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Physical and Technical Principles of Color Doppler Sonography
ab
Fig. 1.26 Vibration artifacts associated with high-grade stenosis and
AV fistula.
a Patient 1: Vibration artifact in the lesser pelvis due to a high-grade
stenosis (angiography demonstrated a 90% transplant renal artery
stenosis).
can lead to spotty color flow (Fig. 1.27) or can mimic thrombosis. Technicalproblems such as too large a Doppler angle or cal-
1
cifications in the scanned region can be recognized in the Bmode image and avoided. It should be noted that acoustic
shadowing varies with the insonation angle and that angling
the color box can produce an oblique shadow (Fig. 1.
False-Positive Flow
Image clutter, pulsation artifacts, or gross motion artifacts (especially in power Doppler) can mimic the presence of intraluminal flow or tissue perfusion.
Pulsation and motion artifacts. Image clutter (flash artifact)
may be caused by setting the gain too high or the PRF too low.
Soft-tissue pulsations or motion artifacts can produce color
28).
b Patient 2: Coarse, continuous vibration artifact located to the right
of the midline, anterior to the aorta.
c Patient 2: Angiogram demonstrates an AV fistula between the
gastroduodenal artery and a branch of the superior mesenteric vein,
causing immediate opacification of the portal vein.
signals that are misinterpreted as flow. Transmitted cardiac
pulsations can produce an apparent flow pattern, especially in
cysts located near the diaphragm. Confetti-like artifacts associated with high-grade stenoses and AV fistulae are caused
by the vibration of perivascular soft tissues and structures in
response to rapid jet currents.
Slice-thickness artifacts and “phantom vessels.“ Slice-thickness artifacts are particularly common in older scanners. They
can cause diagnostic errors in color duplex scanning, as flow
information from neighboring vessels may be projected into an
occluded or thrombosed vessel. “Ghost vessels” or “phantom
vessels” can appear behind strong reflectors such as the diaphragm or pleura (Fig. 1.
not only contains color signals but can even yield a Doppler
spectrum.
c
28), mimicking a second vessel that
22
a
Fig. 1.27 Color Doppler artifacts and pitfalls. I: Effect of color priority.
a Longitudinal scan of an artery with optimum settings (Table 1.3)
shows complete filling of the vessel lumen.
b
b When color priority is decreased, the intraluminal color signal be-
comes spotty.
New Technical Processes and Approaches
b
a
Fig. 1.28 Color Doppler artifacts. II: Shadowing, motion artifacts,
and mirror-image artifact.
a Calcific plaque with a heavy acoustic shadow. The shadow runs perpendicularly in the B-mode image, but the shadow in the color-flow
image is oblique owing to the angle of the color box.
b Longitudinal scan of the posterior tibial artery, with motion artifacts
(flash artifacts) induced by movement of the transducer.
c Mirror-image artifact along the pleural apex. At the top of the image
is the subclavian artery with the origin of the vertebral artery. Below
the subclavian is a “phantom vessel” caused by the reflection of color
Doppler signals from the pleura, which is oblique to the beam.
New Technical Processes and Approaches
Technical advances in ultrasonography have been felt most
strongly in the areas of transducer technology and manufacture; computer hardware with the capabilities of digital highspeed electronics; and new approaches to signal acquisition,
filtering, and processing.
New Developments in Transducer Technology
Transducers with an increasing density of piezoelectric elements are offering higher resolution along with new, flexible
scanning formats based on the steering capabilities of ar rays
(see Principle of Echo Detection and Scanning Techniques,
p. 4).
Trapezoid scan. When a linear transducer is operated as a
phased array, a sector field can be added to each side to expand
the linear scan into a trapezoid scan. This provides a broader
field of view and makes it possible to evaluate even hard-toscan vascular segments such as the origin of the vertebral
artery or common carotid artery from the brachiocephalic
trunk.
c
2 D and multi-D arrays. Other innovations in transducer tech-
nology are two-dimensional (2 D) and multidimensional
(multi-D) arrays. Unlike conventional array designs (one-line
arrays), the 1.5D array consists of multiple parallel rows of elements (n ⫻ m matrix). The rows are driven in the transmit and
receive modes such that the slice thickness (elevation plane)
can be kept optimally thin in both the near and far fields
(Fig. 1.
