward or 135⬚ backward. Since the brain always interprets motion in image sequences as being minimal, more than 180⬚ of
rotation (between two frames) will appear as reverse motion on the
film.
b Aliasing also occurs in the recording of Doppler spectra. When the
frequency to be measured (red curve) is sampled (arrows) at a rate of
less than one-half the pulse repetition frequency, the underlying
frequency cannot be unambiguously identified. The sampled points
trace out a curve of lower frequency (blue curve). This causes a “wrapping around” of the spectral trace in the Doppler spectrum, as high
positive frequencies that cannot be displayed above the baseline are
truncated and appear as negative frequencies at the bottom of the
spectrum. Aliasing in colorDoppler causes the flow velocitiesto be displayed in the opposite color. When the sampling rate is equal to at
least one-half the pulse repetition frequency (the Nyquist criterion),
the curve can be plotted at the correct frequencies.
T
225°
–135°
Fig. 1.17 Aliasing in color and spectral Doppler, before and after adjustment of the pulse repetition frequency (PRF).
a With a PRF of 2256 Hz, aliasing appears as a color reversal in the
color-flow image andas a wrap-around of the truncated spectralpeaks
in the Doppler spectrum.
b When the PRF is increased to more than 5000 Hz for color Doppler
or 4000 Hz for spectral Doppler, a normal, artifact-free color and spectral display of flow information is obtained.
Doppler parameters
Duplex and Color Doppler Sonography
∆f Mean Doppler frequency
σ2 Variance
P Power
A Doppler signal amplitude
a
Noise
–PRF/2
Angle dependence
Θ=90°
P=P1+P2>0
∆f =0
2
A
0
2
A
∆f ∼v· cos Θ
∆f ∼v· cos Θ
b
Θ≠90°
∆f PRF/2
Wall filter
2
σ
P
Physical and Technical Principles
c
Fig. 1.18 Aliasing due to incorrect placement of the spectral baseline.
a Severe aliasing caused by setting the baseline too high. Almost half
of the spectrum has been truncated and displayed below the baseline.
b After an initial baseline shift, most of the spectrum is already displayed normally.
c When the baseline is lowered further, almost theentire PRF is available for sampling, and the Doppler spectrum is free of artifacts. Reverse
flow components cannot be identified, however.
maximum detectable velocity, then, decreases with increasing
depth, and it increases as the transmission frequency is reduced.
Because all of the flow components are integrated over a
time interval, power Doppler is very sensitive in detecting perfusion but is also much more susceptible to relative movements of the organs or transducer and therefore requires considerable user skill.
Comparison of spectral Doppler, color Doppler, and power
Doppler. Therelationship among spectral Doppler, color Dopp-
ler, and power Doppler is reviewed in Fig. 1.
19. The figure
–PRF/2 0 ∆f PRF/2
Fig. 1.19 Doppler parameters in the amplitude–frequency curve. In
both diagrams, the squares of the amplitudes of the flow components
are plotted over the frequency axis.
a Doppler parameters in context. Color Doppler presents the mean
Doppler frequency (
f). The variance (
∆
2
σ
) is a measure of turbulence,
corresponding to the bandwidth of the spectrum. Power Doppler is
based on the area under the curve. Spectral Doppler displays all
frequency components over the time axis.
b Special case of a 90⬚ insonation angle. In color Doppler, a mean flow
velocity of zero is calculated in both directions from the point where
the beam is perpendicular. In power Doppler, on the other hand, the
various directional components are added together, still allowing flow
detection to occur.
shows the amplitudes (squared) of all the Doppler frequencies
that can be measured for each sample volume and time inter-
val:
➤
Spectral Doppler displays all of the amplitudes in the range
gate in a time-varying, brightness-modulated spectral trace.
➤
In color Doppler, only the mean frequency shift ∆f = kx cos Θ
at each point in the image is color-encoded. The variance is a
measure of the bandwidth of the Doppler spectrum, i.e., how
diverse the flow velocities are at the sampling site.
