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1 Physical and Technical Fundamentals of Ultrasound and Doppler Ultrasound
1
Margin Shadow
Margin shadows are often attributed to refraction. As
shown in Figure
impinging on the lateral margins of round, fluid-filled
spaces are diverted into the surrounding tissue. This
phenomenon also explains why some marginal structures such as fetal skulls are not fully displayed.
1.22, mainly parallel ultrasound rays
Side Lobe
The actual sound field does not correspond to our
simplified imaginary model of a thin, focused ultrasonic beam. Rather, in the near field it forms so-called
side lobes. Should a side lobe meet a strong reflection,
this will return echoes in the direction of the side lobe.
Since, however, a received echo is always interpreted
as one returning vertically to the transducer, it is mis-
1.23). In modern ultrasound systems
interpreted (
complex runtime calculations and suppression of
echoes entering laterally can reduce some of the side
lobe artifacts.
Fig.
Slice Thickness Artifact
A slice thickness artifact comes about because of the
finite penetration of an ultrasound beam, i. e., the
“thickness” of the sound field. In an ideal case the
sound field would be infinitely thin, but in reality it extends over one to several millimeters (depending on
Figure
the frequency).
pinging on a cyst, in one instance centrally, in the other
at its margin. The depth of penetration of a sound field
can be reduced by focusing through an acoustic lens.
1.24 shows a sound beam im-
Repetition Artifact
This artifact, also known as reverberation artifact, occurs when ultrasound waves impinge on a strong reflector with a large surface in the near field. Some of
the sound waves arriving there are reflected, return as
echoes to the probe, where they are received. However,
it is possible for some of the echoes to be reflected back
from the probe into the tissues. In this way some of the
sound waves can oscillate back and forth between the
14
Fig. 1.22 Margin shadow. Mode of generation (left) and
simplified diagram of the ultrasound display (right).
Fig. 1.24 Slice thickness artifact. Fig. 1.25 Repetition artifact. Mode of generation (left) and
Fig. 1.23 Side lobe artifact. Mode of generation (left) and
simplified diagram of the faulty ultrasound display (right).
simplified diagram of the ultrasound display (right).
Doppler Sonography
probe and the reflector. Hence the same structure will
be displayed several times at several equidistant
Fig.
depths, with diminishing brightness (
verberation phenomenon is most often seen when interfaces close to the transducer present a large difference in impedance (soft tissue/air).
1.25). The re-
Doppler Sonography
Fundamentals of Doppler Sonography
The Doppler effect is named after Christian Johann
Doppler, who first described it in 1842. A change in distance between an observer and a sound source
changes the observed frequency. If, for instance, we
stand on the sidewalk and, for example, an ambulance
passes sounding its siren, we note that the tone of the
Fig.
siren changes abruptly (
changes is the pitch, not the intensity of the note.
While the vehicle is approaching the intensity increases, but diminishes again once the vehicle has
passed. By contrast, the pitch remains constant as the
vehicle approaches. This remains true as the vehicle
moves away, but the pitch is now lower. If the vehicle
moves at a speed of 60 mph (100 km/h), the difference
1.26). Specifically, what
Geometrical Distortion
When an ultrasonic image is generated from received
echoes, the assumption is that the ultrasonic wave has
a constant velocity and spreads in a straight line. In
fact, however, ultrasound moves at different velocities
in different tissues. The sound ray is also deflected on
its path through the tissues by refraction and diffraction. All these deviations are not accounted for in the
construction of the image. Hence the display may incorporate small geometric errors.
is two whole notes. Of course, in reality the siren emits
a constant pitch. The frequency heard by our ear,
however, depends on whether the vehicle is moving
toward or away from us. The same is true if we are in a
vehicle moving past a stationary ambulance (sender
stationary, receiver in motion) or if both vehicles are
moving (sender and receiver move relative to each
other). When receiver and sound source move toward
each other, the frequency picked up by the receiver is
higher than that sent by the sound source. Correspondingly, the frequency picked up by the receiver is lower
than that emitted by the sound source when receiver
and emitter are moving apart. The difference between
the emitted and received frequency is called the Doppler frequency, or Doppler shift.