29).
The smallest elevation aperture (i.e., the center row of elements) is used for imaging in the near field, while the largest
elevation aperture (i.e., all rows) is used for deeper scanning to
help achieve good penetration depth and slice-thickness resolution. The greatest resolution increase occurs in the plane perpendicular to the transducer (Fig. 1.
Further subdivision and miniaturization of the array lines
has led to the design of 2 D arrays that can scan along as well as
perpendicular to the conventional scan plane. This makes it
possible to acquire data from a pyramid-shaped volume and
generate sectional images with any desired orientation.
29).
Physical and Technical Principles
23
Physical and Technical Principles of Color Doppler Sonography
a
a
b
Elevation
lateral
late
l
ra
xia
axial
l
1
Elevation
Fig. 1.29 Improved resolution with a multi-dimensional transducer
array. Slice-thickness focusing of a conventional transducer with a
single row of elements (a)and a multi-dimensional array (b). While the
axial resolution remains unchanged, the resolution perpendicular to
the transducer plane (elevation plane) can be significantly improved
by focusing.
tions of the array’s various transmitted pulses are temporarily
stored in digital form, digitally focused along with other adjacent subapertures, and utilized for image formation. As an example, Fig. 1.
30 shows the principle and effect of increasing the
aperture by a factor of 2. Other factors can also be achieved.The
more the aperture is enlarged, the better the lateral resolution
and the greater the penetration depth and the ability to recognize details. The practical resolution limit is reached when all
elements in the array are used to compute the information for
every point in the ultrasound image.
Parallel processing. Another advantage of high-spee d digital
electronics is the ability to parallel-process multiple ultrasound lines from the information of one transmitted pulse. The
practical advantages of parallel processing are a faster frame
rate, especially in the two-dimensional Doppler techniques,
and an improved ability to analyze rapid, dynamic processes
and appreciate details.
Special filtering techniques. When special filtering techniques
are applied, nonlinear tissue properties can be utilized to form
the image. One cause of nonlinear tissue propertiesis the effect
of ultrasound pressure on sound propagation velocity in the
tissue, especially when high sound pressures are used. Because
of this mechanism and the inherent differences in sound propagation velocity in different tissues, leading to different transit
times, the pulse emitted by the transducer undergoes distortion in the tissue. If we analyze the frequency content of the received echoes, we find that they contain certain harmonic multiples, or overtones, of the basic frequency (Fig. 1.
31).
Tissue harmonic imaging. Second-harmonic waves, or wave
components at twice the transmitted frequency, are especially
24
New Techniques of Signal Acquisition and Processing
High-speed digital technology. The latest generation of ultra-
sound scanners can fully digitize the received echo signals
while the signals are being acquired. They are also distinguished by their high computer capacity. The high-speed digital technology that is used in these systems combines the advantages of optimum signal scanning and digital technology
(precision, signal fidelity, stability, signal dynamics) with a
very high temporal resolution for transit time and phase correction. These optimal focusing conditions provide excellent
lateral and axial resolution with very high signal dynamics,
making it easier to appreciate diagnostic details.
Synthetic aperture. Other improvements result from enlarging
the active surface area of the transducer (the aperture). This requires accurate, digital interim storage of the high-frequency
echo signals from the individual array elements in unchanged
form. This makes it possible to enlarge the aperture beyond the
conventional theoretical maximum. In the synthetic aperture
(e.g., SynAps) of the Sonoline Elegra system, echoes from por-
Digital
memory
A
1
Parallel
processing
Fig. 1.30 Synthetic aperture scanning: technical principle and effect.
Each aperture consists of 128 elements, for example. With a digital interim memory, it is possible to combine the imaging information from
multiple subapertures (two shown here: A
aperture of 256 elements. The advantages are greater penetration
depth and improved lateral resolution.