➤
Power Doppler encodes each pixel with the sum of all amplitudes, i.e., the area under the curve (amplitudes squared)
averaged over time.
15
Physical and Technical Principles of Color Doppler Sonography
Technical Aspects of the Examination and
Equipment Settings
Numerous technical aspects of the ultrasound examination
and system settings must be considered in the one- and twodimensional Doppler techniques in order to optimize the examination and avoid diagnostic errors.
Insonation angle. The most important scan parameter is the
angle between the Doppler beam and the vessel axis. Because
the cosine of this angle is used in the Doppler formula, the insonation angle should be kept well below 90⬚ during the ex-
amination. When the angle is perpendicular, a Doppler signal
is not recorded. To determine the velocity v from the Doppler
frequency, it is necessary to measure the angle in the B-mode
image and perform an “angle correction.” The accuracy of the
angle correction depends greatly on the size of the beam – vessel angle. Table 1.
velocity is minor at small angles but becomes substantial at
large angles. An accurate calculation of flow velocity is possible
only when the beam – vessel angle is small (see Analysis of
Doppler Information and Artifacts, p. 19).
1
Transmission frequency. Selection of the transmission
frequency for flow detection depends on the sampling depth
and the anticipated velocity. As described under Penetration
Depth (p.5), high ultrasound frequencies are strongly attenuated with depth. This is particularly true of the Doppler
signal, which is considerably weaker than the intensity of the
echoes reflected from vessel walls and organs. At the same
time, there are conditional upper and lower limits to the detectability of Doppler frequencies for technical reasons. Because the Doppler shift frequency is directly proportional to
the velocity to be measured, it is advantageous to use a low
transmission frequency for the measurement of high velocities. By the same token, a high transmission frequency should
be used for low velocities.
Power output and receiver gain (Doppler or color gain). These
settings should be adjusted so that color completely fills the
vessel lumen with no extraluminal “blooming” and the displayed Doppler spectrum is free of noise and superimposed
signals.
2 shows that the effect on the angle-corrected
Velocity scale. In both spectral and color Doppler imaging, the
scan frequency or velocity scale should be set for the anticipated range of velocities. Setting the scale too high makes it
more difficult to detect and analyze the signals; venous signals
in particular may go undetected. If the flow velocity in the vessel exceeds the maximum value set on the velocity scale (PRF)
in one flow direction, this higher velocity will be misinterpreted as a high velocity in the opposite direction. One possible
remedy is to increase the PRF. If this is no longer possible, the
beam–vessel angle can be deliberately increased, or the scanning frequency may be decreased as described above. When
aliasing occurs in the color-flow image, the color changes are
alwaysin the range of higher velocities (e.g., white or yellow).If
the flow direction is actually changed owing to turbulence or
flow reversal, the color changes run through black (Fig. 1.
Power Doppler can mask these phenomena, which are often
helpful diagnostically. On the other hand, the angiogram-like
appearance of the vessels in power mode can aid in the detection of vascular lesions (Fig. 1.
Wall filter and motion-artifact filter. These filters suppress
troublesome pulsations and flash artifacts, but they can also
filter out diagnostically relevant low frequencies causing slow
flows to be missed (Fig. 1.
priority (color balance). If the priority is set too high in the Bmode image, flow may not be detected (see Artifacts and
Pitfalls, p. 20).
Time averaging, color box, line density. Parameters such as
time averaging, which increases sensitivity, and the line density reduce the frame rate. Tokeep the color frame rate in an acceptable range, it is good practice to limit the color-flow information to a color box, which should be of minimal size in its
lateral extent and depth. In some examinations, such as fetal
heart imaging, it is also helpful to reduce the color line density
(at the cost of decreased color spatial resolution) in orderto ensure adequate temporal resolution for cardiac imaging.
Another way to increase the color frame rate is to use a system that offers parallel processing of the echo signals from a
transmitted pulse (see New Technical Processes and Approaches, p. 24). With this type of system, the information
from multiple adjacent scan lines can be processed simultaneously.
