Basic Notions

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Basic Notions

increasing experience, smoothing the image can be useful.

The freeze function is an apparently insignifi- cant function. When this button is pressed, it is easy to note that the image previously visible on the screen in real-time immediately looses a large percentage of its definition. This is only a detail, but if one operator provides a static image, and another operator interprets this image (as done in certain institutions), the full potential of emergency ultrasound is not exploited. The philosophy behind our use stems from deactivating this function. Emergency ultrasound is a dynamic discipline, which should be exploited in real-time only.

The possibility of freezing the image is useful if measurements are taken.

Step 1: Learning to Interpret Spatial Dimensions

Ultrasound has one particularity: contrary to radi- ography, CT or MRI, the operator creates the image. This initial weakness progressively becomes a strength. Spatial learning is the first step, the most important, and without doubt the most deli- cate to acquire, therefore requiring a rigorous approach. The operator must understand how to locate the elements displayed on the screen. The screen is broken down into four parts: upper, lower, left and right (Fig. 1.1).

The upper part of the screen represents the superficial areas. The head of the probe should be imagined at the top of the triangular image. The lower part of the screen represents the deep areas.

This is not a source of problems.

Interpretation of the left and the right parts of the screen is more complex, with two interdepen- dent items to control these areas of the screen: a lateral landmark on all well-designed probes and a left–right inversion button. This button should be configured once and for all so that, when the probe is applied to the abdomen, for instance, with the Notions of the physical properties of ultrasound

are not indispensable for the user. If needed, a reminder will be easy to find in many ultrasound textbooks.

In practice, the mastery of ultrasound follows four steps:

1. Learning to interpret spatial dimensions.

2. The composition of the image: which structures are indicated by gray, dark, and white tones.

3. The descriptive step: this step integrates the first two steps. It consists in the anatomical recognition and the description of the normal structures, then the pathological ones.

4. The interpretative step: this last step depends only on the culture that the operator will acquire, in books or in practice.

Preliminary Note

The ideal material is described in Chap. 2.An ultrasound unit includes several buttons and cursors.

The only functions we deem really useful to know at the beginning and for emergency use are:

∑ The switch-on button (which is not always easy to find)

∑ The gain setting

∑ The zoom (widening of the image)

The sole use of these three settings converts the complex device into a simple stethoscope.

Less useful functions in emergency use are, in our experience, the multiple postprocessing choic- es (we can always use the same one), positive–neg- ative inversions, and annotations. A word must be said on the processing of the image: the best in our opinion is no processing, notably excluding features such as the noise dynamic filter, which is critical for lung analysis. In addition, filters that would make artifacts vanish would also make lung ultrasound impossible. For visual comfort, with

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right-sided landmark, the right organs such as the liver should occupy the left side of the screen.

Conventions in medicine serve the purpose of rapid recognition of an image and establishing habits. As a striking example, when a chest radiog- raphy is held upside down, the image is extremely unusual and hard to analyze, although nothing has been modified. Therefore, the right organs will always be visualized on the left (of a radiograph, a CT scan, and an ultrasound scan). For transverse scans, the landmark is on the patient’s right and the right structures will appear on the left of the screen. For the longitudinal scans, the head should be imagined on the left of the screen, the feet on the right. This is a practical convention in imaging.

In cardiology, the opposite convention positions the head on the right of the screen. In our practice, and in order to work uniformly, the head-on-the- left convention is retained, including the heart. In a longitudinal scan of the liver and the kidney, the liver will be seen on the left of the image, the kidney on the right.

An ultrasound scan can be transversal, longitudinal or oblique. At the beginning, the operators should stick to longitudinal or transverse scans.

Later, with increasing experience, they will enjoy more flexibility.

Two main movements can be described (Fig. 1.2):

∑ Scanning, for example, a transverse scan begin- ning at the epigastrium and ending at the pelvis

∑ Rotation on its main axis: the study of a vessel on its long axis then on its short axis

These movements create significant changes on the screen, which can be unsettling at the beginning, perhaps the major difficulty of ultrasound, but also its strength. One travels through the so- called third dimension. These changes will be in- tegrated and become automatic with practice. A constructive exercise can be to pass from the short axis to the long axis of a large vessel such as the abdominal aorta.

