Learn more about thoracic ultrasound

BTS Pleural Disease Guidelines 2010

Ultrasound physics

Medical ultrasound uses sound waves between 2.5 and 12 MHz generated by a transducer to interrogate tissue. The sound waves are attenuated as they travel through tissue. Some or all of these waves are re
flected at the interface between tissues where a difference between tissue impedance exists. The returning waves are detected by the transducer and converted into an image. An understanding of the physical laws governing the transmission of sound waves in solids and fluids will facilitate an understanding of the acquired image and optimisation of the scanning technique. Fluid is an excellent conductor of sound waves and appears black on ultrasound whereas air effectively blocks all transmission of sound waves and generates a random snowstorm image. Internal organs such as the liver or spleen have variable echogenicity depending on the proportion of sound waves reflected by the structure. The maximal depth and resolution of an ultrasound image is related to the frequency of the sound waves. Lower frequencies have longer wavelengths and hence better tissue penetration but lower resolution. Higher frequencies have shorter wavelengths which provide higher resolution images and at a greater refresh rate but poor tissue penetration.
 
Normal thoracic ultrasound appearance

Ultrasound examination of the thorax is limited by air within the lungs, which is a poor conductor of sound waves, and the acoustic shadow caused by the bony structures surrounding the thorax such as the ribs and scapulae. However, the concept of an acoustic window has allowed for effective ultrasound examination of the thorax in the presence of pleural pathology such as a pleural effusion or pulmonary consolidation or tumour abutting the pleura. The normal thoracic ultrasound appearance is well described. With the transducer held in the longitudinal plane, the ribs are visualised on ultrasound as repeating curvilinear structures with a posterior acoustic shadow. The overlying muscle and fascia are represented by linear shadows of soft tissue echogenicity. The parietal and visceral pleura is usually visualised as a single echogenic line no more than 2 mm in width which slidesor glidesbeneath the ribs with respiration when using a low-frequency transducer. Two separate lines can be visualised when using a high-frequency transducer. Normal aerated lung blocks the progression of sound waves and is characterised by a haphazard snowstorm appearance caused by reverberation artefact which diminishes in intensity with distance from the transducer. Comet-tail artefacts can also be seen due to imperfections within the pleura and are best seen at the lung bases. The diaphragms are bright curvilinear structures which move up and down with respiration. The liver and spleen are readily recognised by their characteristic ultrasound appearance below the right and left hemidiaphragm, respectively.

Abnormal thoracic ultrasound appearance

Pleural effusion:

Ultrasound has a higher sensitivity in the detection of a pleural effusion than clinical examination or chest x-rays including a lateral decubitus film. film. The ultrasound appearance of a pleural effusion is an anechoic or hypoechoic area between the parietal and visceral pleura that changes shape with respiration. Other sonographic characteristics of pleural fluid are swirling echo densities, flapping or swaying tonguelikestructures due to underlying compressive atelectasis of the lung and movable septae. Depending upon its internal echogenicity and the presence of septations, a pleural effusion can be classified into anechoic if totally echo-free, complex non-septated if echogenic swirling densities are present, or complex septated if fine strands are present within the fluid. Anechoic effusions can be either transudates or exudates, but complex effusions are always exudates. The volume of pleural fluid can be calculated using various formulae, but these are mainly applicable to patients receiving mechanical ventilation and are difficult to apply in practice to non-ventilated patients. The following alternative classification has been suggested by Tsai et al: (1) minimal if the echo-free space is within the costophrenic angle; (2) small if the echo-free space extends over the costophrenic angle but is still within a single probe range; (3) moderate if the echo-free space is between a one to two probe range; and (4) large if the space is bigger that a 2 probe range. Furthermore, a pleural effusion is usually considered too small to tap if it is <1 cm in depth.  

Pleural thickening

Occasionally a minimal pleural effusion can be hard to distinguish from pleural thickening which may manifest as an anechoic or hypoechoic stripe. The presence of a chaotic linear colour band between the visceral and parietal pleura using colour Doppler has a higher sensitivity for detecting pleural fluid than grey scale ultrasound alone and this is known as the fluid colour sign. However, the routine application and interpretation of this is likely to be beyond the expertise of the non-radiologist. 