Recommendations on key equipment settings for CW, PW,
and color Doppler examinations are summarized in Table 1.
20).
21). The same applies to color-write
20).
3.
16
Table 1.2 Effect of beam-vessel angle on angle correction errors
Beam-vessel angle Correction factor
Θ)
(1/cos
30⬚ 1.15 ⫾ 3%
45⬚ 1.41 ⫾ 6%
60⬚ 2.00 ⫾ 9%
72⬚ 3.24 ⫾ 15 %
75⬚ 3.86 ⫾ 21 %
80⬚ 5.76 ⫾ 30%
Angle correction errors are calculated by assumption of a detection accuracy
of ⫾ 3%
Correction error
Duplex and Color Doppler Sonography
a
Fig. 1.20 Comparison of turbulence and aliasing in color duplex and
power Doppler imaging.
a Color duplex image of a vascular bifurcation shows local wall thickening and zones of turbulence. Because the lower vessel branches off
at an angle from the main vessel, the change of angle between the
color box and vessel axis causes apparent flow acceleration in the
lower vessel. Color aliasing is recognized by noting that the color
a
b
change is in the range of the brighter colors on the velocity scale (e.g.,
white or yellow). With a true change in flow direction due to turbulence or flow reversal, the color would by running through black.
b Power Doppler shows a homogeneous vessel lumen in which local
wall thickening at the origin of the upper vessel appears dark. Turbulence and aliasing are not visualized, but the local wall lesion is
clearly appreciated in the angiogram-like view.
b
Fig. 1.21 Effect of wall filter setting on frequency detection in spec-
tral Doppler.
a Normal spectrum of an artery with high residual diastolic flow and
low resistance in the distal vascular bed. Wall filter is set at 25 Hz.
b When the wall filter is increased to 225 Hz, velocities in the range
from 0 to10 cm/s arenot displayed (angle correction from the B-mode
image is constant for all settings in a –c).
c When the wall filter is further increased to 400 Hz, velocities in the
range from 0 to 20 cm/s are lost, creating an apparent spectral pattern
of absent end-diastolic flow. Tumor and tissue perfusion and slow
venous flows may go undetected.
Physical and Technical Principles
c
17
Physical and Technical Principles of Color Doppler Sonography
Table 1.3 Recommended system settings for CW, PW and color Doppler examinations
Settings CW PW Color
Doppler
Transmit power, transmit intensity + + + The necessary transmit power depends on the examination conditions
Receiver gain + + + Set the gain to produce a spectrum with minimal noise and no superim-
Pulse repetition frequency (PRF),
velocity scale
Baseline setting + + + Adjust so that the range of measurable frequencies is optimally utilized.
Wall filter and flash-artifact filter + + + Troublesome wall pulsations and motion artifacts are reduced, but low
+ + + Set the PRF to fully utilize the range of measurable frequencies. A PRF of
1
Color priority – – + Postprocessing parameter that controls the balance between gray-scale
Settings, problems, and optimization
(sound absorption) and on depth. The power setting for obstetric examinations should be as low as possible.
posed signals or a color-flow image in which a normal vascular segment
is completely filled with color and there is no color blooming or superimposed color noise.
1000Hz is recommended for veins, 1500–3000Hz for arteries. May have
to increase the PRF if aliasing occurs, or may have to selectively increase
the Doppler angle for deep abdominal scanning. In some cases it may
also be necessary to reduce the scanning frequency (see equation 6).
Shifting the baseline is helpful if aliasing occurs.
velocities may not be detected. Filter setting of 50–200 Hz is recom-
mended for arteries, ⬍100Hz for veins. Use of other filters such as mo-
tion and flash-artifact filters can reduce sensitivity, especially for the detection of slow flows.
echoes and color pixels. If the color priority is set too low, it can suppress
flow signals in the B-mode image. To search for thrombosis or reduce
noise, it should be set for slight gray-scale priority.