Beam
former
128
channels
A
1
Tot a l
*) into a large total
1,A1
A1*
New Technical Processes and Approaches
Transmitted
waveform
Pressure wave
faster than
negative
pressure
component
Waveform that
has passed
through tissue
Waveform
Fig. 1.31 Effect of nonlinear sound propagation in tissue. A sound
wave typically begins as a sine wave. The tissue is compressed,
however, by the positive component of the pressure wave, and so the
sound velocity is temporarily increased. This leads to a more rapid
propagation of the positive part of the pressure wave (right arrow).
During the rarefaction phase of the pressure wave, the tissue can expand again and the sound velocity decreases. As a result, the negative
pressure component of the sound wave is conducted moreslowly. This
phase-dependent change in sound velocity distorts and steepens the
original sine wave, analogously to a wave breaking on the shore. The
effect increases with increasing depth. The result is that the reflected
wave acquires a harmonic multiple of the basic transmitted frequency
(“harmonic energy”). These frequencies, which are troublesome in a
conventional B-mode image,can be specifically utilized for“tissue harmonic imaging” (THI). This technique can be used successfully even in
patients who are technically difficult to scan.
f02f03f
0
Spectrum Depth
prominent in the returned spectrum. Tissue harmonic imaging
(THI) involves isolating these components and suppressing the
basic frequency components. This technique has already been
used to obtain “clearer,” artifact-free images in patients who
are technically difficult to scan.
Phase-inversion or pulse-inversion techniques. The phase-in-
version or pulse-inversion techniques can be used to suppress
the fundamental transmission spectrum and use only the harmonic frequency components over a wide frequency band to
form the image (wideband tissue harmonic imaging, ensemble
tissue harmonic imaging). Instead of a single pulse, two identical pulses are transmitted into the tissue per image line. The
first is a standard ultrasound pulse, and the second is an exact
copy with its phases inverted. The reflected and backscattered
pulse pairs are subtracted (Fig. 1.
32). All of the unchanged,
“linear” signals cancel out. In this way all the signals from the
tissue that are changed owing to nonlinear effects are selected
out and can be used for image formation, independent of filtering techniques. The effect is cumulative and increases with
penetration depth. The THI technique is most effective at moderate and greater depths. Further penetration past a depth of
12–15 cm is limited by absorption. This technique provides significantly greater image improvement than traditional filtering techniques for extracting the nonlinear (harmonic) components of the returned signal. The phase-inversion technique
Pulse 1
Pulse 2
Sum
Linear propagation
Fig. 1.32 Principle of the phase-inversion or pulse-inversion technique. Two identical pulses whose phases are inverted (shifted 180⬚)
are transmitted into the body (top and center rows: pulse 1 and pulse
2). When the returning signals are added, the linear (unchanged)
echoes cancel out (lower left). The signals that have been distorted
due to nonlinear effects (right) add together and can be used for
image formation. This process suppresses the fundamental (basic)
frequencies and the odd harmonic signal components while amplify-
ing the even harmonic components, especially the second harmonics.
Nonlinear propagation
has already proved successful clinically in enhancing the visualization of smaller lesions (Fig. 1.
33). When harmonic imaging
is combined with ultrasound contrast agents, even the smallest metastatic lesions can be identified (Fig. 1.
34).
Programmable image processor. By incorporating a special
programmable image processor into the ultrasound system, it
is possible to raise the computer capacity for image processing
(several billion operations per second) to a level that was pre-
viously reserved for external workstations. In this way, various
image processing tasks that formerly had to be done on separate systems (offline) can now be performed directly (online)
and in real time. An example of this is adaptive contrast enhancement. The result is ultrasound images that are better
suited to the physiology of human vision, making it possible to
conduct the examination even in a bright room.
Panoramic imaging. Another application based on the pro-
grammable image processor is panoramic imaging, first introduced as the SieScape technology in the Sonoline Elegra system. In panoramic imaging, an extended field of view is obtained by manually scanning a standard transducer over the
region of interest without a position sensor. This information is
first displayed on the monitor in standard fashion. Based on the
level of agreement between the individual real-time images,
correlational algorithms are used to determine the transducer
position on the body. This positional information is then used
to seamlessly add the new image to the previous image. Thus
when the transducer is moved along one plane or around an
area, a real-time panoramic image is obtained. In this way the
operator can freely define an image format that is optimum for
the specific anatomy (Fig. 1.
