ACRT Journal

Introduction

Aristotle was the first to discuss the concept of the mind as a “blank slate,” though his theory focused more on the flexibility of the human mind than on its initial purity.

John Locke, a prominent philosopher, later developed the concept of Tabula Rasa, which means “blank slate.” Locke suggested that we are all born with a mind that is free of pre-existing content, like a blank slate on which experiences and events can be written. The Romans used wax-covered wooden tablets as slates, erasing them by scraping with the flat end of a stylus, an image Locke used to illustrate the mind at birth, before it gets knowledge.

However, Locke did not specify which types of memories are stored more effectively. When considering memory types, visual memory often takes precedence. If we consider a baby’s mind, infants initially experience the world primarily through vision. They do not yet understand words, but they can recognize familiar sights. This suggests that visual memory may be one of the first and most effective ways of storing information [1].

Studies show that auditory memory is reliable for immediate recall, while visual memory is more relied upon for information stored in our long-term memory [2].

While years ago, Daniel Lichtenstein had to confront opinions and resistance against the use of ultrasound in diagnosing and treating acute pulmonary diseases, it is now clear that the benefit of lung ultrasound (LU)examination in daily practice is clear, not only for pulmonologists but also for internists.

According to the study that compare LU accuracy with x-ray examination and auscultation, the auscultation has a diagnostic accuracy of 61% for pleural effusion, 36% for alveolar consolidation, and 55% for alveolar-interstitial syndrome. Bedside chest radiography had a diagnostic accuracy of 47% for pleural effusion, 75% for alveolar consolidation, and 72% for alveolar-interstitial syndrome. Lung ultrasonography had a diagnostic accuracy of 93% for pleural effusion, 97% for alveolar consolidation, and 95% for alveolar-interstitial syndrome. Lung ultrasonography, in contrast to auscultation and chest radiography, can quantify the extent of lung injury [3].

In many published studies, the appearance of a normal lung is described as a bat sign: a space limited above by the pleural line, on the sides by the borders of the rib shadows, and below by the screen. This space has been called Merlin’s space by Lichtenstein. Merlin space is considered homogeneous, with twinkling’s that, according to many studies, are caused by pleural sliding [4-9].

From our experience, we saw that when a normal person or patient holds their breath, or in the case of pneumothorax, when pleural sliding is absent, these twinkling’s do not disappear, showing that pleural sliding is not the primary cause of the scintillations. The depth setting of the curved linear probe is crucial, as a proper setting allows for a more correct assessment of the areas of Merlin space as first step.

A depth of 16-18 cm on the curved transducer is reasonable. At this depth, we can visualize not only the pleural surface but also deeper structures, allowing us to see the normal or abnormal structure of the Merlin space. This depth enables us to assess pathology extending beneath the pleural surface and deeper. If a more detailed evaluation of the pleural surface is needed, a depth of 6-10 cm is best.

The density of Merlin’s space is not homogeneous. The structure of Merlin’s space consists of a hyper-echoic twinkling white area that starts from the pleural line and descends downward in a truncated cone shape, bounded laterally by rib shadows, bellow by a hypo-echoic twinkling gray colour within black area. This hypo-echoic twinkling gray colour within black area is bounded below by the screen and above by a hyper-echoic twinkling white area. The hypo-echoic twinkling gray colour within black area is not bordered by the ribs’ shadows, which is an another feature that distinguishes it from the hyper echoic twinkling white area. This feature has its importance because the extension of rib’s shadow to the bottom of the screen is abnormal and sign of “sick bat”.

According to Lichtenstein and other authors, the “bat sign” is a configuration composed of the ribs and their shadows, standing for the bat’s wings, and the pleural line, representing its body. When examining the bat’s wings from beginning to end (where the beginning is the ribs and the extension are the shadows of the ribs), the body cannot be depicted as a simple line but rather as a truncated cone with its base at the pleural line. Thus, the bat’s body aligns with the hyper-echoic, twinkling area in the Merlin space. Therefore, it is crucial not only to see the bat’s wings but also its body. Recognizing the normal appearance of the bat’s body (hyper echoic twinkling white area) along with the bat sign (ribs’s line and shadow, pleural line) is essential for early detection of injuries, known as “sick bat,” in the lungs.

Therefore, we suggest defining the Merlin space as follows: The Merlin space is the hyper-echoic twinkling white area below the pleural line, delimited laterally by rib shadows and inferiorly by the hypo-echoic twinkling black area extending to the bottom of the screen. This definition more accurately reflects the real presentation of the Merlin space and helps in understanding the changes it undergoes in abnormal lung conditions (Figure 1).