In diagnostic medical procedures the Doppler shift is
utilized to measure the velocity of blood flow in blood
Basic Concepts
Fig. 1. 26 Doppler effect. The
frequency heard by the ear depends
on whether a noise is moving
toward or away from the observer.
15
1 Physical and Technical Fundamentals of Ultrasound and Doppler Ultrasound
16
1
vessels, as first described by Satumora in 1959. The
emitted sonic pulse is reflected from the moving blood
corpuscles. The frequency of the echo received by the
transducer differs from the ultrasound pulse it originally emitted. In this case the sender and receiver are
fixed and the frequency shift is due to the movement of
a reflector (erythrocyte). The frequency therefore
shifts twice: once when the ultrasonic pulse strikes the
reflector, and again when the echo with its already
shifted frequency returns. The frequency shift also depends on the angle between the axis of the blood ves-
1.27). This angle is
F
sel and the ultrasound beam (
ig.
known as the Doppler angle.
Mathematically the Doppler formula can be written:
∆f=2
f
v cos α
c
where: ∆f = the Doppler shift frequency
f = transmitting frequency of the trans-
ducer
c = velocity of sound
v = blood velocity
α = angle between the axis of the vessel and
the sound beam
If this formula is solved for v, the velocity of the erythrocytes can be calculated:
v=∆f
c
2 f cos α
The Doppler frequency lies in the range of audible
frequencies.
Table 1.3 shows how the measured Doppler
frequency depends on the Doppler angle. The calculations are based on the assumption that the transmitting frequency is 6 MHz and the blood flow velocity 1
−1
, using the above Doppler formula.
ms
1.3 shows that at a 90° angle there is no Dop-
Table
pler shift, since the distance between erythrocyte and
α
V
Fig. 1.27 Doppler angle. The angle between vessel and ultrasound beam (v = blood flow velocity; α = angle between axis of
vessel and incident beam).
Table 1.3 Relationship between Doppler frequency and Doppler angle
α cos α∆f
0° 1 7.79 0
30° 0.866 6.75 13
45° 0.707 5.51 29
60° 0.5 3.9 50
90° 0 0 100
Table 1.4 Correction factors and correction errors for different
Doppler angles
Angle α Correction factor
1/cos α
30° 1.15 앧 3%
45° 1.41 앧 6%
60° 2.00 앧 9%
70° 2.92 앧 14 %
75° 3.86 앧 21 %
80° 5.76 앧 30 %
MHz
Deviation (%)
Correction error
transducer remains constant, i. e., sender and receiver
move neither away from nor toward each other. Thus,
what is measured is always the velocity vector parallel
to the sound beam. The precise blood flow velocity in
vessels running at an angle to the transducer can be
calculated by determining the Doppler angle. This calculation is often termed angle correction. The Doppler
angle can be measured by a measuring axis, which in
B-mode is often aligned with the axis of the vessel. The
ultrasound system then measures the angle between
the axis of the vessel and the incident sound beam. The
precise Doppler frequency is calculated, taking into account an angle correction factor of 1/cos α. Table
1.4
contains a list of the various correction factors and correction errors that may result from lack of precision in
positioning the measuring axis.
As can be seen in
Table 1.4, as the angle increases the
correction error also increases. Hence during the examination the Doppler angle must be kept as acute as
possible by adjusting the tilt of the transducer.
The frequency of the altered echo received is then
compared to the original frequency of the transmitted
ultrasound pulse to determine the Doppler frequency.
In the ultrasound system the apparatus used for this is
a demodulator. The demodulator delivers electrical
signals that can be transmitted, for example, directly to
a loudspeaker. In order also to determine the direction
of blood flow, phase relationships are compared. Blood
flow toward the transducer is defined as positive, away
from the transducer as negative.