35). Panoramic imaging has been
combined with power Doppler flow imaging to produce a
color-encoded panoramic imaging process marketed as Color
SieScape.
Physical and Technical Principles
25
Physical and Technical Principles of Color Doppler Sonography
a
Fig. 1.33 Effect of tissue harmonic imaging (THI),
illustrated for renal cysts and the gallbladder.
a Longitudinal scan through the right kidney of a
patient with small renal cysts that are barely detectable
in the fundamental mode (left). The THI mode (right)
improves the delineation of the cysts and gives an artifact-free view of the cyst lumen.
b Enlarged view of a gallbladder with a polyp. In the
fundamental mode (left) the lumen contains spurious
scattered echoes, and there are accentuated semicircu-
lar artifacts in the adjacent liver as a result of reverbera-
tions with a curved array. In the THI mode (right), the
gallbladder lumen is free of artifacts and the other artifacts are less pronounced.
1
b
Fig. 1.34 Effect of contrast harmonic imaging in the detection of
small hepatic metastases following the injection of 300 mg/ml
Levovist. After 2.5 minutes the contrast agent has become concentrated in the reticuloendothelial system (RES). The transmit power is
increased, causing the contrast microbubbles to vibrate and burst.
This “acoustic emission” produces hyperechoic enhancement of the
liver parenchyma.
Foci that lack a RES do not enhance, and remain hypoechoic. As a result, even small metastases thatare difficult or impossible to detect on
plain images can be identified by their sharp contrast with the surrounding hepatic tissue.
26
New Technical Processes and Approaches
a
Fig. 1.35 Panoramic images of the breast.
a Breast with a postoperative hematoma. A standard transducer is
scanned over the region of interest. Initially this information appears
on the monitor in the usual form. Based on the degree of agreement
between the individual real-time images, image-comparison algo-
3 D Imaging. If the transducer is not moved just in the selected
plane as in panoramic imaging but is moved or angled perpendicular to that plane, a three-dimensional (3 D) ultrasound
data set is acquired. These movements may be accomplished
by a mechanical apparatus within a special transducer (e.g., a
sweep mechanism in a convex array) or by free manual movements of the transducer. Manual movements may involve
three-dimensional position detection using position sensors or
a method based on uniform, predefined movement patterns.
The “3-Scape” real-time 3 D imaging technique is based on the
latter method and employs panoramic technology with standard transducers to enable freehand volume acquisition by,
say, moving the transducer perpendicular to the scan plane
(sweeping is also possible). In 3-Scape imaging, the volume
b
rithms are used to assemble the separate images into a panoramic
scan. In this way the entire breast can be displayed in one image, and
the location of the hematoma can be seen.
b The exact size of a silicone breast implant can also be determined
following augmentation mammoplasty.
data are collected and computed during the scanning process.
While the transducer is still moving, this data set is used to
generate a perpendicular guide plane that can be use d for
orientation. The three-dimensional data set can then be displayed as a complete volume in a maximum intensity projection (MIP), for example, or surface projection algorithms can be
used to construct a surface-rendered image. The data set can
also be manipulated to display arbitrary planes of section
within the 3 D volume that are not accessible with conventional B-mode imaging (Fig. 1.
36).
If flow information (in the power mode) is also obtained
during data acquisition, the 3 D technique can provide excellent survey views for the complete documentation of organs,
vascular distributions, and tumor blood supply.
Physical and Technical Principles
Fig. 1.36 Quadruplet gestation imaged by real-time 3 D
ultrasound with arbitrary plane selection. The three-dimensional data set can be manipulated to display the
sectional planes that give the best view of an embryo or
other region of interest. The images in this case reveal a
quadruplet gestation.
