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Basic principles
➣ Introduction
Until recent years the applications of ultrasound for studying the chest were irrelevant and
without an effective integration for urgency or critical care clinical specialists. Although echocardiography has been used in cardiology for decades, it has developed as a single discipline.
Indeed, lungs have been judged unexplorable and, excluding the study and the characterization of pleural effusions, only recently other indications have been proposed.
Indeed, in the last decade greater attention to the many clinical applications of ultrasound
has produced a series of information that allow a sufficiently appropriate systematization of
this matter.
is knowledge has not be addressed to a single specialist, but to the needs of all doctors who
have to make diagnosis of chest diseases in critical situations.
Traumatologists, Emergency Physicians and Intensive Care doctors, Surgeons, Pediatricians,
Neonatologists, Intensivists, Cardiologists, and finally all those who have the opportunity
and luck to still consider the patient as a whole, are among these professionals.
On this track, a new semiotics is developing. Despite being instrumental, it strays far from
the classical diagnostic imaging (i.e. instrumental), to become a general cultural heritage for
bedside applications. e instrumental function of ultrasound is similar to the stethoscope
at the beginning of the last century and according to this philosophy, ultrasound is the
stethoscope of our time.
What is emerging is that at every environmental level, we should keep the traditional physical examination as historical culture and basic clinical methodology. But where it is possible,
ultrasound appears today as a cheap technology that must be known to every physician,
especially in today’s context of an increasingly quantitative and accurate medicine.
In today’s technological reality, in fact, many professionals are particularly advantaged by the
use of portable and miniaturized ultrasound that are easy to operate and almost limitless in
their applications. Not secondly, the opportunity to transform images into electronic data
already allows an unprecedented flexibility, even in unusual areas such as the chest and in
non-hospital environments.
is text covers the current concepts of bedside, point of care, focused and goal directed
diagnostics, which give effect to the needs of the clinician to achieve a rapid, economic, efficient and algorithmically simple diagnosis1.
Ultrasound is undoubtedly well suited to the realization of this philosophy. Its non-invasiveness
and repeatability, the miniaturization of machines, but also the chance to respond, particularly
in thoracic ultrasound, to clinical questions in binary terms yes/no, are bringing back to life in a
1
2 oracic ultrasound
modern way the late medieval Occam’s razor*, that is the most basic decision algorithm. is is
not an oversemplification. An opportune improvement of ideas, technique and applications, can
provide more quantitative peculiarities of thoracic ultrasound (Table 1).
Table 1 – Evaluation of ultrasound (qualitative vs. quantitative)
Pathology
Pleural effusion Yes/No Volume of effusion
Pneumothorax Yes/No Retro parietal air extension
“Wet” Lung Yes/No Grading of interstitial disease
Diffuse interstitial syndrome Yes/No Uniformity and symmetry of the
ARDS Yes/No Spread and grading of the syndrome
Pulmonary condensation Yes/No Number. Ventilation and dynamics
Dichotomous
evaluation
Continuous/quantitative evaluation
syndrome
(recruitment)
is book highlights an opportunity: the opportunity for a best diagnosis and for an efficient
clinical monitoring. is feature constantly appears in the pages of the book through original
images and video clips, with the purpose of illustrating specific dynamic aspects of the lung,
not always easy to understand through words.
Moreover the text integrates the knowledge of thoracic ultrasound according to the vision
that the lung cannot be separated from the heart, in traumatology and non-traumatic pathology. We believe that the heart/lung symbiosis (crosstalking) is constant in physiological
and pathological terms, and must therefore be borne in mind by those who study the lungs
with ultrasound.
e final section of the text shows the applications of ultrasound for interventional maneuvers
on the chest, which are much easier and safer with the guide of ultrasound.
e selection of bibliographical references at the end of each section is essential, but particularly cured and updated.
➣ Echography: basic concepts
Echography is a tomographic (from the Greek tomos: cut) survey that employs ultrasound. It
is capable of rebuilding oriented sections of parts of the body without the use of dangerous
energies. e physicians obtains sections (or planes) from a three-dimensional volume in
practically infinite and continuous sequences. is is the strength of the method, but usually
ultrasound needs a mental reconstruction of the original scanned volume. In thoracic ultrasound, however, many scans are tomography only from a technical and operational point of
view. e image itself is often a mixture of anatomical planes and of artefactual components.
is characteristic is the obvious consequence of the presence of air inside a not fully deflated
lung. Only a “solid” lung (without air) generates an anatomic, textured image.
