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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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50 oracic ultrasound
including the visceral pleura. An echogenic surface with shadowing appears when scanning
the ribs or the sternum.
e angle of orientation of these surfaces in relation to the beam is generally close to normal
(90°). Large and relatively smooth layers like these cause a reflection of the incident acoustic
energy, that varies according to the acoustic impedance (indicated by Z) of the means which
are in contact7 (Fig. 4).
Figure 1 – A Lines are examples of artifacts (reverberations), that repeat an impedant plan (in this case
the pleural line) at regular intervals in depth in the image.
Figure 2 – B lines (arrows) are artifactual (not anatomical) images that are projected vertically from the pleural
line to the bottom of the image. Their origin is punctiform and they follow the movements of the pleural line.
Physics of chest ultrasound 51
CONFLUENT B LINES
Figure 3 – An echogenic subpleural plane, due to the confluence of B Lines and masking each horizontal
artifact, is called white lung.
Acoustic interface between similar tissues.
Probe
The reection is poor
Interface between tissues with dierent
acoustic impedance.
The reection is greater
Figure 4 – The reflections occur with perpendicular insonations, interacting on a separation plane
between two acoustic media with different impedance. The greater the difference in acoustic impedance
between the two media, the greater is the reflection. In case B, a large difference in acoustic impedance
allows the transducer to receive a high percentage of acoustic energy reflected.
e latter is a characteristic property of the medium within which the (ultra)sonic wave
propagates and, with a purely resistive impedance, it is directly proportional to the density
52 oracic ultrasound
Z = dc
cos θ
t
cos θ
Z2+ Z
1
of the medium. It is correlated with the speed of acoustic propagation in that medium, according to the equation:
where d is the density of the medium and c is the local speed of sound. is value is measured
in rayls or megarayls (Mrayls).
e concept of acoustic impedance allows us to study quantitatively the processes of transmission within acoustically different media and the reflections on interfaces between them8.
If we consider an ultrasound beam striking with an angle of incidence θ the separation plane
between two media with different acoustic impedance, the coefficient of acoustic reflection
(R) is defined by the trigonometric function:
R =
Z
=
cos θ
Z
2
2
p
r
p
i
Z
1
−
t
+
cos θ
Z
i
1
i
wherein Z1 and Z2 refer to the acoustic impedances of the first and second medium respectively. In this specific case, an angular correction represented by the cosine of the angle of
incidence on the separation plane is necessary (Fig. 5).
Incident
wave
Reected wave
Medium 1
Medium 2
Interface
Figure 5 – When an acoustic wave is transmitted in different media, it undergoes the well-known
phenomena of reflection and refraction.
Refracted wave
For a normal incidence (cosine 0° = 1), more simply, we get:
Z
− Z
2
R =
1
With this equation, we can calculate the reflection coefficient (R) of an acoustic discontinuity,
which theoretically varies between 0 and 1, i.e. between a total permeability and a total reflection.
Physics of chest ultrasound 53
Table 1 lists the physical and acoustic parameters related to water, air and some compact
or variously aerated biological tissues
9,10
. Table 2 lists the reflection coefficients of interfaces
between aerated and not aerated tissues, calculated for a normal acoustic incidence. eir
negative value refers to the fact that the second medium, has an acoustic impedance lower
than the first one, and it acts as a pressure release surface in relation to water
Table 1 – Physical and acoustic parameters in thoracic ultrasound
Acoustic medium Density
(g/ml)
Distilled water (20°C) 0,998 1,5 1480 0,0022
Muscle 1,07 1,7 1585 1,3-3,3
Fat 0,92 1,3 1450 0,63
Tissue (average) 1,05 1,63 1540 0,81
Aerated lung 0,15 0,1 500 50
Not aerated lung 0,5 0,4 733
Consolidated lung (pneumonia) 0,76 0,6 948
Acoustic
impedance
(Mrayls)
Speed of
ultrasound
(m/sec)
11,12
.
Attenuation at
1 MHz (dB/cm)
Pulmonary atelectasis 0,84 1,4 1660
Air (20°C) 0,012 0,00045 343 12
Modified from Soldati G
2011; 37: 1762-1770.
Table 2 – Reflection coefficients relating to normally insonated planes
Interfaces Reflection coefficient
Water / air -0,99
Muscle / air -0,99
Fat / muscle 0,13
Muscle / inflated lung -0,88
Muscle / deflated lung -0,72
Muscle / consolidated lung (pneumonia) -0,61
Muscle / pulmonary atelectasis -0,01
Modified from Soldati G
2011; 37: 1762-1770.
et al.
Synthetic comets: a new look at lung sonography. Ultrasound Med Biol
et al.
Synthetic comets: a new look at lung sonography. Ultrasound Med Biol
54 oracic ultrasound
Table 2 shows that a very high reflection coefficient is generated at the interface between water
and air (99%). Muscle, with density and speed of sound close to those of water, behaves in a
similar way. is is the reason why air collected in the tissues or air bubbles in a liquid behave
as strong reflectors. ey scatter the sound in relation to their size, in the form of echogenic
surfaces, lines or points, returning to the transducer a lot of acoustic energy.
