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Файл:Ординатура / Хирургия / Библиотека им академика М.И. Перельмана / Книга_5769_Библиотеки_им_академика_М_И_Перельмана
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10 oracic ultrasound
Specular echoes
Figure 7 – Interaction of ultrasound with the targets. Above, in the case of relatively large surfaces
compared to the wavelength of ultrasound employed, reflection prevails in a well-defined direction
(mirror effect). At the bottom, in the case of targets smaller than the wavelength of the beam, the acoustic
energy is spread in multiple directions (scattering).
When the ultrasound propagates through a medium, the intensity of the signal degrades
and is attenuated (attenuation). In general, the attenuation from the tissues increases with
the increase of the frequency. e effective propagation of the ultrasound beam is inversely
proportional to the frequency itself, as described by the equation:
1
attenuation (dB) =
(frequency [MHz]) × target distance (cm)
2
Acoustic energy is lost through reflections, scattering and conversion into heat. In practical
terms, the depth reach by an ultrasound is estimated in about 200 wavelengths. For example
with a frequency of 1 MHz the opportunity to explore tissues reaches 30 cm, while with a
frequency of 5 MHz the useful depth reached is approximately 6 cm (reduced to little more
than 1 cm with a frequency of 20 MHz).
Depending on whether the structure to be examined is more or less deep, transducers with
a different frequency (respectively lower and higher) have to be used. High-frequency beams
(7-18 MHz) will be used to study surface structures, while beams at a lower frequency (5-7
MHz) will ensure a good vision of the deepest tissues, as they penetrate in depth (Fig. 8).
e process of the ultrasound image forming is complex. Synthetically the signals received
by the probe are converted into digital data suitable to be amplified and processed for the
production of images. e device produces an electronic manipulation of the image that
makes even minor differences of hue visible (in a gray scale). is allows to reconstruct on the
screen a detailed anatomical map of the section of the body traversed by the ultrasonic beam.
Briefly describing the phenomena involved, each pixel of the ultrasound image is assigned a different brightness, that is proportional to the intensity of the corresponding echoes. As a consequence,
intense echoes appear as white dots, the absence of echoes appears as black (unechogenicity),
finally, intermediate echoes appear as points with different shades of gray (Fig. 9).
Basic principles 11
ULTRASOUND
OPHTALMOLOGY
SMALL PARTS
AUDIBLE SOUNDS
INFRASOUND
ABDOMEN / CHEST
/ HEART
THERAPY
Figure 8 – To the left, the frequency spectrum of sounds. To the right, the frequencies of ultrasound in
clinical applications.
Figure 9 – In ultrasound the echogenicity of an object is described in relation to the field in which it is
located or with the echogenicity of a target tissue. Circle A is anechogenic compared to the background.
In B, it is hypoechogenic. In C it is isoechogenic and in this case it will be visible only if surrounded by a
limit with different echogenicity. Finally, in D it is hyperechogenic.
12 oracic ultrasound
Bone, air, fascial tissue, calcifications generate very high reflection (they possess high echogenicity), and are then represented with white. Confronted with less reflective (less echogenic)
structures, these images are defined hyperechoic. Moreover, the massive reflection makes the
assessment of the underlying structures impossible (as in the case of bone, stones or air):
this phenomenon is responsible for the posterior acoustic shadow seen when the beam hits
surfaces that determine an almost complete reflection.
e opposite occurs when ultrasound passes through structures which are acoustically very
permeable (little or no echoes - hypo-or anechoic) as liquids. is occurs in cysts with fluid
content, in the cholecyst, in the bladder, in blood vessels, and of course in pleural-pericardial
effusions. In these cases the image is black, especially if the fluid is homogeneous and noncorpuscular (transudate, urine, bile, blood). Perfect anechogenicity is also called transonicity
(Fig. 10).
Figure 10 – Schematic illustration of the concepts of posterior acoustic enhancement (A) and of shadow
cone (B). The first one is generally related to acoustically permeable objects (such as fluid collections), the
second one to strongly impedant structures (such as bones and stones).