* William of Occam (1287-1347) was an english scholastic philosopher. His principle, based on the law of
economy, states “Entia non sunt multiplicanda prater necessitatem” that can be translated as: “Entities should not
be multiplied unnecessarily”.
Basic principles 3
c=fλ
During a sonographic examination, the acoustic energy is produced by appropriate probes
operating in contact with the region to explore. e possibility of being realized to the bedside
and its easy repeatability are the most appreciated features of ultrasound.
➣ Ultrasound
Ultrasound are oscillating sound pressure waves with frequencies greater than 20,000 cycles
per second-Hertz (Hz) (20 KHz), higher than those detectable by the human ear (audible
sounds). Frequencies between 3 and 20 million Hz (MHz) are employed for diagnostic purposes. Physically, an ultrasound (like sound) is a wave that requires a transmission medium.
It is characterized by an amplitude, a wavelength and a frequency (Fig. 1).
Direction of the wave
Maximum
pressure
Amplitude
Minimum
pressure
1 cycle
Figure 1 – The acoustic waves. Main parameters.
Time
Wavelength
Frequency is the number of oscillations of the wave in a unit of time (measured in cycles per
second or Hertz). e wavelength denotes the distance between two crests of the wave, while
the amplitude correlates with the energy carried by the wave.
An inverse relationship between frequency and wavelength exists, while the propagation
velocity of ultrasound (c) correlates with the wavelength (λ) and the frequency (f) according to the equation:
Given a standard speed of the ultrasound wave of in the tissues (1540 m/sec), the wavelengths
of usual diagnostic ultrasound vary between 1.5 and 0.10 mm2.
➣ Historical notes on echography
In 1794 Lazzaro Spallanzani, observing the flight of bats, first hypothesized a new method
of animal orientation without the aid of visual stimuli. e acoustic sensitivity of bats allows
4 oracic ultrasound
them to fly in the obscurity by listening to emitted ultrasound waves (100KHz up to 200 KHz)
returning as echoes after hitting environmental obstacles**.
In 1880 Pierre Curie discovered the piezoelectric effect, where a periodic mechanical deformation of a quartz crystal by means of an electric current generates vibrations. An ultrasonic
transducer is a device that converts energy into ultrasound
During clinical sonography the probes containing ultrasonic transducers produce acoustic
energy that propagates through the air and different tissues (but not in a vacuum), at different
speeds. e speed of sound in the air is 340 m/sec, variable depending on the temperature,
while in water (and living tissues) is about 1530-1540 m/sec.
e idea of a medical use of ultrasound derived from the development of sonar in the Navy
in 1928.
In the thirties, Dussik used ultrasound to study the brain and Wild, in 1953, was the first to
employ transducers in contact with the skin for the study of the breast.
Ultrasound in obstetrics and gynecology was developed in the sixties, followed shortly thereafter by the use in cardiology. In 1959 Tanaka and Wagai published in Japan the first study
of Doppler flowmetry.
e innovation that radically changed the practice of ultrasound was the advent of real-time
equipment, whose tomographic power was a kind of technological revolution (Krause and
Soldner, 1965).
e electronic transducers entered the market in the second half of the seventies, while the
image quality improved significantly in the first half of the nineties.
Currently ultrasound is used for numerous purposes in almost all disciplines. We remember abdominal (elective or in emergency) and superficial tissues ultrasound (neck, thyroid,
scrotum, breast, muscles and skeleton), specialist ultrasound (obstetrical and gynecological,
echocardiography, pediatric, neonatal, vascular ultrasound), advanced ultrasound (endoscopic,
cardiac transesophageal echocardiography, transcranial, intravascular, with the use of specific
contrast media), and finally the recent lung ultrasound.
➣ Historical notes on chest ultrasound
A systematization of knowledge relative to chest ultrasound (indications for its use, description of symptomatology, but especially the identification of ultrasound artifacts of the lung)
has a relatively short history, not longer than fifteen years. Part of ultrasound of the chest,
such as heart and diaphragmatic ultrasound along with pleuropericardial effusions, can be
considered a peculiar application of general (abdominal) ultrasound or echocardiography,
and is more dated.
e first reports of the use of ultrasound for the diagnosis of pleural effusions appeared around
19602 and, in the immediately following decade the convincing studies were already a lot,
some of them concerning also thoracentesis procedures. In addition, in those years, the first
pioneering reports of ultrasound findings of intrathoracic solid lesions appeared3.