Malignant pleural effusion

Thoracic ultrasound can facilitate the diagnosis of a malignant pleural effusion. The presence of pleural or diaphragmatic thickening or nodularity or an echogenic swirling pattern in patients with known malignancy is highly suggestive of a malignant pleural effusion.

Pulmonary consolidation

Pulmonary consolidation is sonographically visible in the presence of adjacent pleural effusion acting as an acoustic window or if directly abutting the pleura. It appears as a wedgeshaped irregular echogenic area with air or fluid  bronchograms. On colour Doppler ultrasound, branching tubular structures with colour flow is visible.  

Parapneumonic effusions and empyema

Parapneumonic effusions are usually hyperechoic with septae but can be hyperechoic without septae and even anechoic. Ultrasound is better than CT at demonstrating septae. However, CT is preferred in complex pleuroparenchymal disease as it is better at delineating the relationship between loculated pleural collections, parenchymal consolidation and the mediastinum. The presence of septae does not imply loculations as the fluid may still be free flowing within the hemithorax. In a study of 36 patients with proven parapneumonic effusion or empyema, Kearney et al did not find any correlation between the ultrasound appearance and Lights stages of empyema, the presence of pus or the need for surgical intervention. In contrast, two other studies have shown that septated parapneumonic effusions have a poorer outcome. Chen et al showed that sonographically visible septations were associated with a longer hospital stay, longer chest tube drainage, higher likelihood of fibrinolytic therapy and surgical intervention and Shankar et al found that a complex septated parapneumonic effusion had a 62.5% resolution rate with chest tube drainage compared with 81.5% with a complex non-septated parapneumonic effusion. Ongoing pleural infection despite adequate antibiotic therapy is often due to suboptimal placement of the chest drain, particularly in the presence of loculations. Two studies have demonstrated the utility of ultrasound-guided chest drainage as the principal treatment for parapneumonic effusion or empyema with an overall success rate of 78% and 72%.80 Factors associated with failure were small-bore chest tube blockage, persistent pneumothorax or a pleural peel.

Pneumothorax

The presence of a pneumothorax and hydropneumothorax can be inferred sonographically by the absence of pleural ‘slidingand the presence of reverberation artefact. The utility of thoracic ultrasound for diagnosing a pneumothorax is limited in hospital practice due to the ready availability of chest x-rays and conflicting data from published reports. In a study of 53 patients following a transbronchial biopsy or chest drain removal, thoracic ultrasound using a high-frequency transducer and apical scans had a sensitivity and specificity of 100% for the detection of postprocedure pneumothorax compared with a chest x-ray or CTscan of the thorax. An earlier report comparing ultrasound with CT scanning showed a lesser sensitivity following lung biopsy, and a recently published report suggested that ultrasound was less sensitive and specific in patients with emphysema.  

Thoracic ultrasound technique

The technique for thoracic ultrasound is well described in several review articles and by Koh et al in an online review article containing images and videos. The patient should be positioned either in the sitting or lateral decubitus position if critically ill. The chest x-ray should be reviewed before the ultrasound examination. The ambient lighting should be reduced to maximise screen contrast. In general, a 3.5 -5 MHz sector transducer provides good views of intrathoracic and upper abdominal structures including pleural fluid. A 5 - 10 MHz linear transducer should be selected for detailed examination of the pleura. Acoustic gel should be applied between the transducer and the area to be examined. The transducer should be held like a pen, applying firm pressure upon the skin to maximise acoustic coupling while resting the medial aspect of the palm upon the chest. The image should be optimised by adjusting the depth, gain and focus. The depth should be adjusted until the area of interest fills the entire screen, while the gain should be increased or decreased to maximise the contrast between different tissues. 