In specific situations, the acoustic wave that returns to the transducer (echo) may possess
sufficient energy to be reflected by the transducer and between the transducer and tissues.
If the probe is able to reflect returning pulses in depth many times, it may also be effective
to produce many images of a specular plane. ese representations, which are similar to the
original one, are obviously artifactual, and have a constant distance between them (corresponding to the return time of the first reflection across tissues).
is phenomenon affects every impedant plane encountered by ultrasound, especially if it is
perpendicular to the acoustic beam13. eir magnitude is variable in different body areas, but
it is particularly evident on the chest for the presence, in critical positions, of the musculofascial and pleural planes. e multiple rebounds involving the probe are called “transducer
reverberations” (Fig. 6).
PLEURAL LINE
REVERBERATIONS
Figure 6 – A linear scan of a well-aerated lung in a patient with thin chest wall. The subpleural
(not anatomical) plane is characterized by multiple linear transverse echoes, caused by transducer
reverberations and by mirror effects of the parietal planes.
At pleural plane level, the normal lung behaves almost like a specular reflector, and its reflection coefficient in vivo, with an expanded organ, is approximately 0.88. is means a return
to the transducer of approximately 88% of the acoustic energy sent. e brightness of the
pleural plane is indicative of massive reflection, which produces, coherently with what stated
above, horizontal reverberations with the transducer, defined A Lines. However, only the outer
surface of the pleural plane, whether normal or pathological, must be considered as anatomical
representation. Its deep surface is artifactual, and already belongs to the “artifactual world”.
In addition to the fascial and the pleural planes, other important phenomena of reflection
are produced on ribs, sternum and shoulder blades.
When ultrasound are unable to travel beyond an acoustically impermeable object (bone,
calcification, air), the massive reflection cause the formation of shadowing effects. In this
Physics of chest ultrasound 55
case, horizontal artifacts may appear overimposed on the acoustic shadows, and are typical
reflective phenomena (transducer reverberations).
Exploring the chest, any real image (as consolidation) behind a bone is hidden by its acoustic
shadow, but, in the other hand, the ribs represent a landmark, since the pleural line lies immediately beneath them (Fig. 7).
RIB
Figure 7 – Longitudinal scan. The shadowing rib and the hyperechoic pleural line immediately below
it are evident.
Daily practice and past studies on bioacoustics
14,15
PLEURAL LINE
, show that the normal and the pathological
lung, denser than the normal one but still aerated (to a density of about 0.8 to 0.9 g/ml), do
not permit the expression of a real echostructure. In other words, we can speak of echostructure
only when the density of the organ gets critically closer to that of water.
e scientific evidence shows that the acoustic permeability of the lung is density-dependent16.
e normal lung, through its pleural plane, acts as nearly pure reflector, whereas the consolidation behave as permeable tissue (or acoustic window). erefore, the domain of pathological
artifacts lies between these two extremes density. Finally, because between porosity and density
there is an inversely proportional relationship, the more porous the lung, the greater is its
opposition to the penetration of ultrasound.
In general terms, the artifacts are located in a range of lung density between the normal
pulmonary density, which is slightly different depending on the level of respiratory inflation
(0.15-0.20 g/ml), and that of the lung consolidation (0.8-1 g/ml). Quite simply these artifacts
were originally defined as B Lines17.
In pulmonary pathology, B Lines assume different concentration and homogeneity at pleural
plane level. eir coalescence lets the image assume a characteristic type of echogenicity, generically defined as “white lung”. However, we do not know if white lung truly represents a
coalescence of B Lines or a different phenomenon. What is certain, is that these phenomena
have not been studied enough in their genetic and physical essence. Moreover, we represent
them as if they were real ultrasound images, employing instruments which are not created
to produce this type of image.
In the face of this paradox, some essential points are taking evidence. Although not fully
validated by the scientific point of view, they are enhanced by some logical considerations
extrapolated from current knowledge on general ultrasound.
56 oracic ultrasound
➣ Artifacts in chest ultrasound
We consider B Lines in their various arrangements as pathological artifacts in lung ultrasound.
Similarly, in our opinion, the homogeneous echogenicity called “white lung” is an artifact.
In general terms, artifacts in biomedical images are produced when the electronic system
that processes image signals, “presumes” standardized situations that are violated. erefore,
an artifact is a wrong interpretation of the machine with regard to a signal to be processed as
an image. e first consequence is that this image has no anatomical significance, but it can
carry important information to decrypt.
When processing a signal, some parameters are considered relatively constant by convention,
but in reality it is not so. Among these are, for example, the constant speed of sound in the
media, or the reflection coefficients on the interfaces, admitted in a restricted range. erefore,
when the ultrasound incidence reaches levels where differential acoustic impedance has very
high values (10 X for example), the transducer receives high energy echoes, that can saturate
the system. ese strong echoes can be reproduces as images that have nothing to do with
the actual morphology explored.
B Lines were described as vertical images with pleural punctiform origin, with laser-like appearance, extended in depth to the bottom edge of the image17 (Fig. 8).