18
Line density for B-mode/color flow
(synonym: spatial resolution)
Temporal resolution – – + Sets the number of transmitted pulses used to compute an ultrasound
Correlation (persistence) – – + Affects the composition of color information by the weighted addition of
Spatial averaging – – + One- or two-dimensional filtering of the color image based on the sur-
– – + Improves the lateral resolution of the B-mode or color image at the cost
of a reduced frame rate. If fast motion (e.g., fetal heart) or high velocities
are to be detected, the line density should be reduced.
color line or the number of acoustic lines used in autocorrelation. Increasing temporal resolution improves the signal-to-noise ratio and sensi-
tivity. The disadvantage is a reduced frame rate.
old and new information. The higher the number, the more old informa-
tion is retained. Time averaging the color pixel values helps to image all
flow in the vessel lumen but slows the color presentation. The disadvantage of a high setting is less appreciation of temporal dynamics.
rounding color pixels or color information. Reduces color noise without
affecting the frame rate, but can lead to nonvisualization of very small
vessels.
tain a clear spectral window below the trace. For two-dimen-
Analysis of Doppler Information and Artifacts
sional color flow imaging, the PRF, gain, and any filter settings
should first be adjusted in a normal vascular segment so that
Optimizing the Examination and Analyzing the
Doppler Information
Settings before the start of the examination. After selecting
the proper transducer for the desired information and the
color uniformly fills the vessel lumen. We recommend the following technique. After identifying the vessel and angling the
color box in proper relation to the vessel axis, adjust the power
setting, PRF, and gain until a noisy color image is obtained.
Then lower the gain setting until the color noise is gone.
proper frequency for Doppler interrogation, the examiner
should adjust and optimize the settings listed in Table 1.
obtain a well-defined spectrum that is free of noise and aliasing (Fig. 1.
22). For examinations of the fetal aorta and maternal
arteries, the power and gain settings should be reduced to ob-
3 to
Information. Information on flow obstructions at the sampling
site itself and on conditions in the proximal and distal vascular
beds can be derived from the signal frequencies, which are a
measure of flow velocities; from the amplitudes, which are
Duplex and Color Doppler Sonography
a
Fig. 1.22 Dependence of the Doppler spectrum on resistance in the
distal vascular bed.
a Spectrum of an artery with high resistance in the distal vascular bed.
This resistance leads to a rapid decline of flow after systole, with a
reverse flow component appearing in diastole. All the red blood cells
move at approximately the same velocity during systole, creating a
narrow frequency band with a clear “sonic window.“
b Spectrum of an artery with low resistance in the distal vascular bed,
with persistent residual diastolic flow.
c Venous signal from the inferior vena cava. Unlike the bandlike spec-
trum of peripheral veins or the portal vein, the superior and inferior
vena cava show cardiac modulation of their flow, which can easily be
confused with an arterial flow pattern.
proportional to the number of moving blood cells; and from
the time changes over each cardiac cycle. Color Doppler imaging and spectral analysis are considered to be complementary
in the evaluation and interpretation of Doppler information.
When applied together, both color and spectral Doppler can be
a valuable source of qualitative, semiquantitative, and quantitative information.
b
Physical and Technical Principles
c
Analysis of the Doppler Spectrum
Waveform. The Doppler spectrum can be described in terms of
the spectral bandwidth and the shape of the waveform (en-
velope curve), which is basically characterized by its steepness,
peak systolic velocity,and end-diastolic velocity. Moving left to
right, the trace begins its upstroke at the start of systole and
concludes at end diastole. One or two cardiac cycles are always
evaluated for spectral analysis. The shape of the spectrum depends on local findings and on the organ that is supplied by the
sampled vessel (Figs. 1.
Spectral bandwidth. If all of the blood cells are moving at the
same velocity, the spectrum displays a narrow frequency band
with a typical clear “sonic window” below the trace (Fig. 1.
This is a normal finding in early systole for all larger vessels.
Vessel wall irregularities produce turbulence, causing initial
spectral widening or f illing-in of the spectral window. Starting
at about 50% stenosis, a jet develops owing to local flow acceleration. With proper angle correction, this acceleration can be
identified in the Doppler spectrum.