There is no predominant blood flow inside a blood
vessel. Rather, areas of different velocities form. Flow
rates close to the vessel wall are slower than flow rates
in the vessel’s axis, because of friction between the erythrocytes and the wall. Thus, a laminated flow profile
is formed. These profiles change over time, because of
flow pulsatility, vessel wall elasticity, bends in the ves-
sels, vascular branching, and vascular stenoses. Signals
derived from a blood vessel therefore consist not of a
single Doppler frequency, but of a mixture of frequencies. This frequency mixture can be reduced to its basic
oscillations by the use of the mathematical process of
fast Fourier transform (FFT). The principle of FFT is il-
Figure
lustrated by means of a simple example in
The velocity distribution of the erythrocytes is repre-
sented by a tracing of their spectra on a time axis
(Fig. 1.29). Corresponding to their magnitude positive
frequency shifts are represented above the time axis,
negative below. The rate of occurrence of individual
frequencies is coded by the brightness of each point in
the image.
1.28.
Continuous Wave Doppler Systems
Doppler Sonography
F
F
T
Continuous wave (CW) Doppler systems transmit and
receive sound waves continuously. This requires at
least two elements: one for continuing transmission
Fig.
and one for constant receiving (
systems may be conceptualized as directional and
nondirectional systems. Nondirectional systems cannot distinguish positive and negative flow directions.
Directional systems, on the other hand, can detect flow
direction by determining the phase shift. Since ultrasonic pulses are being transmitted and received continuously, it is not possible to measure the time of
flight between a transmitted pulse and a received echo.
Thus, the depth from which the signals are received
cannot be determined for any scan. It follows that all
vessels on which the ultrasound beam impinges are
scanned, and vessels running together in different
planes are combined in a single display. Thus, it is not
possible to distinguish the individual vessels. The
depth of penetration is determined by the frequency.
CW Dopplers are especially suited for the examination
of vessels close to the surface.
1.30). CW Doppler
Pulsed Wave Doppler systems
The action of pulsed wave (PW) Doppler systems is
similar to B-mode transducers (Fig.
sound pulse is transmitted by an element and the
echoes are received by the same element. Since the
velocity of the sound beam is known, the signals can be
localized precisely by determining the time of flight.
Thus, the ultrasound system first transmits a brief ultrasound pulse. The system is then switched off, i. e., all
echoes entering subsequently are ignored. After a predetermined time the system is switched to receive, and
the echoes are processed. The duration of the receiving
time defines a measured space. As a rule this measured
space corresponds to the diameter of the vessel. It is
called the sample volume, or Doppler window. When
1.31). A brief ultra-
Fig. 1.28 The mathematical procedure of FFT breaks down the
frequency derived from the blood vessel into its component
oscillations.
Fig. 1.29 The different velocities of the erythrocytes are displayed as a tracing of spectra along a time axis.
Transmitting Receiving
V
Fig. 1.30 CW Doppler systems. At least two elements are required to allow continuous transmission and reception
(v = blood flow velocity).
the receiving period is complete, the system again
sends an ultrasonic pulse. The number of ultrasound
pulses transmitted per second is called the pulse repetition frequency (PRF). The maximal transmitted PRF
Basic Concepts
17
1
1 Physical and Technical Fundamentals of Ultrasound and Doppler Ultrasound
Alias Phenomenon in Pulsed Doppler
Since reception in a pulsed Doppler is not continuous
as in a CW Doppler, the pulsed Doppler does not have a
Sample volume
V
Fig. 1.31 PW Doppler systems. Display of the sample volume
or Doppler window, corresponding as a rule to the diameter of
the vessel.
T2
T1
Fig. 1.32 Oscillations with a frequency of 3 Hz being scanned
at points T1 and T2, i. e., with a frequency of 2 Hz.
1s
constant Doppler signal. Rather, the Doppler signal is
composed of a series of samples. The temporal resolution is determined by the pulse interval, which is the
inverse of the PRF. The actual frequency shift is therefore unknown, and the curve between individual
samples is interpolated. The maximal frequency that
can be measured correctly is smaller than half the PRF.