Figure 8 – In the interstitial syndrome B Lines may appear with different density. a: B Lines, easily
countable and spaced 1-3 cm, identifying the so-called septal syndrome. In b and c B Lines increasingly
tend to coalescence. In d they are very close together.
eir artifactual nature is evident at least for three reasons. e first assumes that B Lines can
not represent any structure of the lung, because normal or pathological lung has no vertical
structures, extended for the entire length possessed by the B Lines18. e second point to be
considered is that images similar to B Lines can be reproduced in vitro, by insonating various
substrates (such as foams or porous polyurethane)
18,19
in which the structural linearity is not
present (Fig. 9). Moreover, B Lines are similar to images defined as “ring-down”20, always
considered as artifactual (Fig. 10).
Physics of chest ultrasound 57
Figure 9 – Comet artifacts (similar to B Lines) obtained by insonating a phantom of porous polyurethane
with variable density with the usual frequencies of clinical ultrasound (Soldati
et al.
UMB 2011).
Figure 10 – Ring-down are artifacts similar to B Lines. The image shows ring-down obtained through
the insonation of bullous tetrahedra, according to the model of Avruch and Cooperberg.
58 oracic ultrasound
Finally what goes under the name of white lung, probably represents an artifact, as it masks
every real structure present under it, and this is manifested both in vivo and in vitro.
➣ The reason why pathological lungs show artifacts
Experimental evidence on phantom21 shows that artifacts visible in pathological lungs are
interface phenomena that occur at pleural plane level.
As already discussed, the normal pleural plane is relatively linear and acts as an acoustic reflector with a reflection coefficient of 88%. At subpleural level all of the acoustic insonating
energy is therefore exhausted. e normal pulmonary background is predominantly a shadow
with overimposed transducer reverberations, generated by each impedant plane that the ultrasound beam encounters in its path, but mainly by the pleural plane. e normal pulmonary
background is constituted by A Lines and by a “noise”, probably due to the mechanisms of
differential amplification of the machine22.
It is more difficult to explain the origin of B Lines. Lichtenstein’s
but probably also incomplete. He assumed that B Lines are originating from interlobular
septa of the lung cortex, that are projected vertically on the visceral pleura.
When an interlobular septa is thickened (edema, for example), the ultrasound may encounter at this level a high gradient of acoustic impedance (between air and tissue), adequate to
produce (it was not explained how) the artifact. We think that this event may actually occur,
but it is only one of many ways in which B Lines can reproduce.
In our opinion
19,21
, B Lines can reproduce in every point of the pleural surface in which the
normal specular reflector is “broken”. A loss of local mirroring allows ultrasound, in relation
to its wavelength, to deal with a permissive receptor (a kind of acoustic microhole), which
could allow it to slip between highly reflective surfaces. ese surfaces are the foamy terminal
air spaces. Among these, the ultrasound can undergo multiple reflections (focal reverberations,
possibly modulated by wave interference) which the machine reads as a close succession of
deep echoes (Figs. 11, 12).
17,23
hypothesis is very simple,
Air
Air
Figure 11 – In the scan of a normal lung, air is separated by thin septa, that are not solved by the
ultrasonic frequencies normally used. The surface plane acts therefore as a specular reflector, reflecting
the acoustic energy to the transducer.
Air
Air
Air
Physics of chest ultrasound 59
Air
Air
Air
Air
Air
Air
Air
Air
Air
B Lines
Figure 12 – The interaction of ultrasound with the pleural plane that is no longer specular, is probably the
cause of the images seen in the sonographic interstitial syndrome. In interstitial diseases and, much more
generically, in all situations in which the subpleural lung has a higher density (as in hypoventilation), the
interstitial component becomes relatively greater, causing countless surface acoustic discontinuity, in which
ultrasound undergoes multiple reflections that cause phenomena of additive interference. To the right, a
surface discontinuity of a size consistent with the wavelength used, acts as a kind of hole in which multiple
ultrasonic reflections return to the probe a locally reinforced signal, that the machine “reads” as a B Line.
By analyzing the frequencies of normal use, we can understand how normal interlobular septa
(approximate thickness 100 microns)24 may be irrelevant in producing breaks in the pleural
reflector. However the situation can be vastly different in case of disease or unphysiologic
deflation. Diseases that cause an increase in lung density (the heavy lung), together with
unphysiologic deflated lung (health collapsed lung), can act alike when insonated. ey are
likely to produce different concentrations of B Lines, before the tissue reaches the critical
density to express images of consolidation (Figs. 13, 14).
e consequences of these observations seem to us extremely important:
• e septal hypothesis of Lichtenstein appears only as a partial aspect of the complex phe-
nomena, that occur at the pleural surface.
• ere is no need for a dominant anatomical structure to produce B Lines, but these are
generated whenever the specular pleural surface appears acoustically interrupted (collapse
of the reflector).
• Since the acoustic interruption of the pleural line is realized when the lung becomes denser,
it is logical to admit that the production of B Lines indicates a variation in density, i.e. a
lower porosity of the lung.
• e production limit of B Lines (and of white lung) is the critical density, which generates
the consolidation. Pure consolidation has a density very close to 1 g/ml25.
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