Ultrasound of the pediatric chest

Abstract

Cross-sectional imaging modalities like MRI and CT provide images of the chest which are easily understood by clinicians. However, these modalities may not always be available and are expensive. Lung ultrasonography (US) has therefore become an important tool in the hands of clinicians as an extension of the clinical exam, which has been underutilized by the radiologists. Reinforcement of the ALARA principle along with the dictum of “Image gently” have resulted in increased use of modalities which do not require radiation. Hence, ultrasound, which was earlier being used mainly to confirm the presence of pleural effusion as well as evaluate it and differentiate solid from cystic masses, is now being used to evaluate the lung as well. This review highlights the utility of ultrasound of the paediatric chest. It also describes the normal and abnormal appearances of the paediatric lung on ultrasound as well as the advantages and limitations of this modality.

Introduction

Ultrasonography (US) has until the recent past been underutilized for evaluation of the lung. The bony thorax and presence of air within the lungs were thought to interfere with the transmission of ultrasound waves.¹ US was initially used only for detection and evaluation of pleural fluid, later for differentiation of cystic and solid lesions abutting the chest wall. Its further role in imaging the lung and extra cardiac mediastinum has evolved in the recent past.^2,3

Although plain radiographs still remain the initial modality for paediatric chest imaging, there has been a significant increase in the clinical utility of chest ultrasound in recent years. Improvement in transducer technology and colour flow imaging, widespread application of the ALARA principle and following the dictum “Image gently” have contributed to this trend. In a patient with consolidation/ atelectasis or a lung mass, colour Doppler can help visualize the presence of vessels within the lesion and hence evaluate the lesion without the requirement of contrast administration.⁴

Ultrasound is economical, easily available, portable and lacks ionizing radiation. It can be performed after careful evaluation of the chest radiograph. Contrast administration and sedation are not required. The investigation can be performed at the bedside and allows real time visualization in various planes.

The main indications for lung US in children include:

1. Evaluation of the neonatal lung – neonatal point of care ultrasound

2. Follow up and evaluation of antenatally detected lung abnormalities

3. Evaluation and follow up of consolidation.

4. Assessment of an opaque hemithorax

5. Misleading chest X-ray findings to differentiate cystic from solid lesions

6. Evaluate palpable chest wall lesions

7. Clarify inconclusive plain film findings.

Method of scanning

The ribs in neonates and small infants have low mineral content, thus allowing trans osseous scanning, especially in the parasternal region where the ribs are cartilaginous.⁵ This can be done through the trans-sternal and trans costal approach. A large thymus allows excellent visualization of the mediastinum as it provides an acoustic window for the sound waves.⁶ A drawback in older children is the inability to obtain a panoramic view of the chest, since the bones are ossified and the acoustic window available is limited. The type and frequency of transducer used would vary with the age of the patient and the location of the lesion. Linear transducers with high frequency and a small footprint are preferred to perform a sagittal and intercostal scans in neonates.

Acoustic windows used for evaluating the chest include (Figure 1A)

Figure 1.

(A) Diagrammatic representation of acoustic windows used for evaluating the chest include- (1) Suprasternal notch (2) Parasternal region (3) Intercostal spaces (4) Trans diaphragmatic approach and (5) Sub costal/ Sub xiphoid approach. (B) Line diagram of normal appearance of the lung on a transcostal longitudinal US scan showing the chest wall, ribs, pleura, and A-lines (C) Corresponding transcostal longitudinal US image of normal lung with linear transducer showing the ribs with distal shadowing (R), intercostal spaces (IS), pleural line (arrow) and multiple horizontal echogenic lines, equidistant from each other suggestive of A lines (arrow head). (D) M mode ultrasound in normal lung showing the seashore sign. The cursor is placed on the pleura while sampling. The normal lung movement termed as “lung sliding” generates a “twinkling” or grainy appearance seen distal to the pleural line, distributed uniformly. The chest wall above the pleura shows no significant movement. This appearance mimics the sea and shore hence the term “seashore” sign. (E) Lung point in pneumothorax. Small pneumothorax showing separation of the pleura cranially and the normal pleura apposing caudally. (E inset) Diagrammatic representation of M-mode ultrasound in pneumothorax depicting the Bar code or stratosphere sign due to lack of movement wihtin the lung because of the presence of air. (F) Line diagram depicting small pneumothorax causing divergent visceral and parietal pleura at a point called the lung point.

1. Suprasternal notch,

2. Parasternal region

3. Intercostal spaces

4. Trans diaphragmatic approach

5. Sub costal/ Sub xiphoid scans

Evaluation of the diaphragm, the subdiaphragmatic space as well as the liver and spleen should form part of the protocol as lung consolidation and empyema may be secondary to a liver abscess.