Higher frequencies contain too few samples to be displayed accurately. The PRF must therefore be at least
twice the Doppler frequency being measured. Figure
1.32 shows an oscillation with a frequency f. If we as-
sume that the segment of the curve shown corresponds to one second, the oscillation has a frequency
of 3 Hz. Suppose the signal is scanned at points T1 and
T2. The oscillation is scanned twice in one second, i. e.,
the scanning frequency is 2 Hz. The signal constructed
from the two sample measures has a frequency of
쏝1 Hz (Fig. 1.33). To reconstruct the true frequency we
would have to measure with a scan frequency of at
t
least 6 Hz in this example. The maximal frequency that
can be displayed without error is called the Nyquist
limit. If the Nyquist limit is exceeded, the result is an
aliasing (Latin: alias = otherwise, at another time) phenomenon. This effect can be observed on a rotating
wagon wheel. When the wagon begins to move, the
direction of rotation of the wheel is seen correctly. As
the wagon accelerates, the wheel’s motion is seen correctly up to a certain maximal speed. As the speed continues to increase, the motion is suddenly seen as
backward motion. As shown in
phenomenon can be seen in spectrum analysis when
the Nyquist limit is exceeded. The maximal Doppler
frequency is reflected in the opposite direction.
1.34, the aliasing
Figure
18
T2
T1
Fig. 1.33 Signal with a frequency of 쏝1 Hz, reconstructed
from the two measurements T1 and T2 in Fig. 1.32.
depends on the depth of penetration: The deeper the
sample volume lies in the tissue, the longer is the wait
for the returning echo, i. e., the greater is the interval
between successive pulses. Thus, the maximal PRF
diminishes with deeper sample volumes.
1s
t
Baseline Shift
The PRF cannot be raised indefinitely if the Doppler
signals are to provide accurate localization. A limited
remedy is offered by the baseline shift. The Nyquist
limit is divided equally between a positive and a negative component. By shifting the reference axis (base
line) for the direction of flow upward or downward,
the range of the display can be extended in one direction. Such an extension can reach twice the Nyquist
limit. However, during this process the opposite flow
cannot be displayed.
shifting of the baseline during the recording of a Doppler curve.
Figure
1.35 shows a stepwise
Wall Filter
A wall filter is designed to eliminate low frequency
noise artifacts and movement of vessel walls from the
frequency spectrum. It is conceived as a high pass filter
(HPF), i.e., high frequencies can pass the filter, low
frequencies are filtered out and rejected. The setting of
the wall filter is variable and can be changed by the
user. The wall filter determines the minimal frequency
range that can be displayed.
Color-Coded Doppler Sonography
As described in the section on PW Doppler, a pulsed
Doppler system can derive Doppler signals with a defined sample volume from a precisely defined location.
In order to obtain a flow velocity analysis from several
locations simultaneously, many sample volumes are
arranged along adjacent ultrasound lines of sight
ig.
F
1.36). This results in an analysis of flow velocity
(
over a defined area, called a region of interest (ROI). In
this situation a simultaneous spectrum analysis of
each sample volume by FFT would require too much
time and therefore cannot be applied. Moreover, the
Doppler spectra obtained could not all be displayed
simultaneously. Scanning with 20 lines of sight each
with 50 sample volumes would after all encompass
1000 samples! In color-coded Doppler sonography
readings are therefore obtained not by FFT but, among
other procedures, mostly by means of an autocorrelation procedure. In this procedure the Doppler signals
for all the sample volumes in the time of one scan are
collected and compared (correlated) with those of the
following scan time. While 128 scan points are required to analyze a signal by FFT, just four suffice for an
autocorrelation procedure. The information about the
phase difference so obtained is a direct measure of the
mean velocity distribution in a sample volume. The results of the scan are displayed in color. In B-mode flow
toward the transducer is displayed in red, away from
the transducer in blue. The velocity of flow is indicated
by degrees of brightness in the colored pixels. A bright
pixel indicates high velocity, a dark pixel slow blood
flow. Color-coded Doppler sonography follows the
same rules as PW Doppler. It is dependent on the Doppler angle and shows aliasing. Aliasing is clearly shown
in the color display by color reversal.
Doppler Sonography
Fig. 1.34 Aliasing in a spectral analysis. When the Nyquist limit
has been exceeded the maximal Doppler frequency is recorded
in the opposite direction.
Basic Concepts
Fig. 1.35 Stepwise shift of the baseline during a Doppler recording.
Fig. 1.36 ROI.. A number of sample volumes are constructed
along beams with their axes lying side by side to obtain a flow
analysis measured at several locations simultaneously.