27
Physical and Technical Principles of Color Doppler Sonography
References
1 Bonnefous O, Pesque P: Time domain formulation of pulse Doppler ul-
trasound and blood velocity estimation by cross correlation. Ultrason.
Imaging 8 (1986) 73–85
2 Campbell S, Diaz-Recasens J, Griffin DR: New Doppler technique for
assessing uteroplacental blood flow. Lancet 1 (1983) 675–678
3 FitzGerald DE, Drumm JE: Non-invasive Measurement of human Fetal
Circulation using Ultrasound: A new Method. BJM 2 (1977) 1450–1451
4 Gosling RG, King DH: Ultrasound Angiology. In Macus AW, Adamson J
(eds.): Arteries and Veins. Churchill-Livingstone, Edinburgh 1975
5 Haerten R: Verfahren der Farbdoppler-Sonographie: Ein Methoden-
vergleich. Ultraschall Med. 14 (1993) 225–230
6 Kasai C, Namekawa K, Koyano A, Omoto R: Real Time two-dimensional
Blood-flow Imaging using autocorrelation technique. IEE Tran. Soc.
Son. Ultrason. SU 32 (1985) 458–463
7 Kay SM: Modern Spectral Estimation: Theory and Practice. Prentice
Hall 1988
8 Klews PM: Physik und Technik der farbkodierten Duplexsonographie.
In Wolff K-J, Fobbe F (eds.): Farbkodierte Duplexsonographie. Thieme,
Stuttgart 1993
9 Liu D, Kim J, Schardt M: Modified autocorrelation method compared
with maximum entropy method and rf cross-correlation method as
mean frequency estimator for Doppler. IEEE Ultrasonic Symposium
1991, 1285–1290
1
10 Maulik D, Nanda NC, Saini VD: Fetal Doppler Echocardiography:
methods and characterization of normal and abnormal hemodynamics. Am. J. Cardiol. 53 (1984) 572–578
11 Pourcelot L: Applications clinique de l’examen Doppler transcutane. In
Pourcelot L (ed.): Velocimetric Ultrasonore Doppler. Iserme, Paris 1994
12 Satomura S: Ultrasonic Doppler Method for the Inspection of Cardiac
Functions. J. Acoust. Soc. Am. 29 (1957) 1181–1183
13 Seitz KH, Kubale R: Duplex-Sonographie der abdominellen und retro-
peritonealen Gefäße. VCH, Weinheim 1987
14 Soldner R: Physikalische Grundlagen der sonographischen Bildge-
bung, Ultraschall-Gewebe-Interaktion und Sicherheitsaspekte. In
Bogdahn U, Becker G, Schlachetzki F (eds.): Echoverstärker und transkranielle Duplex-Sonographie. Berlin 1998
15 Stuart B, Drumm J, FitzGerald DE, Diugnan NM: Fetal blood velocity
waveforms in normal pregnancy. Br. J. Obstet. Gynaecol. 87 (1980)
780–785
16 Taylor KJ, Burns PN, Woodcock JP, Wells PN: Blood flow in deep
abdominal and pelvic vessels: ultrasonic pulsed Doppler analysis.
Radiology 154 (1985) 487–493
17 Thompson RS, Trudinger BJ, Cook CM: Doppler ultrasound waveforms
in the fetal umbilical artery: quantitative analysis technique. Ultrasound Med. Biol. 11 (1985) 707–718
18 Weiser HF, Birth M: ViszeralchirurgischeSonographie. Springer,Berlin
Heidelberg New York 2000
19 Hetzel G: Neue technische Entwicklungen auf dem Gebiet des Ul-
traschalls. Der Radiologe 10 (2003) 777-792
28
2 Safety Aspects of Doppler and Color Doppler Sonography
H.-D. Rott
Doppler sonography, with its ability to define and evaluate the
fetal blood supply, has been an important addition to diagnostic ultrasound in obstetrics. Because these techniques require
power outputs and ultrasound intensities that are considerably higher than in B-mode imaging, the essential safety that
Mechanisms of Tissue Effects
The biological effects of ultrasound are based largely on heating and cavitation. These effects are dependent on different
sound field parametersand tissue properties and can therefore
occur separately from each other. Other primary physical effects have no bearing on the clinical safety of ultrasound.