** “ Lazzaro Spallanzani (1729-1799) was an Italian biologist and physiologist. He wondered why bats could
navigate at night and catch insects as they flew. Abbot Spallanzani’s report about some species of bats, which after
being blinded, regularly perform during the flight the same reflexive movements that they perform when they can
see and that other birds can not, if they are blinded”.
Basic principles 5
e seventies and eighties marked the birth of ultrasound of the chest and pulmonary consolidations which, although not competitive with the prevailing radiography and the rising
computed tomography, allowed complementary diagnostic and interventional applications,
which became the capital of several pulmonologists and internists4. It is a type of ultrasound
that, with greater clarity, completeness and utility, also appears today in large parts of classical
texts about ultrasound, such as Mathis5.
e history of ultrasound diagnosis of pneumothorax is interesting because ultrasound changed
a consolidated paradigm of static medical imaging. Over twenty years ago particular attention
was paid to real scan thoracic (i.e. intercostal and targeted to the pleural level) looking for a
dynamic sign more than a morphological one. is sign was named “pleural sliding” and it
actually represents the movement of the pleural plane whose nature is mainly artifactual (i.e. it
is an acoustic reflex). Curiously, the diagnosis of pneumothorax by ultrasound in humans has
developed from veterinary experiences.
In 1986 Rantanen6 described the absence of pleural sliding as an sonographic finding of pneumothorax in horses. e following year Wernecke7 published a study on humans, identifying
the absence of sliding in eight subjects with radiological evidence of pneumothorax. From that
date until 1996 a dozen clinical trials have appeared in the literature involving a total population
of over 130 patients, allowing the identification of other sonographic signs of pneumothorax.
However over the past five years sonographic experiences in this field have multiplied. e studies published in this period have involved over 1500 patients and helped define the excellent
diagnostic accuracy of ultrasound8.
e use of ultrasound for the diagnosis of pneumothorax, overcoming a common wrong idea,
introduced a more consistent and useful comprehension of ultrasound of aerated organs and
of lung artifacts.
After pioneering and undeveloped reports, in 1997 Lichtenstein9 observed that the lung with
interstitial disease (mainly with edema) showed peculiar sonographic images called, with a
questionable (and questioned) term, comet tails. ese images, then defined as B lines, have
certainly marked the development of lung ultrasound until today. In fact, the dichotomy between
A Lines (normal) and B Lines (pathological) immediately appeared extremely attractive from
a diagnostic point of view. is basic distinction has unequivocally shown, in the beginning of
this century, that a certain amount of pulmonary diseases (especially in emergency) could leave
a clue constituted by the presence of Lines B. It also pointed out that ultrasound of the lung is
largely a particular ultrasound (perhaps not even ultrasound). In simplistic terms it is a “non
real” sonography or an ultrasound of artifacts. In our opinion, this is a crucial point: if it is not
well developed and understood, it can seriously affect everything that has been done, perhaps
hastily, up to now. It is a topic that will be particularly developed in this book.
➣ Echography today
e processes involved in the production and reception of the ultrasound signal and the image elaboration by the machine will be briefly described. Any further information is left to
the citations in the bibliography.
e ability of suitable transducers, placed in contact with the skin, to emit ultrasound and
receive from the explored tissues acoustic frequencies as echoes, is basic for producing sonographic images.
6 oracic ultrasound
Ultrasonography is therefore a technique that returns tomographic gray scale images (such
as slices – salami slices), oriented and adjustable in different spatial planes. In simple words
it produces anatomical images of body sections in modulated tones of black / white.
e ultrasound machine, regardless of its size and sophistication, is basically made up of a
body, the control panel, transducers (or probes) and peripherals (such as monitors, printers,
systems that can be connected with USB ports, the writer for CD etc.) (Figs. 2, 3, 4, 5).
Figure 2 – Ultrasound machine for fixed location.