Diffusion phenomena explain the various shades of gray of different parenchyma in echography (liver, spleen), but also of images of the lung when it loses air to a critical level, very
close to the density of solid tissues (or water) (Fig. 11).
Atelectatic lobe
Figure 11 – In complete atelectasis, the absence of air gives the lung parenchyma a density and the
distinctive appearance of solid tissues, not differentiable from spleen or liver.
Liver
e intensity of the ultrasound beam progressively decreases while it passes through the tissues.
Basic principles 13
is process is called attenuation and has already been described. Because of the attenuation,
echoes coming from the deep structures are gradually weaker than those coming from more
superficial levels and the effect is compensated by a differential amplifier, which enhances
more farther echoes. e function of depth compensation (defined as time gain compensation or TGC) is an automatic function of the machine, but can also be adjusted manually by
the operator, who can vary the amplification, globally or selectively, according to the depth
(Fig. 12).
Figure 12 – The system of TGC cursors in an ultrasound machine allows to selectively amplify the
echoes coming from the planes of the scanned object, from its surface to its deepest part.
➣ The Doppler effect
In pleuropulmonay echography color Doppler, continuous and pulsed Doppler are currently not widely used and there are no consistent data in literature for their extensive use.
e use of Doppler in its various applications is only possible on thoracic structures during
the exploration of non-aerated solid parts, and not moving by themselves for physiological
events (breathing, pulse).
While not strictly necessary in this context, we shall refer to the characteristics of the Doppler
function, for its extensive use in echocardiography and for possible future applications for
the study of the moving pleura and of solid lesions of lung and pleura.
An acoustic signal produced by a moving object (approaching or moving away from the listener
- or transducer) undergoes a frequency shift (it appears respectively more acute or deeper).
According to this principle, described by Christian Andreas Doppler in 1845, it is possible to
insonate moving structures (such as red blood cells) to measure their speed with the frequency
shift of the returning echoes, according to the equation:
14 oracic ultrasound
c(F
− Ft)
v =
S
2Ft(cos θ)
where c is the speed of ultrasound in the tissue, Fs - Ft the frequency shift (returning frequency
minus frequency emitted) and Cos θ the cosine of the angle between the ultrasound beam and
the flow studied. Considering the angle of insonation (the optimal cosine of this angle is less
than 60°) and the speed of ultrasound in tissues (1540 m/s) as constant, the shift correlates
with the speed of the target (in the case of blood, red blood cells), generating an explorable
speed/time curve.
ere are three types of Doppler ultrasound: color Doppler (CD), pulsed wave (PW) and
continuous wave (CW) Doppler. e color Doppler is a pulsed wave Doppler technique, where
a color code is assigned to the signal (shift) received and superimposed on a two-dimensional
image. With this application numerical data are not acquired, but the flow is represented with
different colors (with different intensity). Flows approaching to the probe are represented by
shades of red, flows receding are in blue tones. e homogeneous saturation of a color generally represents laminar flows, mosaics of different colors represent turbulence (Figs. 13, 14).
Flows
approaching
Flows
receding
Figure 13 – The color Doppler representation of the flow in the vessels is coded with red for the
approaching flow and blue for the receding one from the exploring probe.
Mosaic of
dierent colors
Figure 14 – The color saturation is related to the slow and laminar movement of the flow. In the image
the homogeneous red of the flow approaching indicates laminarity, while the mosaic of colors of the
receding flow indicates turbulence.
e graphical and panoramic representation of color Doppler helps for a comprehensive assessment of flows in the studied area and to direct further action on specific points of the image.
Basic principles 15
Pulsed and continuous wave Doppler help determine (positioning specific cursors on the CD
image) quantitative traces speed/time of the flows. CW Doppler simultaneously transmits and
receives an ultrasonic signal using two crystals. PW Doppler uses a single crystal. It therefore
transmits and receives intermittently. In practice, the difference is that CW Doppler measures
with accuracy high flows along the entire line sampled, not allowing to determine the depth
of the specimen acquired. e PW Doppler, on the contrary, determines the exact flow at a
specific point (depth) of the image on which the sample is drawn, but it has limits on sampling
faster flows. e measurement with PW of flows over the semplable speed threshold, causes
the appearance of artifacts defined aliasing.