Figure 1: New concept of Merlin space [10].

Knowing this normal structure which to our knowledge has not been described in this way in details but Merlin’s space has been considered homogeneous—allows us to use ultrasound to discern when “something” is not normal at the lung periphery. If we are not familiar with the “healthy bat” image, it will not be easy to recognize the “sick bat’s” images with various diseases, especially in the initial stages.

Since the alveoli at the apices of the lungs are more expanded than those at the bases, and the air interstitial tissue ratio at the interface is greater, this results in the twinkling white area of the Merlin space being less dense and having a shorter length. Given that the alveoli in the middle and caudal regions of the lungs are less expanded due to increased trans pulmonary pressure, the air/tissue ratio at the air– interstitial tissue interface is smaller than in the apices, resulting in the twinkling white area of the Merlin space being denser and longer. Typically, the length of the hyper-echoic twinkling area ranges from 3.5 cm to 6.5 cm. This area is a result of reverberations, scattering, and reflections of ultrasound passing through the secondary lobule and both pleurae [10].

The pulmonary interstitial tissue is closely next to the visceral pleura. The density of the interstitial tissue varies from the apex to the base of the lungs and changes with different depths of breathing. The density of the interstitial tissue is distinct from that of the visceral pleura. In both healthy and diseased subjects, deep inspirations are followed by a decrease in density, shortening of the length, and a change in the shape of the Merlin space’s twinkling white area. The opposite happens in deep expiration.

The hyper-echoic twinkling white area seen in severe air trapping is characterized by lower density at the apex and higher density at the bases compared to normal individuals or those with mild obstruction. Unlike in normal individuals or those with mild obstruction, the size of this hyper-echoic twinkling area remains consistent between maximal inspiration and expiration. Indirectly, through abnormal contour images of the twinkling white area of the Merlin space, we can deduce the presence of past or present active changes. Irregularity of the white twinkling area borders can be the only finding of lung abnormality. The homogeneity of the white twinkling area of the Merlin space is particularly important. The presence of hypo-echoic or hyper-echoic micro-densities within it should prompt consideration of post-inflammatory fibrosis or latent chronic inflammatory processes [10]. We continue to think that the hyper-echoic twinkling white part of the Merlin space is not an artifact, but a hyper-echoic density created not only by lung sliding but also by the presence of peripheral interstitial tissue in secondary lobule, which has a different impedance than the pleura itself and, of course, air. In the twinkling part, we see the A-line, but also a bright white area between them. Furthermore, in not a few cases, this hyper-echoic twinkling white space exists alone, without the A-line.

The shape, intensity, and length of the Merlin space’s twinkling white area depend not only on the thickness of the thoracic wall and the angle of the ultrasound probe but also on changes in the subpleural peripheral interstitial tissue, which is thicker anteriorly than posteriorly [10].

Ultrasound of the lung has its “butter side” and “seamy side.”

The “butter side” of LU

The butter side consists in the indisputable finding of lung ultrasound (LU) in acute diseases, that there is something wrong in the periphery of lung even when physical examination and X-ray did not show anything. While the x-ray had the sensitivity of 49% (95% CI, 40–58%) and specificity of 92% (86–95%), lung ultrasound had an overall sensitivity of 95% (92–96%) and specificity of 94% (90–97%) [11].

This is particularly true for ground-glass opacities, acute and chronic inflammation of the periphery of the lung (increase in the length of hyper-echoic density of the Merlin space), and minimal pleural effusion. LU also detect tiny peripheral lung inflammatory pathologies that “escape” X-ray examination. LU can help us make a brief list from a lengthy list of findings. When combined with cardiac ultrasound, we can have a more precise and early diagnosis. LU can eliminate the need for X-ray, which is not a free lunch, avoiding radiation exposure and the inconvenience of transporting patients to the radiology department, which may be far from the clinic [12-14].

The tilting technique is extremely helpful and useful for discovering more details about lung lesions. In one position of the transducer, we can get an image that differs from the one obtained from another position without displaying it. Through ultrasound, we can detect diaphragmatic paresis and paralysis in patients with respiratory failure and normal alveolar-capillary gradients or mediastinal infiltration of lung cancer.

LU can help diagnose COPD in smoker patients with more than 20 pack-years. The length of Merlin’s space hyper echoic twinkling white area is increased in acute exacerbation of COPD and asthma. The length of Merlin’s space hyper echoic twinkling white area is increased in cases of lung cancer, atelectasis, and subacute or post-inflammatory events. The dilemma lies in distinguishing between increased Merlin’s space hyper echoic twinkling white area or posterior enhancement. Merlin’s space hyper echoic twinkling white area length is decreased in cases of severe obstruction of COPD and asthma in stable condition due to increased alveolar air trapping.