Examination should commence with the transducer placed within an interspace on the posterior chest wall on the side of interest. The transducer should be moved obliquely along the interspace (avoiding the acoustic shadow cause by reflection of the ultrasound by the ribs) in both the transverse and longitudinal planes, thereby minimising interference from the acoustic shadow from the ribs. It is imperative that the diaphragm is unequivocally identified before any invasive procedure to avoid inadvertent intra-abdominal penetration. The thorax should be examined posteriorly, laterally and anteriorly, particularly when a loculated pleural effusion is suspected. The thorax should be examined using grey-scale real-time ultrasound, paying particularly attention to location, sonographic appearance and echogenicity. The echogenicity of a lesion is defined relative to the liver which, by definition, is isoechoic. The contralateral thorax can be used as a control except where there is bilateral pleural pathology.

Ultrasound-guided pleural aspiration and chest drain insertion

The identification of a site for pleural aspiration using physical examination can be straightforward in the presence of a large free-flowing pleural effusion, but image guidance is recommended for all procedures as discussed above. When using ultrasound to select a site for aspiration of a pleural effusion, the site chosen should have (1) sufficient depth of pleural fluid (at least 10 mm), (2) no intervening lung at maximal inspiration and (3) minimal risk of puncture of other structures such as the heart, liver and spleen. It should be noted that ultrasound will not prevent inadvertent laceration of the intercostal neurovascular bundle, particularly where they run within the intercostal space medial to the angle of the rib. Once a site has been localised, it should be marked either with an indentation or indelible ink and a mental note made of the maximal depth of fluid present and the required angulation of needle insertion. It is preferable to perform the aspiration at the time of the ultrasound rather mark a spot for subsequent aspiration as any alteration of the patients position may significantly alter the relationship between the skin marker and the underlying pleural fluid. Real-time guidance using a freehand approach may be necessary in small or loculated pleural effusions. The technique of ultrasound-guided chest drain insertion is similar to that for pleural aspiration. The main purpose of ultrasound is to identify a safe site for aspiration of fluid followed by insertion of the chest drain. The procedure is rarely performed under real-time guidance

Pleural procedures within the critical care setting

Ultrasound guidance reduces the complications associated with pleural procedures in the critical care setting and its routine use is recommended. (C) Thoracic ultrasound within the critical care setting is especially useful due to the portability of the equipment when treating and diagnosing relatively immobile patients. Erect and, less commonly, decubitus chest x-rays are frequently used to diagnose pleural effusions. However, these views are rarely possible in critically ill patients. Diagnosis of pleural effusions on supine films is much more challenging and frequently inaccurate. The use of bedside ultrasound by appropriately trained intensivists has been shown to safely identify and guide aspiration of pleural effusions in mechanically ventilated patients. Of the 44 effusions that were aspirated during this study, the pleural effusion was not evident on a supine chest x-ray in 17 cases. Ultrasound guidance is strongly recommended in this setting, not only because the diagnosis of pleural effusions is more difficult but also because the consequence of complications is often more serious. With ultrasound-guided procedures the complication rate is similar to procedures undertaken in other settings.

Thoracic ultrasound training

At least level 1 competency is required to safely perform independent thoracic ultrasound. (U) Thoracic ultrasound is a very operator-dependent procedure where imaging acquisition and interpretation are carried out simultaneously. There is little evidence to specify the length of training required for a non-radiologist to become competent in basic thoracic ultrasound. In the UK the Royal College of Radiologists has published guidelines establishing the minimum standards required to achieve basic or level 1 competency in thoracic ultrasound. Although the guideline defines a minimum number of supervised procedures, it should be recognised that some individuals may require more supervision to achieve competency in thoracic ultrasound. An additional 100 scans to achieve level 2 standard or 2 years further experience at level 1 standard would allow the individual to train others to level 1 thoracic ultrasound standard. In practice, more scans are required beyond level 1 competency to achieve a reasonable level of expertise in thoracic ultrasound, particularly where there is loculated pleural fluid. It is advisable for the novice to start with patients with simple free-flowing pleural effusions before moving on to patients with complex pleural or pleuroparenchymal disease. The images should be correlated with the CT scan of the thorax or advice should be sought from a radiologist if the individual is unable to interpret the acquired images.
 Resources:
 
 
BTS Guidelines:
 
 
 
Royal College of Radiologists:
 
 
 
Britich Medical Ultrasound Society
 
 
 
Point of Care Ultrasound