Step 2: Understanding the Composition of the Image

How does one master the white, gray and black nuances of the images?

Gain

The gain control influences the gray scale. Optimal control of gain is crucial to obtain an interpretable image. Decreased gain will give a black image, and details will be masked. Increased gain will give a white image, and details will be saturated (Fig. 1.3).

In the units we use, the proximal, distal and global gains can be adjusted. It should be remembered that only practice provides efficient control of this function. That said, we modify the global gain from time to time, rarely the proximal and never the distal.

A liver–gallbladder scan can be used to establish a proper basis for gray-scale interpretation. The 4 Chapter 1 Basic Notions

Fig. 1.1. This figure shows spatial configurations. The probe is applied transversally on the abdomen. The right structures of the patient are displayed to the left of the screen (R). The top of the screen corresponds to the superficial area, i.e., the skin (S), the lower part of the screen to deep areas (D). The right diagram shows a reference CT scan

Fig. 1.2. The two main movements of the probe in the space. (A) Rotation along its long axis: the transversal scan becomes longitudinal scan. (B) Scanning: in a transversal approach, the upper then lower structures are displayed

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liver must appear gray, the reference for solid structures. The gallbladder bile must be black, the reference for fluids (Fig. 1.3C), on condition that there is no sludge (see Fig. 8.9, p 50).

Basic Glossary

An anechoic structure yields a black image, since no echo is generated. In the pioneering times of ultrasound, the images were inverted, i.e., anechoic images were represented as white. Logically, increasing the gain does not affect this state.

Inversely, an echoic structure generates echoes, and will give a gray image. It can be more (closer to white) or less (closer to black) echoic.

An image defined as hypoechoic, isoechoic or hyperechoic assumes that a reference image has been defined, usually the liver.

The term »acoustic window« designates a structure that is easily crossed by ultrasound. It thus allows analysis of deeper structures. This window can be physiological (the liver for the study of the kidney or the heart, the bladder for the analysis of the uterus) or pathological (pleural effusion

used as an acoustic window to study the thoracic aorta).

The basic term »echoscopy« is used to designate a dynamic sign: cardiac dynamics, ripples, changes in shape, lung sliding, and many others).

Artifacts

An ultrasound image is composed of real and unreal structures. Real structures are anatomical, unreal ones are artifacts. Artifacts are traditionally a hindrance, since they spoil the image. In our opinion, artifacts provide vital information which can be lifesaving. Artifact analysis is, for instance, the basis of lung ultrasonography. Their precise analysis is therefore crucial. Artifacts are created by the principle of propagation of the ultrasound beam. They are stopped by air and bones and accelerated by fluids.

All artifacts converge to the top of the screen, i.e., the head of the probe, like parallels or meridi- ans. They move with the probe movements. They are always regular, straight, i.e., totally different from the majority of the anatomical structures, A

C

B

Fig. 1.3. A Longitudinal scan of the liver, the inferior vena cava and the gallbladder. The near gain is too high:

superficial areas are saturated. B Same scan. The near gain is too low: superficial areas are now underexposed and again escape analysis. C Same scan. The gain is opti- mal. The hepatic parenchyma is now homogeneous, and a good-quality analysis is possible

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which exceptionally are strictly straight and strictly parallel or meridian.

An acoustic shadow is a regular, echo-free (i.e., black) image, which arises from a bony structure (Fig. 1.4). Information behind an acoustic window is thus hidden.

A reverberation echo, repetition echo, or impure shadow is generated by an air structure. A succes- sion of roughly horizontal (or slightly curved) lines generates a striped pattern, alternating dark and clear lines at regular intervals. They can be large (i.e., on the screen, they extend from the left to the right) or very narrow (see Figs. 16.1–16.3

and 16.11, pp 105). A reverberation echo hides the information below, as does an acoustic shadow. At the lung level, acoustic shadows of ribs and reverberation echoes of air regularly alternate, and no information is available under them. If the user is interested not in what happens under the pleural line (a domain that escapes ultrasound) but in what happens at the pleural line, he will realize that artifact analysis can be a discipline in itself.