Heating
Ultrasound energy that is transmitted into tissue is partially reflected, partially scattered, and partially absorbed and con-
verted to heat. The degree of heating depends on different
properties of the ultrasound field and the exposed tissue. A key
factor is the spatial peak time average intensity (I
pulsed Doppler flowmetry, however, the power output of the
device is a more important factor than field intensity
sound frequency is also a factor, since higher frequencies are
absorbed more strongly, and therefore the applied energy is
concentrated over a smaller volume.
4, 23
). In
SPTA
. Ultra-
has been established for diagnostic ultrasound in general cannot be assumed for all applications. Consequently, the sonographer should be familiar with potential bioeffects and their
physical mechanisms in order to avoid any risks that may be
associated with the examination.
sion in different tissues and organs varies substantially, and so
the cooling component in different tissues can only be roughly
estimated. With brief ultrasound exposure, perfusion contributes little to tissue cooling
Hyperthermia. An increase in temperature may be a risk, since
cell division is inhibited above 30 ⬚C, and temperatures higher
than 41 ⬚C can cause cell death with sufficiently prolonged exposure. A key factor besides the amount and duration of tissue
heating is the sensitivity of the affected tissue
fetal brain is extremely sensitive to temperature increases. Although brain tissue has a low absorption coefficient, secondary
heating from the more strongly absorbing cranial bone must
also be taken into
that whole-body hyperthermia above 41⬚C can be teratogenic,
depending on the duration of exposure
account
5
.
3
. The embryo-
4
. Animal experiments have shown
11, 2 4
.
Cavitation
Physical and Technical Principles
Absorption coefficient. Sound absorption in biological tissues
increases with the protein content of the tissue
body fluids such as urine and amniotic fluid absorb almost no
sound energy, and their absorption coefficient
0.002 and 0.003 dB cm
kidney, and muscle have approximately the same absorption
(
α = 0.4–0.6 dB cm
sorber (
risk. The secondary heating of soft tissues in close proximity to
bone should also be considered when evaluating risk
Acoustically, the human embryo is equivalent to soft tissue. By
the end of the first trimester, however, absorption increases
owing to early mineralization of the bone. Thereafter it continues to increase during the rest of the fetal period.
Heat conduction and perfusion. Heat conduction and blood
flow contribute to the elimination of tissue heat. They are the
major factors that determine the final temperature that is
reached, especially on prolonged exposure. Whereas heat conduction can be calculated to a good approximation, the perfu-
α =5–10dBcm
–1
MHz. Soft tissues such as brain, liver,
–1
MHz). Bone is by far the strongest ab-
–1
MHz), placing it at greatest thermal
19
. Water and
α is between
21, 25
Noninertial and inertial cavitation. Cavitation refers to the
sound-induced formation and dynamic behavior of cavities
and gas bubbles, which can produce a variety of physical,
chemical, and biological effects in the medium
cavitation occur: noninertial cavitation, which describes the
prolonged resonant oscillations of preexisting gas microbubbles that do not collapse; and inertial cavitation, in which
preexisting microbubbles (“cavitation nuclei”) rapidly expand
in the rarefaction phase of the ultrasound wave and collapse
again in the subsequent compression phase. This process can
.
generate local pressure amplitudes greater than 1000 MPa and
local temperature peaks in excess of 1000 ⬚C. This may b e associated with electrical arcing (“sonoluminescence”) and the
formation of free radicals (OH
properties that are indistinguishable from the radicals produced by ionizing radiation
stable and transient cavitation.
Sound-pressure amplitudes. Cavitation is a threshold effect.
The critical field parameter that determines whether cavita-
–,H+
,H2O2, etc.) with mutagenic
15, 25
. A continuum exists between
25
. Two types of
29
Safety Aspects of Doppler and Color Doppler Sonography
tion occur is the negative sound-pressure amplitude p–(by
consensus, this amplitude is stated as a positive value in MPa).