Amplitude-Coded Flow Display
Beside color-coding described above, an amplitudecoded flow display has become available in the last few
years. This principle was first described in the literature in 1994 and is available for modern ultrasound
systems under various names, depending on the manufacturer. Amplitude-coded procedures are variously
known as (among others) ultrasound angiography,
power Doppler, power Doppler angio, color angio,
angio color, color Doppler energy, or color perfusion
19
1 Physical and Technical Fundamentals of Ultrasound and Doppler Ultrasound
20
1
imaging. To distinguish between echoes of ultrasound
signals from tissue and blood a HPF is inserted into the
system in color-coded Doppler sonography (cf. Wall
p.
Filter,
nals from tissue and erythrocytes are additionally
coded by their intensity (amplitude). As a rule signals
from tissue display an intensity 1000 times higher
than those from erythrocytes, enabling significantly
better separation between signals from tissue and erythrocytes. While in color-coded Doppler sonography
erythrocyte echoes are processed by frequency analysis, in amplitude-coded flow displays color attribution
depends directly on the intensity of the echoes, the
way intensities are displayed by a corresponding gray-
19). In an amplitude-coded flow display, sig-
Safety Aspects
The possible biological effects of medical ultrasound
diagnosis have been discussed since their beginnings
in the 1960s. Two dif ferent phenomena associated
with the passage of ultrasonic rays through the body
are known to influence biological systems. These are
thermal effects, since part of the energy insonated into
the body is transformed into heat and mechanical effects due to changes in pressure on the molecules.
Thermal Effects
The rise in temperature in the tissues during insonation can be attributed to absorption of energy, absorption being the transformation of ultrasound energy
into heat. The property of absorption differs greatly in
different tissues. The ability to absorb energy is poor in
fluids (amniotic fluid, blood, urine), but very high in
bones. The bones of an adult can absorb 60−80 % of the
ultrasound energy impinging on them. Ultrasound
frequency is the most important physical parameter
influencing absorption, for the higher the frequency
the faster ultrasound energy will be absorbed. A rise in
temperature of 2.5 °C or more can cause severe damage
to biological tissues, while temperature rises of 1 °C are
not considered to be significant. However, experimental studies (AIUM Bioeffects Committee 1988) suggest
that in the diagnostic range no damaging thermal effects are likely to occur.
Mechanical Effects
Biomechanical effects almost always refer to phenomena related to the generation and possible implosion of
microscopic bubbles in tissues. The interaction between ultrasonic rays and these bubbles is known as
cavitation. An ultrasonic oscillation gives rise to nega-
scale value in B-mode. Since amplitude-coding determines only intensity and not frequency shifts of the
signals received from the erythrocytes, the procedure
of amplitude-coded flow display does not produce
color reversals due to changes in flow direction or aliasing. Changes in blood flow velocity or direction cannot be distinguished in an amplitude-coded display.
The advantages of amplitude-coded procedures include in particular the ability to display flow velocities
that are distinctly lower than those that can be displayed by conventional Doppler methods. Amplitude
signals are also less prone to noise and are almost independent of the probe angle.
tive as well as positive pressure phases. The positive
phase is known as the compression phase, while the
negative phase is called the cavitation phase, or rarefaction. If the negative pressure is sufficiently large, it
is possible to generate microscopic bubbles or to enlarge bubbles that are already present. The appearance
and effect of cavitation depends not only on the
selected ultrasound frequency and intensity, but also
on the focus of the sound field.
For instance, cavitation appears in a stable form in
vibrating, gaseous bodies that are stable in themselves,
but have been set to oscillate by the ultrasound field.
Once oscillations have reached a certain level, a fluid
medium incorporating gas microbubbles is set in motion. This is called microstreaming. It has been demonstrated that microstreaming exerts an extremely
strong pressure that can cause cell membranes to
burst.
Another aspect of cavitation is the so-called transient cavitation in which existing microbubbles or cells
undergoing cavitation inflate under the influence of
negative pressure and then implode abruptly. These
processes happen in microseconds. The implosion can
then cause a sudden surge in temperature or a sudden
rise in pressure in an area as small as a square micrometer. In the end this may result in cell and tissue destruction. However, research has shown that transient
cavitation is purely a threshold phenomenon that only
occurs in the presence of extreme pressure and ultrasound frequency when cavitated cells are already present. If the pressure is below the cavitation threshold,
it will never by itself lead to cavitation even during extremely lengthy insonations. To this day there has been
no known case in which the diagnostic insonation of a
human body has led to even transient cavitation.