Neonatal point of care ultrasound

Normal appearance of the lung

The skin, subcutaneous fat, pectoral and intercostal muscles do not generate artefacts since they conduct sound waves. The chest wall and pleural surface as well as the air within the lung reflect the sound waves, which are seen as artefacts. These artefacts are interpreted to look for pathology. The ossified ribs block the transmission of sound waves. The waves are reflected causing a bright (hyperechoic) appearance of the superficial cortex with shadowing distal to the ribs spreading up to the edge of the screen.

Pleural line and lung sliding:

The “pleural line” is a single hyperechoic line seen below the internal intercostal muscles formed by sound waves reflected from the parietal and visceral pleura. The pleural line “slides” i.e. shows a “ to and fro” movement with respiration described as lung sliding. The chest wall does not move or moves in a direction opposite to the lung. The sliding movement at the lung base is more than at the apex where it is imperceptible. This is due to the inflation and deflation of the lung and subsequent movement of the pleural surface.⁷ The presence of “pleural/lung” sliding should be looked for and documented (Figure 1B, C).

A lines:

The lung and the soft tissues differ in their acoustic characteristics causing reflection of the ultrasound waves from the lung surface creating reverberation artefacts. These are seen as echogenic horizontal lines parallel and equidistant from each another, known as “A” lines which indicate the presence of normal aerated lung⁸ (Figure 1B, C).

Seashore sign:

On M mode imaging, the cursor is placed over the pleura. The superficial parietal layers are stationary and appear as a horizontal pattern of lines. The normal lung parenchyma consists of air containing alveoli which scatters the sound waves with no through transmission. The areas deeper to the pleural line appear “coarse” and irregular since the movement of the pleural line is reflected all over this area. This is known as the “seashore sign”⁹ (Figure 1D).

B lines

B- lines are seen as vertically oriented artefacts and indicate an abnormality in the interstitial or alveolar compartment. They indicate the presence of interstitial fluid content. They are seen arising from the pleura, extending towards the edge of the screen. They erase the A-lines and move with respiration.¹⁰ The fetal lung has a high fluid content hence B-lines may be seen even on the first day of life in neonates without respiratory distress. These usually disappear by the third day.

The B lines have to be differentiated from comet tail artefacts, also called Z lines. These are echogenic lines perpendicular to the pleural surface, extend for a short distance from the pleural line hence do not reach the edge of the screen and unlike B lines do not erase the A lines (Table 1) (Figure 2A, B).

Table 1.

Differentiating B-lines and comet tail artifacts.^11,12

Comet tail	B-lines
They are ultrasound artifacts	They indicate presence of interstitial fluid
Short, vanishing after 2 to 4 cm	Spread right up to the corner of the screen
Ill defined	Well defined
Less echogenic than the pleural line	Isoechoic to the pleural line
Not erasing A lines	Most of the time erases A lines
Does not move with lung sliding	Moves with lung sliding

Figure 2.

RDS and TTN. (A & B) Line diagram and corresponding longitudinal US image depicting B-lines (Black arrow) and comet tail artefacts (arrow head/ short arrow). B- lines are lines vertical to the pleural surface, extending to the edge of the screen, deleting “A” lines. They are due to fluid rich interstitium and need to be differentiated from "comet tail" artefacts, which as the name suggests are artefacts which are vertically oriented ill defined lines perpendicular to the pleura. They are short, not reaching upto the edge of the screen, not erasing A lines and do not move with lung sliding. (C) Chest Radiograph supine view in a child showing extensive bilateral uniformly distributed fine granular opacities suggestive of respiratory distress syndrome (surfactant deficiency disease). (D) Subcostal US scan showing absence of mirror image of liver in the lung. Densely packed B-lines are seen instead, consistent with respiratory distress syndrome (RDS). (E) Subcostal scan (US) depicting mirror image artefact of the liver in the lung indicating normal aerated right lung base. Note presence of few “comet tail” artefacts (arrow), which are vertically oriented short lines which need to be differentiated from B lines. (F) Longitudinal US image of the chest demonstrating double lung point seen in Transient Tachypnoea of Newborn; SLF – Superior lung field, ILF – Inferior Lung field. (Image Courtesy - Dr Pradeep Suryawanshi, HOD Neonatology, Bharati Hospital, Pune)

Respiratory distress syndrome (RDS)