Acoustic enhancement (Fig. 1.5) creates a more echoic pattern behind a fluid element. For example, the liver parenchyma is more echoic behind the gallbladder than lateral to it. Fluids give acoustic enhancement. In our emergency practice, this familiar artifact is rarely of use.

Last, a small fluid structure such as a blood vessel, surrounded by strongly echoic tissues such as fat will contain various parasite echoes within its lumen (see Chaps. 12–14 devoted to the vessels).

Elementary Anatomical Images

The approach from the anatomical structure (Table 1.1) means:

∑ A solid tissular mass is echoic: parenchyma, muscle, thrombosis, alveolar consolidation, or tumor.

∑ A pure fluid mass is anechoic (with acoustic enhancement: circulating blood, vesicular bile, urine, pure fluid collections).

∑ A pathological fluid mass can be rich in echoes:

abscess, hematoma, thick bile, necrosis, etc. If the collection contains tissular debris or bacter- ial gas, it can be highly heterogeneous. Varia- tions in shape, possible acoustic enhancement, 6 Chapter 1 Basic Notions

Table 1.1. Elementary ultrasound images Real structure Artifact

Black tone Pure fluid Acoustic shadow Gray tone Parenchyma Acoustic

Alveolar enhancement

consolidation Thick fluid Thrombosis

White tone Bone or calculus Gas Fat

Valve Interface Fig. 1.4. Posterior shadow (asterisk) generated by a lar-

ge calculus (white arrow) in the gallbladder. The bile appears anechoic

Fig.1.5. Acoustic enhancement (X) arising from the gall- bladder (G). This sign is of interest when the examined site is poorly defined, since its fluid nature is demonstrated. This is example of a figure that provides the answer to a clinical question (fluid or solid mass?), in spite of the extreme bad quality of the image

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and detection of a dynamic movement within the mass indicate a fluid.

∑ A gas structure is hyperechoic with posterior echoes of reverberation: air or microbial gas.

∑ Deep fat is hyperechoic such as mesenteric fat.

∑ An ossified structure is hyperechoic with poste- rior shadow: bone or calculus.

∑ An interface between two anatomical structures results in an hyperechoic strip: pleural layers, diaphragm, cardiac valves, interface between liver and kidney, etc.

The approach from the encountered echostructure means:

∑ An anechoic image can be:

– An artifact: the shadow of a bone or calculus – A real image: pure fluid

∑ An echoic image can be:

– An artifact: acoustic enhancement – A real image: normal

Parenchyma

– A real image: pathological Solid

Thrombosis

Alveolar consolidation Hematoma

Necrosis

Fluid: thickened bile, abscess, noncirculat- ing blood

∑ A hyperechoic image can be:

– An artifact: reverberation of gas structure – An anatomical structure: surface of a bone,

surface of a calculus, surface of a gas bubble, deep fat, cardiac valve, or interface

Step 3: Ultrasound Anatomy: Descriptive Step

Ultrasound anatomy is easy to study when the basic rules of ultrasound and general anatomy are acquired. One difficulty of ultrasound (which also makes it thrilling and unique) is that this anatomy can be studied in the three dimensions. The operator’s hand must (or can) make rotating, pivoting or scanning movements, and she thus creates an image to a certain extent. Some clues can be given here. A tubular structure (vessel, bowel loop) will be followed along a certain length when the area is scanned. The practice, at the beginning, of strictly transverse or strictly longitudinal scans will make the images more quickly familiar.With experience, the probe will follow the true axis of the structures.

As an example, the analysis of the inferior vena

cava can begin in a transverse scan of the epigastrium, probe scanning from the top (head) to the bottom (feet). Once the vein is located, it can be aligned on its long axis by rotating the probe. With experience, it is possible to directly begin by a longitudinal approach to this area, the probe scanning from left to right until the vein is found.