With the limitations described above, these techniques allow to measure with relative accuracy
vascular and intracardiac flows in terms of speed/time, and, with appropriate mathematical elaborations, to derive pressure gradients or speed/time integrals, i.e. distances. is is
the basis of many non-invasive hemodynamic assessments, which have important practical
implications (so-called echodynamics).
➣ Contrast agents in chest ultrasound
Contrast agents used in ultrasound are liquids injectable intravenously (or into body cavities),
containing suspended gas microbubbles able to amplify the backscatter signal, and to enhance
the echogenicity of the fluid that contains them. e physical properties of an ultrasound
contrast agent relate to its gaseous content, to the microbubble dispersion, to their resonance
capacity and lifespan.
Modern contrast agents contain microbubbles of such a size (3-10 microns) as to overcome
the pulmonary circulation and thus provide perfusion images in the systemic circulation.
ese agents must meet the demands of intravenous administration, stability in the passages
through the capillaries and heart, persistence in the circulation and acoustic response.
Here we refer to compounds suitable to be insonated with low mechanical index. ey must
resonate with appropriate frequencies, producing harmonics and vascular real time images,
similar to the vascular enhancement of computed Tomography (CT).
Among the products available, microbubbles of sulfur hexafluoride encapsulated in phospholipid shell (Sonovue®, Bracco, Italy) are extensively used in abdominal diagnostic and had
applications in both cardiac and thoracic sonography. e product is reconstituted prior to
intravenous use adding 5 ml of saline solution to 25 mg of powder, obtaining a white suspension. e suspension is administered in boluses of 2.5 ml. Its half-life in humans is 6 minutes
and more than 80% of the injected product is cleared in 11 minutes by the pulmonary route.
In general, ultrasound contrast agents have a high safety profile and show no renal, liver
or nervous system toxicity. Local adverse events are rare and hypersensitivity reactions are
exceptional. Caution is recommended in cases of heart disease and/or severe lung disease
(heart failure, coronary artery disease, pulmonary hypertension, diffuse parenchymal lung
disease, COPD). However, many studies on cardiac patients in different settings, including
stress-testing ones, showed no specific risks related to these substances.
In pleuropulmonary pathology contrast agents are used only when the lesion is solid and not
covered by aerated parenchyma. In other words, the contrast enhanced ultrasound of the chest
is possible only in relation to actual, parietal, diaphragmatic, pleural or pulmonary images.
16 oracic ultrasound
Under these premises, each solid lesion identified in terms of shape, borders, echogenicity
and echo-structure can be studied in its dynamic macro-and microvascular components after
administration of Sonovue.
In our experience optimal images are obtained after administration of a bolus of 2.5 ml of
contrast into an antecubital vein, followed by a bolus of 10 ml saline. Normally, the enhancement of the healthy lung is not visible due to the presence of the air. e atelectatic or
consolidated lung usually shows a rapid, homogeneous and lively (comparable with that of
the spleen) enhancement through the pulmonary artery (7 seconds on average). Necrotic or
colliquated lesions shows no enhancement. Neoplastic lung lesions cropping out of the pleura,
show a dominant bronchial (systemic) vascularization which is often uneven or anarchist, with
arteriovenous shunts. Neoplastic nodules and masses may have a sufficiently typical behavior
with systemic (after 20 seconds), irregular enhancement and often with rapid washout.
Under these premises, the interest of the ultrasound contrast agents for the diagnosis of
pulmonary embolism is absolutely clear. Pulmonary embolism may lack positive ultrasound
images (i.e. lesions detectable by ultrasound). In these cases, lung echocontrastography is of
no interest, or can be used as enhancement of the right ventricular cavity. However, if the
pleural plane is broken by a consolidation compatible with an embolized area, the administration of a bolus of Sonovue can show minimum/absent enhancement and/or only by a
peripheral echogenic rim (Fig. 15).
Figure 15 – Pulmonary infarction. In the upper left corner baseline image. Then images acquired
respectively after 45, 90 and 180 seconds from intravenous administration of 2.5 ml of sulfur hexafluoride
microbubbles with CEUS technique. No enhancement.