When increased Merlin’s space length is found in obstructive syndromes (such as COPD and asthma), we must consider subacute or acute concomitant inflammation, whether due to infection, autoimmune causes, or malignancy.

The length and width of the hyper-echoic twinkling white area of Merlin space, as well as the rib’s hypo-echoic shadow, also depend on the angle of the probe. The length of Merlin’s space hyper-echoic twinkling white area is longer in the longitudinal axis than in the short one. Shaggy, irregular borders of Merlin’s space’s hyper-echoic twinkling white area are characteristic of lung cancer, pneumonia, chronic inflammation, or post inflammatory residue.

Irregularity in the rib’s acoustic shadow of Merlin space is another indirect sign of an abnormality, whatever its nature.

When we are in front of a waterfall hyper-echoic density image, let’s try to “dig” by tilting to discover if there is a hypo-echoic densities inside the hyper echoic density. By finding the waterfall sign, we may trace the real lesion. Thus, the waterfall sign may be the only sign of pulmonary damage, or it may be a secondary, accompanying sign.

What is our diagnostic approach in the case of peripheral hypo-echoic densities?

Differential diagnosis must be done according to the location, shape, size, and structure of the hypo-echoic density. C-lines usually appear in diffuse lung diseases and chronic inflammatory conditions or as residual fibrotic lesions. Multiple larger hypo-echoic densities, resembling “deep bites” of lung periphery and sometimes looking like a “front sight in rear sight” of a gun, suggest lung infarcts due to pulmonary embolism.

Peripheral sizable round hypo-echoic densities suggest the possibility of peripheral tumors, which must be differentiated from lobulated pleural effusion or cystic formation using Doppler mode. Doppler ultrasound showing vascularization of hypo-echoic densities, along with heterogeneity, supports a diagnosis of lung tumor.

There are three types of atelectasis: 1- obstructive atelectasis, 2- contraction atelectasis, and 3- compressive atelectasis. Each has different ultrasound presentations [15]. Obstructive atelectasis appears as a hyper-echoic density, light gray in colour, with tubular hypo-echoic densities representing the vascular tree inside lung tissue. During deep breathing, discrete hyper-echoic densities, like small dots, may appear, standing for air in the bronchi.

Compressive atelectasis has a similar appearance but moves like a jellyfish during respiration, hence the “jellyfish sign.”

In contraction atelectasis, hyper-echoic densities are combined with hypo-echoic areas, and these densities are heterogeneous due to chronic contractive inflammation of lung tissue. Unlike obstructive atelectasis, its colour ranges from solid gray to black, and the vascular tree’s hypo-echoic densities are absent.

By determining the dilatation of the inferior vena cava and assessing the presence or absence of collapse, lung ultrasound can narrow down a lengthy list of diagnostic possibilities or help of showing comorbidities. LU findings are particularly useful when combined with each other specifically lung, heart, and abdominal ultrasounds [16,17].

For tracheal infiltration, ultrasound can detect septal rocket B-lines without the deletion of the A-line.

 Pulsating of lung in LU may be an indirect sign of atelectasis, lung consolidation caused by inflammatory, malignant processes or absence of normal lung ventilation. The presence of lung pulse rules out the diagnosis of pneumothorax [18].

The absence of a sinusoid sign in M-mode in cases of pleural effusion is a sign of fibroproductive pleural effusion or an echinococcal cyst mimicking pleural effusion. The combination of the jellyfish sign with hyper-echoic densities in pleural effusion, reduced diaphragmatic movements, or lung hepatization is characteristic of malignant, parapneumonic effusion or empyema. 

Alternating hyper-echoic and hypo-echoic densities within the pleural space suggest the diagnosis of septated effusion characteristic of empyema, chylothorax, malignancy, or pulmonary echinococcosis.

 Lung ultrasound (LU) has an irreplaceable value in diagnosing echinococcal cysts in the first stage, where its sensitivity and specificity are higher than those of computed tomography (CT). The double-layered wall sign and the identification of “sand” inside the cyst are pathognomonic findings for the diagnosis of echinococcal cysts in the early stage.

In the ultrasound examination of the superior lobes, particularly in the posterior regions, signs such as pleural thickening, the delineation between the parietal and visceral pleura, C-lines, B-lines, and an increase in the density and length of hyper-echoic twinkling white areas should primarily suggest active tuberculosis or post-tubercular fibrosis.