However, biological effects are difficult if not impossible to demonstrate, since the negative effects of
these phenomena may in some circumstances affect
only individual or a few cells, while cavitation may
occur in any location in the tissue. Currently available
ultrasound systems are designed in such a way that
their settings allow the operator to avoid such mechanical effects as cavitation at peak pressures by limiting average pulse intensity.
ously. The sample volume is therefore distinctly
smaller than in B-mode, but the PRF at ca. 1 kHz is also
clearly lower. As far as damage to the patient is concerned, M-mode scanning may also be considered safe.
CW Doppler
Important Definitions
Acoustic Output
Acoustic output is defined as the ultrasound waves
that are insonated by the transducer into the body of
the patient under examination. The intensity of ultrasonic waves is usually measured in fluids with known
absorption coefficients.
Acoustic Power
Acoustic power is defined as the total acoustic output
(in mW) insonated by the transducer into the tissue.
Intensity
Intensity is defined as the energy acting on a specific
area. Intensity is measured in mW/cm
over time (I
).
ta
2
and is averaged
Intensity Special Peak Time Average
Intensity special peak time average (ISPTA) measures
the maximum of the intensity averaged over time. Its
unit is mW/cm
2
.
Risks of Individual Ultrasound Procedures
B-Mode
As in M-mode, the insonated tissue volume in a CW
Doppler procedure is relatively small. Energy is transmitted not as ultrasound pulses, but continuously. The
insonated energy therefore attains a power of ca.
100 mW. Thus, thermal effects can no longer be ignored in all cases. However, despite the higher energy
output, the mechanical effects remain far below those
of B-mode or M-mode procedures. In CW Doppler the
transmitted energy must be adjusted to the depth of
the examination. It is important to keep examination
times as brief as possible.
PW Doppler
The insonated tissue volume is also very small in this
procedure. Moreover, PW Doppler involves very high
PRFs. The pulse time is often double that of B-mode or
M-mode. Therefore, considerably elevated ultrasonic
energy values may be attained. The implied danger of
temperature effects can no longer be ignored. Since the
intensities of the stimulating energy are similar to
those of B-mode and M-mode, mechanical effects may
be ignored.
Color-Coded Doppler Sonography
The energies used in color-coded Doppler sonography
lie between those used in B-mode and those used in
PW Dopplers. Mechanical effects can be ignored. The
insonated ultrasound pulses are distributed over a
relatively large volume. Thermal effects are higher
than in B-mode, but lower than in PW Doppler.
Basic Concepts
The insonated ultrasound energy is very small in Bmode (쏝10 mW/cm
possible very brief (쏝5µs) single pulses are transmitted at a PRF under 5 kHz. The insonated energy is
distributed over a large sample volume and generates
immeasurably small temperature changes in the tissues. As far as damage to the patient is concerned, Bmode scanning may be considered absolutely safe.
2
). To attain as great a resolution as
M-Mode
In this case an elevation of temperature might be considered in view of the higher energy used. As opposed
to B-mode, a single ultrasound ray is emitted continu-
Summary
To date there have been no indications of any injuries
due to the use of ultrasound procedures anywhere in
the world with the currently used sound pressure intensities of less than 100 mW/cm
Nevertheless, each user is advised to adhere to the
ALARA principle (as low as reasonably achievable), i.e.,
to use as low an intensity as possible for no longer than
necessary. Consequently, Doppler procedures using
higher intensities should be avoided during the first
trimester of pregnancy.
2
.
21
1 Physical and Technical Fundamentals of Ultrasound and Doppler Ultrasound
Important Instrument Settings
1
A great many ultrasound instruments of various types
with a variety of different controls are available on the
market. The most important parameters that can be
controlled will be described in the following.