Under certain physical conditions, however, the signs of the
pressure values may change when an ultrasound wave is reflected, so that the reflected wave acquires a negative pressure
amplitude equal to the positive pressure amplitude of the incident wave. As a result of this, the positive pressure amplitude
may also be a relevant quantity. The cavitation threshold increases at higher frequencies.
Cavitation nuclei and cavitation threshold. Human tissue is
fairly resistant to cavitation because generally it does not contain cavitation nuclei. Such nuclei are presumed to exist,
however, following infusions, administration of echo contrast
agent, gas gangrene infections, and open injuries. The negative
sound-pressure amplitude necessar y for cavitation to develop
is unknown. In shock-wave lithotripsy, cavitation is known to
occur at negative pressures that exceed 10 MPa, but modern
sonographic techniques employ fields whose negative peak
pressure does not exceed 5 MPa (Fig. 2.
large scale would seriously affect imaging because the induced
bubbles would cause extreme increases in scattering and absorption. So far these effects have not been observed in ultra-
2
sound examinations. Cavitation, then, is unlikely to be a risk
factor in the ultrasound techniques in current use. It should be
added, however, that echo contrast agents greatly lower the
threshold for cavitation (see Ultrasound Contrast Agents,
p. 31).
1a). Cavitation on a
a
12
(MPa)
–
p
8
4
0
b
100
)
2
10
(Wcm
SPTA
I
1
0.1
0.01
0.001
Fig. 2.1 Minimum (min), maximum (max) and mean values of the
negative peak pressure p
Max
Mean
Min
Imag VagM Col
CW
Imag VagM M Col
–
(a) and I
PD
SPTA
Ther
Litho
CW Ther
PD Litho
intensity (b) of various European
ultrasound scanners in various modes. The corresponding values of
therapeutic and lithotripsy devices are also shown for comparison.
Imag: B-mode imaging; VagM: M-mode with a vaginal probe; M: conventional M-mode; Col: color Doppler; CW: continuous-wave Doppler;
PD: pulsed Doppler; Ther: physiotherapy; Litho: lithotripsy. (After reference 6.)
30
Risk Assessment of Various Ultrasound Techniques
The risks associated with various ultrasound techniques depend chiefly on the likelihood of a biologically significant
temperature rise and the potential for inducing cavitation. The
field parameters of current diagnostic instruments that have a
bearing on these effects are reviewed in Fig. 2.
1. Recent
measurements have essentially confirmed these values while
also showing that newer systems tend to have higher field intensities but do not have higher negative peak pressures
14
.
Duplex Sonography
Duplex sonography combines B-mode imaging with pulsed
Doppler flowmetry. B-mode provides the sectional image for
locating the site of interestand positioning the Doppler sample
volume, while flowmetry yields the desired information on
blood flow velocity.
Heating. Biological risks have been ruled out for B-mode imaging. Flowmetry requires the additional use of a Doppler beam
of greater pulse length and higher intensity (I
sity of this stationary Doppler beam may fall within the upper
range of therapeutic intensities. Animal experiments have
). The inten-
SPTA
shown that Doppler pulses can produce a biologically significant temperature rise in soft tissues, and that this effect can no
longer be considered harmless
can become particularly hazardous when bone located below
the Doppler sample volume is exposed to the ultrasoundbeam,
since bone has a high absorption capacity and undergoes rapid
heating. It was found, for example, that when fetal guinea-pig
brain was insonated within the intact skull, the temperature
rise was greater near the bone on the opposite side of the skull
than at the center of the brain
conceivable that instruments with a high power output and
prolonged scan time (⬎30 seconds) can induce temperature
rises during flowmetry that cannot be considered harmless.
Cavitation. Because the negative peak pressure is no greater in
Doppler ultrasound than in B-mode and does not exceed
5 MPa, cavitation should not occur.
4, 23
(Fig. 2.2). The heating effect
4
. Based on these findings, it is
8
7
6
5
4
Temperature rise (°C)
3
2
1
0
0 60 120 180 240 300 360
Fig. 2.2 Temperature rise over exposure time at the center of isolated guinea-pig brain.