Lung US if performed as the first modality tends to over diagnose RDS. Pneumonia, transient tachypnoea, pneumothorax and meconium aspiration syndrome were mistaken for RDS in one study.¹³ In neonates suffering from RDS, abnormal B lines are seen at lung ultrasound. These are compact, widespread and seen symmetrically in both lungs similar to a “white lung” seen on a chest radiograph (Figure 2C, D). The pleural line is thickened and irregular since pleural involvement occurs. Associated sub pleural basal consolidation may be present and is seen posteriorly and laterally. Sometimes larger areas of consolidation may be seen posteriorly. US is helpful in the follow up of patients with RDS after the administration of surfactant by monitoring reduction in the B lines and reappearance of A lines, hence reducing the number of radiographs done and the radiation dose.¹⁴ A mirror image of the liver is commonly seen within the chest on the right and of the spleen on the left on sub costal scans indicating presence of normal aerated lung in the bases. This occurs due to reflection of the ultrasound beam when it strikes the diaphragm¹⁵ (Figure 2E). In RDS, this mirror image artefact is absent and replaced by dense B- lines (Figure 2D).

Transient tachypnoea of newborn (TTN)

TTN occurs due to delayed clearance of fluid from the alveoli and interstitium. It is more commonly seen in neonates born by caesarean section compared to those born vaginally and is thought to be due to lower levels of catecholamines and absence of thoracic compression during a vaginal delivery. The condition may be difficult to diagnose on plain chest radiographs which may show perihilar streaking due to engorged lymphatics and presence of fluid within the fissures. The ultrasound findings in TTN include partial or complete disappearance of A-lines. Condensed B lines are seen caudally in the lung in neonates with TTN whereas they are not so compact in the superior lung field. This forms a sign called as the Double lung point (Figure 2F) described by Copetti and Cattarossi.¹¹ The ultrasound findings are thus simple to interpret and reliable as there is a difference in the echogenicity of lower and upper lung areas, which helps differentiate TTN from RDS where B lines are seen uniformly across the entire lung (Table 2). This improves the accuracy of LUS for diagnosis (sensitivity 93%, specificity 97%). The pleural line is normal in TTN with presence of normal lung sliding as compared to RDS where the pleural line is thickened and irregular. Associated pleural effusion may be seen which may be either unilateral or bilateral.¹⁶ The changes of TTN usually resolve in 72 h.

Table 2.

Differentiating RDS from TTN; RDS: Respiratory distress syndrome, TTN: Transient Tachypnea of Newborn.^11,16

	RDS	TTN
B Lines	Bilateral confluent B-lines No double lung point	Very compact B lines in the inferior pulmonary fields, not so compact in superior lung field - “Double Lung point”
Pleural Line	Thickened and irregular	Normal regular echogenic
Evolution of B Line	Persists, no change even after surfactant	Disappears by day two coinciding with clinical improvement
Lung Consolidation	Associated lung consolidation may be seen	No lung consolidation

Meconium aspiration syndrome (MAS)

The diagnosis of MAS does not need radiographs or lung US, but is made by aspirating meconium below the vocal cords. Lung US however is very helpful to indicate the involvement of the lung and therefore the severity of the disease.

B-lines are seen indicating the presence of interstitial fluid. These may be coalescent or sparse. There may be associated consolidation or atelectasis.¹⁷

Pneumothorax

The accuracy of US as a first line investigation for detection of pneumothorax is approaching the accuracy of CT and far exceeds the accuracy of plain radiographs. Signs, which indicate the presence of a pneumothorax, are absence of lung sliding, presence of a lung point (Figure 1E) and absence of B lines. The Barcode or stratosphere sign on M mode (Figure 1E inset) also confirms presence of a pneumothorax. This occurs as there is no motion of the chest wall and no motion of the lung due to the presence of air in the pleural cavity. This is seen as multiple parallel horizontal lines resembling a bar code.¹⁸

Normal lung sliding and B -lines originating from the visceral pleura are obliterated at the site of the pneumothorax (Lichtenstein et al.¹² The lung point is the point at which normal findings diminish. It demarcates the presence of air in the pleural cavity (pneumothorax) and is associated with 79% sensitivity, 100% specificity.

Pneumothorax can be a complication of RDS along with pneumomediastinum and pneumopericardium.^19,20 Due to the delicate nature of the surfactant-deficient lung and frequent oxygen therapy requirement, neonates with RDS have an increased risk of air-leaks.²¹ These can occur spontaneously, or due to inadequate mechanical ventilation causing alveolar rupture and subsequent escape of air beyond the terminal airways.²²

Meconium aspiration syndrome can result in pneumothorax due to airway constriction around aspirated meconium. This prevents air from escaping on exhalation increasing the resistance of expiratory airflow, causing a ‘ball valve’ effect. The air, which is, trapped causes hyperinflation leading to alveolar rupture (air-leak).^23,24 The role of US in detecting pneumomediastinum and pneumopericardium is debatable. The chest radiograph is hence mandatory as the initial investigation in a neonate with respiratory distress. The neonates may be followed up with US.