A common electronic probe is formed by a series of small ceramic crystals (transducers) with
piezoelectric properties, generally of lead titanate or barium titanate zirconate. ey are in
number between 64 and 128. When a piezoelectric material is subjected to a mechanical strain,
it produces an electric charge. However, when speaking about ultrasound generation, the
reverse process is relevant. A longitudinal train of acoustic waves is the result of a mechanical
strain when an electric field is applied to the material. is sinusoidal field of compressions
and rarefactions is transmitted through the media and it is progressively attenuated.
e frequency of the acoustic signal that the transducer emits is determined by the thickness
of the crystal and by the potential difference which it is subjected to. Every single transducer
alternately sends and receives a series of impulses at a predetermined frequency, which cause
pressure variations in the time of the medium, characterizing the propagation of a wave.
Basic principles 7
e very short emission is interspersed with a short latency time, which is necessary for the
propagation of the ultrasound beam in the tissues and for the subsequent recording of the
reflected echoes. e period pulse/pause is called the pulse repetition period (PRP), the number
of signals emitted in one second is called the pulse repetition frequency (PRF). e time devoted
to the impulse is only a small fraction (0.1%) of the PRP. A relatively long period of listening
allows ultrasound to come back in the form of echoes, where the transducer is ready to receive.
Figure 3 – Modern portable ultrasound machine endowed with three main probes.
e ultrasound beam passes through the tissues explored, interacting with them. In essence
these interactions are based on the size of the targeted scatterers and can be expressed by class.
Within these classes the relationship between absorption, reflection and transmission accounts
for the appearance of biological tissues. In part the sound is attenuated, in part it comes back
to the transducer whenever it strikes acoustic discontinuities. ese discontinuities act as large
(specular) or small (diffusive) reflectors relatively to the exploring wavelength. Finally the probe
receives the returning echoes enabling the production, again through the piezoelectric effect, of
electric currents interpreted by the software of the ultrasound machine and converted into images.
e interaction of ultrasound with tissues, in relation to their acoustic impedance, is the basis
for the formation of echoes within the organs.
e acoustic impedance (Z) is a property of all mediums and corresponds to the forces that
oppose the transmission of the ultrasonic wave inside it (Fig. 6).
It is equal to the product of density (ρ) of the medium multiplied the speed of propagation
of sound through the medium itself (c):
Z = cρ
8 oracic ultrasound
Figure 4 – Accessories (peripherals) of an ultrasound machine. Monitor (a), recorder (b), printer and
burner (c), probes or transducers (c).
Figure 5 – Main types of probes. A: sector probe with relatively low frequency. B: linear probe with
high frequency. C: convex probe with intermediate frequency.
Basic principles 9
Incident
wave
Reected wave
Medium 1
Medium 2
Interface
Refracted wave
Figure 6 – An acoustic wave transmitting in different media, undergoing the well-known phenomena
of reflection and refraction.
e ability of the transducer to receive return signals from the medium, allows the electronic
system of the machine to interpret them in terms of tissue density and depth. Ultrasound
(and especially lung ultrasound) is therefore a densitometric investigation. e higher the
density of the structure investigated (compared to the contiguous one), the greater is the
signal received (echo), and much more brilliant is the corresponding pixel on the screen.
Much deeper the target, the greater is the time for the impulse to return to the transducer
(go/return time), according to the equation:
1
Depth =
(go/return time) × 1540 m/s
2
1540 m/s is the standard speed of ultrasound in biological tissues as assumed by the machine.
In reality this is slightly different depending on the various tissues traversed. Indeed the sound
runs faster through muscle tissue (1585 m/s) than through adipose tissue (1450 m/s).
During the listening phase the ceramic transducer receives the returning echo to the probe,
which is strongly linked to physical properties of traversed tissues. Ultrasound is therefore
mainly (electronic) listening.
Like any ondulatory energy, acoustic waves can interact with tissues and generate phenomena
classified in reflection, refraction and scattering (Fig. 7).
In the first case a tissue interface (a separation between two tissues with different acoustic
impedance) is sufficiently linear and large compared to the wavelength. If the incidence of the
wave is perpendicular (normal), a portion of energy is specularly reflected to the transducer
and not transmitted in depth.
In the second case, the wave transmitted through two physically different layers, undergoes
a deflection in relation to the variation of speed in the two media (Snell’s law).
Finally, if the surface of separation between two tissues (between two acoustic media) is irregular
or wrinkled, or the internal structure of the explored organ is finely irregular, the incident
energy is divided into multiple small fractions. ey are refected in various directions, each
of which can interact with different weight with the transducer (scattering).
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