Ultrasound field parameters: frequency 3.3 MHz, pulse duration
µs, PRF 4 kHz.
6.25
Upper curve: total power output 1120mW, I
Lower curve: total power output 260 mW, I
(After reference 4.)
SPTA
SPTA
1120 mW
260 mW
Time (s)
2.5 W/cm.
2.9 W/cm
Color Doppler
In color Doppler systems, blood flow velocities are determined
not just in a single sample volume but over a larger “region of
interest” as in B-mode imaging. These velocities are color-encoded according to their direction and magnitude and are superimposed over the B-mode image.
Heating. Because the Doppler beam is not stationary as in duplex sonography but is swept across the region of interest, the
absorbed energy is distributed over a larger tissue volume, resulting in a substantially lower time average intensity. A useful
rule of thumb is that the I
intensities in color Doppler are 10
SPTA
times higher than in conventional B-mode imaging, while the
intensities in duplex scanning are 100 times higher (Fig. 2.
1b).
There are considerable differences, however, among different
scanners and modes of operation. For example, as the region of
interest is increasingly narrowed in its lateral dimension, the
thermal effect is increased. When the region of interest is narrower than 2 mm, the thermal conditions are practically the
same as in duplex scanning. As a general rule, however, the
thermal risks associated with ordinary clinical use are small.
Cavitation. As in duplex scanning, cavitation should not occur
in color Doppler imaging.
Power Doppler
The ultrasound exposure in power Doppler imaging is the
same as in color Doppler. The processing mode is different,
however, as power Doppler displays signal amplitudes rather
Risk Assessment of Various Ultrasound Techniques
than blood cell velocities. This type of signal processing can detect even slow flow velocities, providing a more accurate display of vessel volumes. The thermal effect corresponds to that
of color Doppler.
Color Velocity Imaging (CVI)
While this technique does not employ the Doppler principle, it
does bear some similarities to color Doppler. The velocity and
direction of blood flow are determined by the computer comparison of successive B-mode echo images. The principle is
that B-mode can detect acoustic inhomogeneities (“streaks”)
in flowing blood and can track their motion. CVI requires a
large computer capacity but uses low ultrasound intensities
and is thermally harmless.
Transvaginal Scanning
A frequent concern in transvaginal scans is excessive exposure
of the embryo owing to the proximity of the endovaginal
probe. The opposite should be true, however, since the short
range leads to less absorption, and therefore a lower power setting can safely be used. However, manufacturers have offset
this advantage by using higher frequencies, which, while improving image resolution, also cause greater sound absorption
that necessitates higher intensities. As a result, the average intensities in transvaginal B-mode scanning are somewhat
higher than in abdominal scanning, but they are slightly lower
in transvaginal pulsed Doppler
7
. The thermal exposure to the
embryo or fetus is approximately the same as in transabdominal sonography.
Ultrasound Contrast Agents
Gas-containing ultrasound contrast agents are being used with
increasing frequency. They appear to be immunologically safe,
and microbubble sizes on the order of 10
capillary blood flow.
Cavitation. At the same time, ultrasound contrast agents considerably lower the threshold for cavitation. It is unknown
whether administration of echo contrast agent is sufficient to
induce cavitation by diagnostic ultrasound. Stable cavitation
would increase the echogenicity of the contrast agent, but local
hemolysis would also be expected to occur. So far, however,
there have been no clinical reports of hemolytic effects
tial cavitation would also be associated with free radical formation. This would contrast with the similar effect of ionizing
radiation in the distribution pattern in the tissue: presumably
the radicals would form chiefly in the intercellular compartment owing to the lower viscosity and would form intravascularly only at sites where the contrast agent is excited by ultrasound. It is unclear at present whether this is apt to produce
mutagenic effects. Since no experimental data have yet been
published on this issue, a definitive risk assessment cannot be
made. On the whole, the risk does not appear to be particularly
13
high
. There are no concrete objections to using ultrasound
µm do not affect
18
. Iner-
Physical and Technical Principles
31