Pulsatility index (PI). Several techniques have been devised for
analyzing the spectral Doppler waveform. Gosling
first to describe an index for analyzing pulsatility. This is a
22,1.23).
4
was the
22).
Fig. 1.23 Doppler spectrum with parameters for spectral analysis
(S/D, RI, PI). This spectrum is from a vessel with a low-resistance distal
bed. After the operator manually marks the starting and end points of
two cardiac cycles, most scanners will automatically calculate the key
flow parameters and Doppler indices. While the velocity measure-
ments are angle-dependent and require corresponding angle correc-
tion in the B-mode image, all of the indices listed below are independent of the beam anglebecause in each case the correction factor cancels out of the equation.
S: peak systolic velocity (here: 55.3 cm/s)
D: end-diastolic velocity (here: 32.3 cm/s)
Tav: time-average peak velocity (here: 39.4 cm/s)
Pulsatility index PI = (S–D)/TAV (here: 0.59)
Resistance index RI = (S–D)/S (here: 0.42)
Stuart index S/D (here: 1.72)
Maulik D/TAV
19
Physical and Technical Principles of Color Doppler Sonography
measure of the difference in flow between systole and diastole during the cycle; it was first determined by calculation
from the Fourier analysis. A simplified version is the Pulsatility index (PI). This is calculated by dividing the difference between the peak systolic and end-diastolic frequencies (D) by
the time average of the maximum frequency shift (TAV)
(Fig. 1.
23).
Resistance index (RI). Pourcelot introduced the resistance
index, an angle-independent measure of pulsatility in which S
describes the peak systolic frequency and D the end-diastolic
frequency. Many sonographers use simplified indices like that
of Stuart and Drumm
frequency S and end-diastolic frequency D. Maulik proposed
the ratio D/TAV, in which the maximum end-diastolic
frequency is normalized to the mean Doppler shift frequency
of the outer envelope curve.
S/D ratio, A/B ratio. Other parameters for waveform analysis
were introduced in 1983 by Campbell et al.
17
al.
. These were based on curve fitting and on descriptions of
steepness, maximum frequency, and other defined slopes. But
from the many indices that have been devised for semiquanti-
1
tative analysis,only the S/D ratio (also called also the A/B ratio),
the resistance index (RI), and the pulsatility index (PI) have become widely used. The formula RI = 1 – 1/(S/D) is used to convert between the RI and S/D ratio.
15
, which is the ratio of the peak systolic
2
and Thompson et
and can be confirmed and described semiquantitatively by analyzing the Doppler spectrum. Apparent changes are caused by
changes of insonation angles within a vessel (Figs. 1.
and by the occurrence of aliasing when the PRF is set too low.
Other pathological changes are seen in association with malformations, stenoses, and occlusions.
Stenoses. The oldest indication for Doppler scanning was the
detection of stenoses. These sites are recognized in color duplex sonography by typical local flow acceleration, which is
usually manifested by a bright color jet or aliasing (Fig. 1.
Measurements of maximum flow velocity help to confirm and
quantitate the stenosis. Velocities of 140–180 cm/s are suspicious, while higher values confirm that a stenosis is present.
Additional criteria include eddy flow located within and just
past the stenosis and poststenotic dilatation, which is often
visible in the B-mode image. Other, indirect criteria are the
presence of distal poststenotic damping and increased prestenotic resistance, which causes a “flickering” of the color
pixels. Poststenotic damping causes a loss of pulsatility in the
color image and decreased brightness modulation in the spectrum.
Whereas the diagnosis of an intra-abdominal stenosis is
generally based purely on hemodynamic criteria in the colorflow image, the color duplex scanning of superficial vessels
with a high-resolution transducer can additionally define the
morphology of the stenotic lesions (Fig. 1.
25).
15,1.24)
25).
20
Quantitative information. Measurements of the Doppler shift
frequencies with angle correction supply quantitative information on flow velocity and also on volume flow when the
cross-sectional area is considered. Owing to errors in angle determination, the effect of filters, inaccuracies in estimating the
true center frequency, and problems in determining the exact
vascular cross section used in calculations, the importance of
volume measurements has dwindled in favor of angle-independent semiquantitative parameters (PI, RI, S/D).