Selecting the Most Suitable Transducer
As shown in the section on “Resolution” (cf. p. 9), the
axial as well as the lateral resolution in addition to the
depth of penetration are dependent on the frequency
of the sound wave. The higher the frequency of the ultrasound waves, the higher the resolution, but the
lower the depth of penetration. The optimal transducer
frequencies for abdominal and obstetric examinations
lie between 3 MHz and 5 MHz.
B-Mode Settings
Depth of Penetration
The depth displayed by an ultrasound system can be
varied within the maximal depth of penetration set by
the frequency of the transducer. This depth should be
set to correspond to the areas of diagnostic interest,
i.e., the depth of the display should correspond to the
depth of the anatomical structure to be examined.
Gain
For optimal diagnostic accuracy, the tissue must appear uniformly bright over the whole depth of the
image. If the gain is too weak, the result is a nonhomogeneous display pattern with inadequate echoes. Subtle tissue differences cannot be distinguished. If the
gain is too high, the individual structures are insonated
excessively, so that they cannot be distinguished from
each other. Electronic noise will also be displayed
when the gain is too high and will be seen, for example,
as spontaneous echoes in fluid-filled spaces.
The intensity of echoes diminishes with increasing
penetration. Therefore, the echoes from the deepest
penetration must be intensified relative to those from
p.
the near field by using the TGC (cf.
7).
Focusing
Setting the Doppler Parameters
Sample Volume
In PW systems it is of paramount importance to adapt
the size of the sample volume to the diameter of the
vessel to be examined, for if the sample is too small not
all parts of the flow can be encompassed.
PRF and Baseline Shift
The sample volume must also be adapted to the type of
flow to be measured in the vascular segment to be examined. If the flow is displayed in only one direction, it
is possible to increase the sample volume by shifting
the baseline to the Nyquist limit (cf. Baseline Shift,
p. 19). Next, PRF must be adapted to flow velocity. Setting the PRF too low results in the phenomenon of aliasing described on page 18. If, on the other hand, the
PRF is set too high, the tracing of spectra displayed on
the monitor is too small. The result is that markers for
measurements cannot be set accurately, leading to errors in the determination of the Doppler indices. PRF
and baseline should be set so that the tracings of spectra displayed on the monitor are as large as possible
without aliasing.
Scaling the Time Axis
The scale of the time axis can also be set. If it is too
long, the tracings of the spectra are too compressed,
again resulting in inaccuracy in applying markers for
measurements. If, on the other hand, the time axis is
spread too wide, it becomes difficult to evaluate the
temporal course of several cardiac cycles.
Wall Filter
The wall filter defines the minimal frequency range
that can be displayed. If the wall filter is set too high,
low-frequency end-diastolic signals can be cut off and
are not displayed. If end-diastolic signals are not displayed in the tracings of the spectra, the setting of the
wall filter must be shifted toward zero in order to exclude any pathology artificially created by an error in
the setting of the system.
22
As described in the section on focusing (cf. p. 9), the
highest resolution is attained at the focal point. The
focal zone is displayed as a mark beside the image on
the monitor. The position of the focus can be adapted
to the plane of diagnostic interest using electronic
transducers.
Orientation of the Tracings of Spectra
The basic setting of ultrasound systems generally displays positive flows (toward the transducer) above the
line and negative flows (away from the transducer)
below the line. As a rule, however, it is desirable to dis-
Important Instrument Settings
play and measure the deflection of spectra above the
baseline, regardless of the direction of flow. To accomplish this, the spectrum often needs to be inverted.
Color-Coded Doppler
Size of the Color Window
In color-coded Doppler procedures it is often necessary to adapt the size of the color window (ROI) to the
size of the region to be examined. The greater the
selected ROI, the smaller is the refresh rate of the
image. If the refresh rate is too small, a dynamic flow
display becomes impossible.
PRF
Color-coded Doppler also requires that the PRF be
adapted to the type of flow in the examined area. Too
low a PRF here, too, leads to aliasing, which in this case
is seen as color reversal.
Color Gain
If color gain is exaggerated, noise artifacts are displayed as a mosaic of color evenly filling the whole
color window. For optimal color gain, it is recommended that one raise the gain above the noise limit,
and then reduce it gradually until noise artifacts are no
longer displayed.
Basic Concepts
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