➣ Ultrasound machine
e ultrasound equipment consists of probes, and the machine itself. e machine contains
the power supply, the control panel, the electronic circuitry for signal elaboration, a monitor
and peripheral devices (such as printer or image recorder).
Extremely compact battery powered devices, which are easily transportable are particularly
interesting, because they can easily reach the patient in a variety of circumstances (in the
ward, emergency room, operating room, intensive care). Nowadays these devices have the
Basic principles 17
opportunity to have all probes available, to operate in color and power Doppler and with
contrast agents. Moreover they are comparable to the more cumbersome and expensive
ultrasound machines.
e probes exhibit various shapes and operate with elective frequencies, which translates into
images showing various depth penetration, different form and especially different definition.
e 7.5-18 MHz linear probe is a high frequency, high-resolution and low penetration probe.
It provides a rectangular image and it is used for the study of superficial organs.
e 3.5-5 MHz convex probe is a medium frequency, medium resolution probe. It provides
a trapezoid image and it is used for the study of deep organs. A convex or microconvex probe
is useful to explore lung consolidations, pleural effusions and diaphragm.
e 2-3.5 MHz sector probe is a low frequency, low resolution but high penetration probe. It
provides a triangular conical image, and it is used for the study of the heart. In our opinion it is not
suitable for pleural exploration, because it does not produce detailed images of the lung surface.
e control panel is essential for the acquisition of adequate images. It consists of various
commands, especially in the most advanced equipment, with color and power Doppler regulations and functions for three-dimensional images. However, few commands for a normal
two-dimensional grayscale ultrasound are necessary and they will be briefly described.
Starting, patient identification, preset and examination type: this system allows to enter patient
data into the machine and on the screen, with the aim of documenting the individual subject
with appropriate images. It also allows to define the operating setting chosen with the appropriate probe. e operator usually enters the name and the surname of the patient. Other
data can be represented on the screen, such as the type of scan, the organ explored and other
indicators. A keyboard system is normally used to insert data, and the appropriate group of
measurement commands is employed for the dimensional estimates.
Scan depth: through a dedicated command it is possible to define the depth of the scan to be
performed, which appears as a graduated scale on the screen next to the image.
Gain control: this command increases or decreases the amplitude of the returning echoes to
the machine. In the first case, the overall picture is clearer, in the second case it is globally
darkened. e adjustment of the overall gain is displayed on the screen in decibels or as a
percentage of the maximum gain.
TGC (time-gain compensation): a brief description has already been given. e echoes coming to the transducer from the deepest parts of the scan plane are normally more attenuated
than those coming from the surface. In the absence of a control system, therefore, the deeper
structures tend to appear darker than the more superficial ones. By itself, the machine tends
to “favor” the echoes that come from far fields, in order to maintain uniformity of the image.
If this is not sufficient, the operator can intervene through a series of cursors (TGC) that
reduce or amplify the returning echoes in a selective way from various depths of the explored
section, correcting the brightness features of the image.
Focusing: it is the function that allows you to optimize the focusing of the ultrasound beam
to the desired depth. In some machines it is also possible to focus more points at different
depths. e focused areas appear to the side of the image on the screen.
Freezing: it allows the “freezing” of the image on the screen for a more selective study, for
measurements or to make photographic copies. Many machines also possess a function to store
a series of static images (from 8 to several hundred), which then can be recalled in sequence,
and then suitably selected for measurements, study or copy (Fig. 16).
18 oracic ultrasound
Gain
Focus
Figure 16 – The most important controls of an ultrasound machine are the gain cursor, the adjustment
of the Image depth, the adjustment of the position of the focal point (or points), and the one that allows
the blocking of the image in static form (freezing).
Depth
Freeze frame
Of course there may be many other commands on the ultrasound machine: to adjust brightness, contrast and smoothing of the picture, for tuning Doppler functions, and for zooming
operations. ese commands are variously arranged and available and, although useful, are
not essential to the correct execution of basic ultrasound examination.