Pulmonary hemorrhage

A sudden drop in haemoglobin with ground glass haziness of the lungs on chest radiograph should raise a high index of suspicion for pulmonary haemorrhage. There may be hemorrhage seen in the endotracheal tube. On ultrasound condensed B lines are seen akin to the findings seen in RDS (Figure 3A and B ). Also consolidation (of different size) containing small hyperechoic spots may be seen.

Figure 3.

(A) Chest radiograph in a neonate with sudden drop in hemoglobin. Note the ground glass haziness over both lungs simulating RDS. (B) Longitudinal US image of the same neonate showing condensed B-lines similar to those seen in RDS. Diagnosis- Pulmonary haemorrhage. Note the pleural line is thick and irregular as also seen in RDS. (C) CPAM versus sequestration. Post natal colour Doppler Transverse image shows a branch from the aorta directly supplying the hypoechoic lesion in the left lung base. An antenatal Fetal MRI done at 38 weeks gestation revealed a wedge shaped hyperintense lesion near the left lower lobe (image not shown). (D) Coronal contrast enhanced CT image arterial phase confirmed the arterial supply from the aorta. (Inset) Operative photograph showing the wedge shaped lesion (arrow). Note the appearance of the normal lung in contrast.

FOLLOW UP AND EVALUATION OF ANTENATALLY DETECTED LUNG ABNORMALITIES

Antenatally detected abnormalities in the chest include congenital diaphragmatic hernia, congenital pulmonary airway malformation (CPAM), formerly called congenital cystic adenomatoid malformation (CCAM) and pulmonary sequestration.²⁵ The diagnosis of these would already have been established in utero in most cases with ultrasound and fetal MRI. In children where the condition has not been detected antenatally, recurrent pneumonias seen as a homogenous mass especially at the lung base on a chest radiograph are often the mode of presentation.

A chest radiograph is mandatory before doing the initial ultrasound examination. In CPAM a chest radiograph may be inconclusive. Lung US can be inconclusive if the lung lesion does not reach the pleura.

On ultrasound, these lesions may be seen either as a complex echogenic mass which may be multicystic or as a homogenous solid mass. It is important to interrogate the lesion with colour Doppler imaging to demonstrate an anomalous vessel arising from the aorta extending into this lesion, thus confirming a lung sequestration²⁶ (Figure 3C and D). Distinguishing a CPAM from sequestration may sometimes be difficult on antenatal imaging in the absence of demonstration of a vessel. It should be remembered that both CPAM and sequestration can coexist in hybrid lesions. CPAM is seen on foetal ultrasound as a fluid containing anechoic or cystic lesion which may be uni or multiloculated. In Type III CPAM, where the cysts measure <5 mm, the lesion may appear echogenic. (Figure 4A to C).^27,28

Figure 4.

Antenatally detected thoracic lesion (A) Antenatally detected cystic lesion left lung Postnatal chest radiograph in a 3 day old child revealed a homogenous opacity in the left hemithorax causing contralateral mediastinal shift. (B) US image showing the presence of a cystic lesion. Colour flow was seen in the mediastinal vessels medial to the cystic mass (dopper image not shown). (C) Axial contrast enhanced CT image revealed a cystic lesion in the left hemithorax. The child was operated on. Histopathological examination – CPAM.

Congenital diaphragmatic hernia, which also presents with an abnormal opacity in the hemithorax on the chest radiograph, is usually a straight forward diagnosis however at times it can be puzzling especially if the presentation is late and has not been detected antenatally. In these patients movement/ peristalsis within the bowel loops in the hemithorax would confirm the diagnosis. Moreover the contents of the hernia other than bowel can also be evaluated.

EVALUATION AND FOLLOW UP OF CONSOLIDATION

Ultrasound helps evaluate the location and nature of an area of increased opacity on the chest radiograph. On US, when the lung is airless as in consolidation, this simulates the appearance of the liver, also termed hepatisation (Figure 5A, B). The branching echogenic foci within it, which represent the residual air within the bronchi as well as within some of the alveoli, are given the nomenclature of a sonographic air bronchogram, similar to the air bronchogram seen on a chest radiograph.²⁹ The presence of a dynamic air bronchogram, i.e. movement within the sonographic air bronchogram on inspiration and expiration, rules out bronchial obstruction.

Figure 5.