Analysis of Color Information
While the Doppler spectrum portrays the time course of flow
at a selected sampling site, the color information supplied by
two-dimensional Doppler techniques describes the flow conditions that prevail within a volume element at a given point in
time. Color changes, like spectral shape, are influenced by
physiological and pathophysiological states in the sampled
vessel and in the proximal and distal vascular beds. Besides
qualitative information (flow: yes/no), color image analysis focuses on the time course of color saturation and the color distribution:
Reverse flow components. Physiological changes in flow and
flow direction occur at vessel branch points such as the carotid
bifurcation, consisting of flow reversal or a reverse flow component in high-resistance vessels such as the aorta and the iliac
arteries (Fig. 1.
bral artery and umbilical artery (in the second and third
trimesters) show unidirectional flow that persists even in diastole, appearing as slightly darker residual flow. The presence of
a reverse flow component in these vessels is a pathological sign
24). Low-resistance vessels like the middle cere-
Thromboses and arteriovenous (AV) fistulae. Color Doppler
may also demonstrate voids in the color-filled lumen signifying thrombosis or a coarse, mosaic-like vibration artifact
(“confetti sign”). This may be an accompanying feature of a
high-grade stenosis, or it may signify an AV fistula (Fig. 1.
The lesions are differentiated by performing a spectral analysis
and checking for secondary fistula signs such as increased flow
in the feeding artery and/or an “arterialized” spectrum of the
draining vein.
Tumors. Thedetection of blood flow aids in identifying tumors,
and the specific blood flow pattern can advance the differential
diagnosis. Newer approaches, such as calculating the inflow
rate of ultrasound contrast medium in tumors or in the kidney,
can furnish quantitative information on tissue perfusion and
renal function.
26).
Artifacts and Pitfalls
Artifacts and possible diagnostic errors can result from improper settings or from unexpected physical or equipment
limitations. Some artifacts cause loss of sensitivity, while
others give a false-positive indication of flow.
Absent (False-Negative) Flow Detection
Besides the power and gain settings, the detection of flow depends on the wall-filter and motion artifact-filter settings
(Fig. 1.
21) and on the selected PRF. Other system-specific pa-
rameters are often underestimated in their significance. These
include the color priority, which controls the representation of
color in the B-mode image. Setting the color priority too low
Duplex and Color Doppler Sonography
ab
Fig. 1.24 Difference between physiological flow reversal, reverse
flow component, and aliasing.
a Physiological flow reversal in the carotid sinus. Zones of eddy flowat
the carotid bifurcation are encoded in blue.
b Reverse flow component in a vessel with high diastolic resistance.
The cranially directed flow is encoded in red. The reverse flow component, encoded in blue, is bordered by a dark zone representing the
zero baseline (compare with spectrum in Fig. 1.22 a).
c Aliasing due to apparent flow acceleration (“pseudojet”). The cause
is the smaller Doppler angle that results from the curvature of the vessel. When angle correction was applied, a normal flow velocity was de-
tected in all segments.
Physical and Technical Principles
a
Fig. 1.25 Color duplex images of stenoses in the neck and in abdominal arteries.
a Longitudinal scan through the liver and aorta shows a high-grade
stenosis of the celiac trunkwith aliasing and moderate poststenotic dilatation. The aorta and the origin of the superior mesenteric artery appear normal.
b Transverse scan through the vena cava (encoded blue), aorta, brachiocephalic trunk, splenic artery, and occluded common hepatic
b
c
c
artery. The insonation angle is large, but scans at all angles confirmed
the absence of flow signals.
c Seventy-percent stenosis at the origin of the internal carotid artery,
with a short jet phenomenon. Angle-corrected Doppler measurements showed arise in the peak frequency tomore than 400 cm/s. The
cause is a tubular wall thickening 1.5 cm long with a proximal filiform
constriction.
21