➣ Probes in chest ultrasound
Currently there are no indications supported by scientific evidence, on the use of specific probes
for chest ultrasound. Clinical studies have used probes that were usually used by experimenters
in relation to their own fields of interest or habits. Cardiology studies have employed sector
probes. For EFAST a common convex probe was used because chest scans preceded or followed abdominal scans. Finally the microconvex probe was considered optimal for its ability
to adapt well in intercostal spaces. According to, clinical and biophysical assumptions of
thoracic ultrasound, it is possible to differentiate the probes, with their different use frequencies, in relation to what the clinician wants to explore and what he or she wants to observe.
ere is no doubt that the study of the parietal planes of the chest must be executed with
linear probes operating with high frequencies for the reduced thickness which are studied
(2-4 cm). e pleural plane is well represented with linear probes for the study of surface
structures. e normal visceral pleura is the last level that can be explored through a real
ultrasound, while the deep plane of the visceral pleura, which upholsters a normally aerated
lung, is simply a reflection and not a real image.
An acoustic “breakthrough” of a normal lung beyond its parenchymal millimetric plane below
the visceral pleura is therefore useless. What appears as a structure falsely attributable to the
Basic principles 19
lung, which is just a field of specular reflections, is exclusively important for the observation
of the type of artifacts produced.
Different is the situation when a pleural effusion, pulmonary atelectasis or consolidations
appear in the ultrasound scan. In these cases an acoustic window is generated, that allows the
extension of the real field (explorable by ultrasound) well below the pleural line. Whenever
an acustic window is present it is logical to sacrifice the surface definition for a deep exploration (also of several centimeters) in order to display the extension of the fluid collection, the
bronchograms or the remaining aeration of a hepatized lung. Even the use of ultrasound contrast agents, which requires a “real organ” to explore, must rely primarily on the convex probe.
e study of the pleural dynamics10 can be performed with convex or linear probes. e
convex probe is adequate for detecting the pleural sliding in trauma, when the transducer is
moved over the chest during EFAST. e panoramic view of the convex probe is definitely
superior to that obtained with the linear probe. However, intercostal scans performed with
the linear probe provide detailed images of the pleural line and its sliding. A linear probe is
extremely efficient for detecting the lung point in subjects with pneumothorax.
Diaphragm dynamics is studied with the convex or sector probe and cardiac imaging relies
on the sector probe.
➣ Principles of two-dimensional ultrasound
ere are some basic principles that each operator (basic or advanced) using the ultrasound
machine must know.
Beyond the knowledge of ultrasound physics, it is important that the clinician is familiar
with the characteristics of the probes and the equipment he uses. e use of a few basic
commands must be clear, in order to obtain interpretable and defined images. is is now
facilitated by the use of small compact devices, where control systems are limited to the essentials. Equally fundamental is the use of a precise terminology in terms of echogenicity,
echo-structure, planes, artifacts and anatomy. In practice, two-dimensional ultrasound is the
reconstruction of anatomical plans (salami slices), in which tissues and organs appear in real
time, with their real morphology and in a gray scale. Operating over the chest, the production of artifacts or, rather, not anatomical images11, whose physical basis will be the subject
of a separate discussion is enhanced.
Echography is therefore a topographical and, especially, a tomographic anatomical reconstruction. It requires, on the one hand, dexterity to perform optimal tomographic scans
(slices with the probe) and on the other hand, a knowledge of topographic anatomy that
allows to orientate within the “slice” acquired. During the exploration of the pleuropulmonary complex, this methodology is extremely simplified. In fact, the normal pleural cavity
is virtual, then sonographically non-existent, while in case of disease, ultrasound permits to
recognize the characteristics of the intracavitary effusions12. With a systematic, algorithmic,
or symptom-guided approach, the coordinated movements of the probe allow the elective
visualization of one structure after the other. In the case of the chest, the graphic representation
is only a sequence of parietal planes, up to the plane of the visceral pleura, which represents
the boundary of “the real world” in a healthy patient. e visceral pleura, in fact, does not
allow an anatomical representation of the underlying lung, replaced, as we shall see, by an
artifactual image13. Each transition from a cross section of the explored area to a sagittal (or
longitudinal) one occurs with the clockwise movement of 90 degrees of the probe. Once
acquired the necessary experience, these operations tend to become automatic. e operator
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