Consolidation and atelectasis (A to D) 14 year old child with breathlessness and cough. CXR (not shown) revealed homogenous opacification of the right hemithorax with blunting of the right CP angle. Air bronchogram was noted suggestive of consolidation. (A) Longitudinal US image shows normal “A” lines replaced by consolidated lung. Associated small rim of pleural effusion is seen adjacent to the diaphragm. (B) Echogenic lines showing a branching pattern are seen within the consolidation due to the air within the bronchioles. This is termed “hepatisation” as the appearance mimics the appearance of the liver. No air bronchogram is seen distally, due to fluid filled bronchioles. (C) On colour doppler, branching pattern of the vessels is seen within the consolidation. This helps to differentiate consolidation from a mass. (D) Axial post contrast CT image confirming the ultrasound findings. No mass was seen. In retrospect the CT was probably not needed.

Sometimes the air in the bronchi is replaced by fluid. This is also known as a “fluid or mucus bronchogram”. This also indicates lung consolidation and is akin to the air bronchogram.³⁰ The fluid or air bronchogram may not be visible in the peripheral lung at times. In these cases colour flow imaging helps demonstrate a normal branching pattern of vessels within the consolidated lung, thus differentiating it from a mass (Figure 5C).

In atelectasis the lung appears hypoechoic, triangular in shape with crowding of the bronchi due to loss of lung volume. Only a few of these may have air within them.³¹

US is useful in the follow up of children with consolidation and in monitoring resolution. Areas of breakdown can be detected on US far earlier than they are detected on a chest radiograph.³² They are commonly seen in staphylococcal pneumonias, where pneumatoceles can occur, as well as in acute necrotizing pneumonias (Figure 6A to F). However it should be noted that lung US has low sensitivity and specificity for the aetiology of pneumonia.

Figure 6.

Three year old female child with pneumonia. (A) Transverse US image shows consolidation with air bronchograms. Overlying pleural effusion is seen with echoes within suggestive of an empyema. (B and C) Follow up longitudinal ultrasound images showing areas of breakdown within the consolidation bilaterally not appreciable on the chest radiograph taken at the same time. (D) Chest radiograph one month later revealed well defined cystic areas in the right mid and lower zones and ? left lower zone. (E) Longitudinal ultrasound image left lung base showing a hypoechoic lesion within the consolidation suggestive of an abscess. (F) Coronal CT lung window image reveals multiple thick walled cystic areas which had air fluid levels within them bilaterally suggestive of abscesses. Diagnosis – Necrotizing pneumonia.

In a child presenting with empyema it is imperative to see the subdiaphragmatic region, the liver as well as spleen to rule out an underlying pathology. In south east Asian countries where amoebiasis is common an amoebic liver abscess rupturing the diaphragm resulting in an empyema and consolidation is not uncommon (Figure 7A to F).

Figure 7.

Subdiaphragmatic pathology involving the chest. 4 year old child presenting with fever, chills, dry cough and breathlessness. (A) Frontal chest radiograph revealed right lower zone opacity wth blunting of the CP angle suggestive of pleural effusion with associated consolidation. (B-D) US images showing consolidation of the right lower lobe and part of the upper lobe. Small pleural effusion with moving echoes within was also seen. A large well defined hypoechoic subdiaphragmatic intrahepatic lesion was seen suggestive of an abscess ? amoebic. Suspicious diaphragmatic discontinuity seen posteriorly ? abscess ruptured into the thorax. (E&F) Axial and sagittal post contrast CT images reveal a well defined round thick walled lesion with irregular enhancing internal margins suggestive of an abscess.The right hemidiaphragm shows discontinuity posteriorly (arrow). The child did not respond to antibiotics and had to be put on Metrogyl confirming the amoebic aetiology of the abscess.

LUNG ULTRASOUND AS A DIAGNOSTIC TOOL IN THE EMERGENCY ROOM (ER)

If lung US is performed as the first imaging tool in children suspected to have a community acquired pneumonia (CAP), there can be a significant reduction in the radiation dose received due to a decrease in the number of chest radiographs performed. Signs of pneumonia on lung US include absence of A lines, with one or more of the following findings, i.e. presence of multiple B lines indicating interstitial pneumonia, thickening of the pleural line, absence of lung sliding, presence of lung hepatisation and air bronchogram. These findings may be seen with or without an associated pleural effusion.