Dimensions can be accurately measured by freezing the image and adjusting electronic land- marks, or calipers. In the figures of this book, the edge of the image displays a centimeter scale, making it possible to measure any structure therein.

How Should One Optimize the Quality of the Image?

Several maneuvers are available and experience plays a major role. Knowledge on how to adjust the gain, for instance, is acquired only with experience.

The operator should always attempt to find an optimal acoustic window. For instance, interposi- tion of liver parenchyma between the probe and the heart will optimize, in some cases, subcostal cardiac analysis. In other words, it is sometimes wise to move away from the target to better visual- ize it. Bones and digestive gas should be avoided.

Subcostal organs will be best exposed when the probe is applied strictly against the lower rib.

It is important that the probe remain absolutely still, because the great majority of signs studied in emergency ultrasound is based on dynamic signs.

Holding the probe like a pen, with the hypothenar eminence firmly resting on the patient’s body, makes this absolute immobility possible and pre- vents fatigue during a prolonged examination. In addition, this absence of motion will prevent what we call the out-of-plane effect, an effect that creates the illusion of dynamics caused by the movement of the probe alone.

Impediments to Ultrasound Examination

An ultrasound image in a patient with clearly echogenic structures is gratifying, because a defi- nite diagnosis can be made and therapeutic man- agement can be immediately instigated. However, particularly at the beginning, several factors can lead to something of an esoteric fog, which will give this method an unwarranted sense of inacces- sibility.

Gas and ribs interrupt the image. This is one of the major drawbacks of ultrasound that is not found with CT and MRI.

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Bowel gas is, per se, an inescapable obstacle.

However, an acoustic window may exist between two gases. A gas can move, like a cloud hiding the sun. Therefore, before concluding that the examination is impossible, the approaches must be diversified: one must sometimes wait a few min- utes and try again. In addition, the operator’s free hand may be able to shift the gas.

Air at the lung level is considered an absolute obstacle. We will see that this dogma should be revised.

Bones are absolute obstacles. The adult brain should therefore not be examined with ultrasound.

We will see, however, that fine bones (maxillary bones, scapula) are transparent to the ultrasound beam. Using these windows, ultrasound will extend its territory throughout the entire body.

In certain patients, the liver and spleen are entirely hidden by the ribs and cannot be analyzed using the abdominal approach. One must proceed by the intercostal approach, which often results in incomplete images.

Obese patients are not good candidates for ultrasound. This is true for deep structures, where a 3.5-MHz probe will often be inadequate (and a 5-MHz probe even less so). However, paradoxical- ly, critical data can be extracted from analysis of superficial structures. An edifying example is the anterior pleural line, which remains accessible even in extremely obese patients.

A patient covered with extensive dressings will be difficult to examine. This is mainly the case in surgical intensive care units.

In rare young and thin patients, the skin is not a good conductor of ultrasound, and the examination can be disappointing.

In daily practice, an examination that con- tributes nothing is almost never seen. In fact, ultrasound always answers a clinical question in one of two ways:

1. The item was clearly analyzed, its normal or pathological features are clearly demonstrated, a situation encountered in 80%–90% or more of cases.

2. The conditions are suboptimal, there is a risk of making a mistake, or it is totally impossible to explore a structure, a situation seen in 10%–20%

or less of cases.

Step 4: Interpretation of the Image

Only the operator’s familiarity with the field, enriched by reading the literature and personal experience (an empirical approach, of interest in a field where much remains to be said), will indicate which conclusions can be drawn from, for instance, a gallbladder wall measured at 9 mm. Previously, this operator has carefully learned to switch on the ultrasound unit, check for the proper gain, locate the gallbladder and take an accurate measurement of its wall.

When no training structure is available, or while waiting for personalized training, two practices can allow one to progress in ultrasound:

∑ Reading any anatomy textbook will be a good reminder to understand the location of the organs in space.

∑ Making as many examinations as possible alone and comparing one’s conclusions with those of more experienced operators.

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