CAP are infections acquired outside the hospital where children depending on their age, classically present with fever, tachypnoea, recession, breathlessness, cough, tachycardia, wheeze, headache, abdominal pain, or chest pain.³³ According to the international guidelines, chest X-ray (CXR) should be performed only in severe cases of CAP requiring hospital admission or in cases unresponsive to appropriate medical therapy. It should not be routinely performed in all children with clinical signs of CAP. They have also recommended that lateral x-rays should not be performed routinely.^34,35

Routine follow-up chest radiographs are not warranted in children who show an uneventful recovery. It must be remembered that radiological resolution lags behind a clinical resolution. Certain radiological abnormalities may persist but rarely alter clinical management. Patients with lobar collapse or recurrent pneumonia involving the same lobe may benefit from follow up US to rule out abscess formation, necrotising pneumonia and other complications. US also helps exclude a chest mass mimicking a pneumonia. The presence of a dynamic air bronchogram confirms bronchial patency. In cases where there is ongoing clinical concern, a repeat chest radiograph may be performed to exclude airway obstruction either due to an extrinsic compression or an intrinsic cause like a foreign body. When US is used alone, limitations include inability to detect abnormalities which are deep seated and not subpleural. Conversely smaller areas of subpleural consolidation may not be detected by a chest radiograph. US can help follow up in patients with subpleural lesions and assess response to therapy.

Current evidence supports lung US as an imaging alternative for the diagnosis of childhood pneumonia. Recommendations to train paediatricians to perform lung US for diagnosis of pneumonia may have important implications in different clinical settings. Lung US was found to have a sensitivity of 96% (95% confidence interval [CI]: 94–97%) and specificity of 93% (95% CI: 90–96%), and positive and negative likelihood ratios of 15.3 (95% CI: 6.6–35.3) and 0.06 (95% CI: 0.03–0.11), respectively.³⁶

Assessment of an opaque hemithorax

In children presenting with breathlessness with or without fever where the chest radiograph reveals an opaque hemithorax, US helps in differentiating between pulmonary, pleural and mediastinal lesions and their morphological evaluation. Absence of air bronchograms on the chest radiograph is a clue to evaluating the child further with US. Metastatic tumours are more common than primary tumours. Primary malignant tumours in children include a pulmonary blastoma, mucoepidermoid carcinoma, bronchogenic carcinoma, hemangiopericytoma and rhabdomyosarcoma.³⁷

Pleuropulmonary blastoma is the most common and is seen as a peripherally located solid mass in the lung.³⁸ These are usually very large at presentation occupying the entire hemithorax (Figure 8A to F). Askin tumours can also occupy the entire hemithorax simulating consolidation and effusion (Figure 9A to F). Absence of air bronchograms within the consolidation on US and loss of normal vascular branching pattern favour a tumour.

Figure 8.

(A) Frontal chest radiograph of a three year old child with dyspnea showing homogenous opacification of the left hemithorax with contralateral mediastinal shift. No air bronchograms seen. (B-C) Longitudial US images left hemithorax reveal a large predominantly solid mass left hemithorax showing absence of air bronchograms with abnormal vascularity within. A small pleural effusion is also seen. (D) Transverse US image shows the mass in left hemithorax causing contralateral mediastinal shift. (E & F) Contrast enhanced axial and coronal reformatted CT image shows a large poorly enhancing heterogenous mass in the left hemithorax surrounding a collapsed left upper lobe. Diagnosis - Pleuropulmonary blastoma.

Figure 9.

Eleven year old child with breathlessness. (A) Chest radiograph showing a homogenous opacity in the left hemithorax with no air bronchograms. Left CP angle is obliterated. Contralateral mediastinal shift is seen. (B) US image show a large pleural effusion with echoes within it as well as septae. As the patient was afebrile, and an intrathoracic mass was seen, this was although to represent a haemothorax rather than an empyema. (C and D) US images showing a large hyperechoic predominantly solid mass with few small cystic areas within it was also seen with no air bronchograms. Abnormal vessels were seen within the mass. (E) Axial post contrast CT image showing a large enhancing mass with few poorly enhancing areas suggestive of necrosis, bulging through the intercostal spaces. (F) Histopathological examination report - Malignant round cell tumour of the chest wall - PNET.

Mediastinal evaluation

Indications for mediastinal ultrasound include -

• To evaluate mediastinal widening and rule out a normal thymus as the cause

• Evaluation of vascular anomalies

• Search for lymph nodes

• In the evaluation of oesophageal atresia and tracheo-esophageal fistula (TOF)

• To assess complications in patients with indwelling catheters

• To assist mediastinal biopsies

Thymus or a mass

The thymus is seen in the superior mediastinum, anterior to the great vessels. Due to its varied appearance in size and shape, which may be age related as well as secondary to stress or disease, it may mimic a mediastinal mass, lung consolidation involving the upper lobe or atelectasis. The normal thymus has a characteristic appearance on US. It appears hypoechoic with tiny echogenic foci.³⁹ The gland shows no significant vascularity (Figure 10A and B). US helps in differentiating an enlarged or persistent thymus from a mediastinal mass especially in young infants with varied configurations of the normal thymus. It is not unusual to see the thymus in the neck while doing an US of the neck in children. The characteristic stippled appearance helps in recognizing it.

Figure 10.

Thymus (A and B) A 3 day old male child born at term who cried weakly after physical stimulation, bag and mask ventilation given subsequently developed pneumomediastinum. (A) Frontal chest radiograph showing an opacity extending on either side-of the mediastinum touching the chest wall laterally, showing well defined margins. Pneumomediastinum was seen. (B) US axial image showing normal stippled appearance of the thymus confirming the opacity, is the thymus. (C & D) six month old child with fever for a week. (C) Digital chest radiograph showing superior mediastinal widening more to the left of the midline. (D) US image showing the opacity is due to the thymus which shows a classical stippled appearance (arrow). Chemical shift imaging (not included) showed signal loss on the out of Phase images confirming benign thymic enlargment.

Thymic hyperplasia is commonly seen in children. It follows chemotherapy in 10–25% patients and occurs within two years of initiation of chemotherapy.⁴⁰ Its diagnostic challenge is to distinguish it from tumor recurrence in patients with prior lymphoma (Figure 10C). On CT it may be difficult to differentiate a thymoma from thymic hyperplasia, however US shows the characteristic sonographic appearance of the thymus (Figure 10D). Of course, thymomas are rare in childhood. Nodular contour of the gland, presence of necrosis or calcified foci favour a neoplasm. In cases in which there is ongoing concern, the child may be further evaluated with MR with chemical shift imaging - in and out of Phase imaging helps confirm the diagnosis, as hyperplasia shows signal loss on out of Phase imaging. This is due to the presence of microscopic fat within the normal thymus and thymic hyperplasia.⁴¹

The thymus may be enlarged due to benign pathologies like a thymic cyst, intrathymic haemorrhage, lymphangiomas and haemangiomas. Lymphatic malformations (LMs) are uni or multiloculated lesions which can vary in size. They may undergo haemorrhage in which case internal echoes will be seen within the cysts. Thymolipomas and thymomas are rare in childhood. However leukemic and lymphomatous infiltration of the thymus is fairly common. In these cases the gland shows an irregular or lobulated margin with a coarse heterogenous echotexture and presence of calcifications. Langerhans cell histiocytosis is one of the conditions where there is thymic involvement which can manifest as heterogeneity of the gland with or without calcification.⁴²

Mediastinal masses

US of the mediastinum has a role to play in evaluating mediastinal widening. It helps by differentiating a mass from thymic hyperplasia. It is also useful in resolving equivocal findings seen on the chest radiograph. A bedside ultrasound can additionally be useful in evaluating the mediastinum in a patient who is critically ill.³⁸ Visualisation of the mediastinum is easier in neonates and infants as the sternal ossification centres have not fused and their bone mineral content is low. Hence the acoustic windows available are far better than in an older child.

Mediastinal adenopathy as a cause of mediastinal widening can be further evaluated on ultrasound. The nodes can be imaged for presence of necrosis or cavitation, which would favour a tubercular aetiology.⁴³ These nodes are usually confluent. Accessible nodes can be biopsied under ultrasound guidance.

A mediastinal opacity seen on the chest radiograph is further resolved as a posterior mediastinal mass with the help of Felson’s signs - the silhouette sign, cervicothoracic and abdomino-thoracic signs or with the help of a lateral radiograph.⁴⁴ This lesion may be further evaluated on a chest US. Its cystic or solid nature and vascularity as well as any cavitation or necrosis within it can be determined. Common posterior mediastinal masses include neurogenic tumors and neurenteric cysts.⁴⁵ Neurogenic tumors, are seen as a well defined lobulated hypoechoic mass which may show fleck like calcification within them (Figure 11A, B). Cysts are seen as well-defined, anechoic lesions with thin walls. The cyst may show internal echoes within due to mucous, blood or proteinaceous fluid.

Figure 11.

Six month old child, chest radiograph (not shown) revealed a well defined opacity in the posterior mediastinum on the right. (A) Ultrasound done revealed a solid oval mass with few foci of calcification and vascularity within it. (B) T₂ weighted coronal MR image revealed the mass had no intraspinal extension and confirmed the solid nature. US guided biopsy done. Histopathological examination – Ganglioneuroma.

EVALUATION OF PLEURAL EFFUSION

The role of US in confirming the presence of an effusion and differentiating it from pleural thickening is well established.⁴⁶ The type of pleural effusion depends on the nature of the fluid collection. Serous fluid is usually a transudate and is a clear fluid or simple effusion. It may also be cloudy with or without swirling particles. An exudate or empyema is purulent fluid also classified as a complicated effusion. This shows multiple septa within and is multiloculated. In these patients thoracocentesis may be difficult. Multiple septa in the fluid favours infections like tuberculosis. Long standing haemothorax will also show multiple thick septa within it, however the child will have a history of fall in haemoglobin and haematocrit levels. Internal echoes would be seen within the fluid within an acute haemothorax as well as an empyema. The thickness of the pleura can also be evaluated in children with empyema where decortication is planned. Quantification of the fluid and marking a site for diagnostic and therapeutic aspiration are well-established indications of US.^47,48 When marking a site for pleural drainage it is essential to mention in the report if the patient was scanned in the upright or supine position. In some hospitals, marking a site for another clinician to aspirate / insert a drain is not allowed. If this is to be done, the mark should be made ideally in the presence of the clinician who will be performing the test and the procedure should be carried out promptly after the mark has been made, with, as suggested, the patient in the same position.

A thickened echogenic rim of pleura represents pleural thickening or a plaque. Movement of the fluid with change in posture confirms amenability of the fluid to aspiration. Although internal echoes or debris within the fluid is well seen on B mode ultrasound, colour Doppler can help in confirming the presence. Colour signal within a fluid collection in the pleural space, is a useful sign for detection of an effusion with floating debris. This is called the “fluid colour” sign and occurs due to scattering of the sound waves by the moving internal echoes or debris within the effusion producing colour Doppler signals.⁴⁹ The multiloculated nature of pleural fluid may be better appreciable on US than on CT.⁵⁰

In patients with an elevated hemidiaphragm and blunting of the CP angle where subpulmonic effusion is suspected on a chest radiograph, ultrasound confirms the presence of fluid. It also detects lung or pleural masses masked by a large pleural effusion.

Diaphragmatic evaluation

The diaphragmatic outline can be traced on sonography and the movements in inspiration and expiration assessed.⁵¹ This is better seen in the right and left lateral positions in the longitudinal plane, using a transthoracic approach. In congenital diaphragmatic hernia US helps in evaluating the contents of the hernia. It may also be able to demonstrate the size and location of the diaphragmatic defect, although this may not be possible in all patients.

US helps in determining the cause of an elevated diaphragm detected on the chest radiograph. Paradoxical movements of the diaphragm would suggest phrenic nerve palsy as a cause.

CHEST WALL LESIONS

Abnormal swelling over the chest wall, which is either palpable or seen, clinically may be further evaluated on ultrasound. This may appear as an unusual area of increased opacity on chest radiographs. Ultrasound helps by determining if the lesion is cystic or solid. The underlying ribs can also be evaluated for destruction. The cortex of the bone is seen as a uniformly echogenic structure with smooth margins. In case of involvement this becomes irregular, it may show thinning or partial absence, periosteal new bone formation or a soft tissue mass lesion.⁵²

In India and South East Asia where tuberculosis is common, there can be involvement of the chest wall secondary to the tubercular infection with a cold abscess.⁵³ Empyema necessitans may occur which is well appreciated on ultrasound (Figure 12A to D).

Figure 12.

A 7-year-old female child with cough, expectoration, fever and a left chest wall swelling for 10 days. (A)Frontal chest radiograph showing a well defined opacity in the left mid zone also involving the upper zone. No obvious rib destruction was seen. (B and C) US images confirm the consolidation. A large complex collection was seen overlying it anteriorly extending into the chest wall showing a heterogenous appearance. (D) Axial contrast enhanced CT image of the thorax at the level of the lesion confirmed the ultrasound findings. Diagnosis - Empyema necessitans.

In children where history of the swelling being present since birth can be elicited, a possibility of a veno-lymphatic malformation needs consideration. Injection of the sclerosant may be done under US guidance.

The most frequent malignant tumour of the chest wall in the paediatric age group is the small round cell tumour. This includes Ewings sarcoma, PNET, lymphoma and rhabdomyosarcoma.⁵⁴ These tumours present as large intra or extra thoracic masses on chest radiographs. The large intrathoracic masses have to be differentiated from a massive effusion. US confirms the solid nature of the mass, helps determine its epicentre, presence of associated pleural or pericardial effusion / haemothorax and rib involvement (which may also be visible on the chest radiograph).⁴⁷

Conclusion

US is invaluable in evaluating diseases of the lung, pleura, and mediastinum. Rapid assessment of the patient with an opaque hemithorax is possible. It may help in determining the cause of mediastinal widening, evaluates a mediastinal mass if present and may rule out or confirm thymus as the cause of mediastinal widenening. It has of late evolved in its role in the evaluation of the neonatal lung. Follow up on ultrasound in neonates with RDS reduces the number of chest radiographs thus reducing the radiation dose.