Soft Tissue Neck Study

Inspiratory stridor resulting from an upper airway obstruction is the most frequent indication of a soft tissue neck study in pediatric patients.³ The common causes of inspiratory stridor include croup, epiglottitis, a foreign body, and an upper airway mass in infants and children. To obtain optimal diagnostic quality, the neck of the patient should be extended and images should be obtained at full inspiration. The standard soft tissue neck radiographic views consist of both an anteroposterior (AP) view and a lateral view. An additional expiratory lateral view of the neck subsequently may be obtained for assessment of subglottic stenosis, which can be beneficial in differentiating a fixed large airway disorder (e.g., subglottic stenosis) from a dynamic large airway disorder (e.g., tracheomalacia).

Use of the high kilovoltage technique with added filtration and coned magnification may facilitate visualization of the upper airway and adjacent soft tissue. Immobilization may be necessary for infants and young children (<5 years) who may not be able to follow verbal instructions. However, pediatric patients with respiratory distress should not be immobilized, especially in a position that may further aggravate the airway obstruction. For instance, the upright position is preferred and the supine position is contraindicated in the setting of acute epiglottitis. When necessary, careful manual immobilization of the head and neck during the study by the child’s parents or by an experienced technologist may be considered. A clinician should accompany the child with respiratory distress, and an emergency kit should be ready for immediate use in the examination room. In most conditions resulting in upper airway obstruction in pediatric patients, a soft tissue neck study usually is sufficient to make the diagnosis. However, when the underlying cause of upper airway obstruction is not evident, fluoroscopy, CT, or MRI may be necessary as a next step in the imaging evaluation.

Chest Radiography

Chest radiography is the imaging study obtained most frequently to evaluate the respiratory system in pediatric patients. Immobilization is usually required in uncooperative infants and young children (<5 years) to achieve consistent image quality by decreasing motion artifacts and position deviations.⁴ Chest radiography is obtained during quiet inspiration in uncooperative infants and young children and during full inspiration in cooperative older pediatric patients. The standard chest radiographic views consist of the AP view in infants and young children (<5 years) and both the AP and PA views in older children, in addition to a lateral view. AP, PA, and lateral views of the chest can be obtained with the patient in the supine or erect position. Exposure parameters of the chest radiography should be appropriately optimized. However, unnecessary radiation to nonthoracic structures such as the lower neck, proximal upper extremity, and upper abdomen should be avoided by using appropriate collimation and shielding.^5,6

In contrast to a radiographic screen-film system, image acquisition and display are decoupled in digital radiography, which allows increased versatility of image manipulations.7,8 One of the most commonly used image processing techniques in digital chest radiography is unsharp mask filtering or edge enhancement, which facilitates the detection of thoracic fine linear abnormalities or structures such as pneumothorax, vascular catheters, and the interlobar fissures, even in newborns.⁹ Recently, optimization of digital radiographic techniques has been emphasized to reduce potentially unnecessary overexposure to pediatric patients.^6,10 The increased likelihood of overexposure in digital radiography, the so-called dose creep, is attributed not only to the lack of a standardized exposure index but also to the difficulty in recognizing overexposed radiographs by radiologic technologists and radiologists. Fortunately, a new standardized exposure index for digital radiography was proposed recently.¹¹

Optimized techniques of digital chest radiography in children often are different from those in adults. For instance, automatic exposure control and grids are not particularly helpful in small pediatric patients. Additional views such as expiratory, decubitus, and oblique views sometimes are necessary to clarify abnormalities detected on standard AP or PA views or to solve unanswered clinical questions. Expiratory views may be used to confirm air trapping due to an underlying airway obstruction. The lateral decubitus view generally is used to detect freely shifting pleural effusion or air; it is used infrequently to demonstrate an air-fluid level in an intraparenchymal cavitary lesion. In addition, the lateral decubitus view may be used to demonstrate air trapping in the dependent lung or to clarify poorly defined lung opacities in the nondependent lung in uncooperative infants and young children.12,13 Oblique views may be helpful for evaluating rib, soft tissue, hilar, carinal, and peripheral lung abnormalities.

Fluoroscopy-Guided Airway Study

A fluoroscopy-guided airway study may be helpful in demonstrating dynamic airway abnormalities, such as laryngomalacia, tracheomalacia, obstructive sleep apnea, and vocal cord dysfunction syndrome.15–17 Additionally, a fixed airway disorder such as stenosis or stricture also can be identified and differentiated from a dynamic airway disorder. Compared with endoscopic procedures for evaluating large airways, a fluoroscopy-guided airway study generally is inexpensive and less invasive.¹⁸ In recent years, the diagnostic role of the fluoroscopy-guided airway study has begun to be replaced by dynamic airway CT with a low radiation dose technique.¹⁹ Dynamic airway CT is more accurate than a fluoroscopy-guided airway study for evaluation of the location, degree, and extent of both the dynamic and fixed airway abnormalities. Furthermore, dynamic airway CT can provide additional information about other intrathoracic structures, which is a valuable benefit.¹⁹

Evaluation of Lungs

The main clinical indication for ultrasound of the lungs in pediatric patients is to characterize a peripheral lung opacity (i.e., parenchymal versus pleural disease) that has been detected with chest radiography.²⁰ Frequent underlying etiologies include atelectasis, consolidation, lung necrosis, a lung abscess, congenital lung lesions, and a primary or metastatic lung neoplasm (Fig. 49-1).^20,21 For evaluation of these abnormalities, conventional chest radiography should be carefully reviewed to localize the area of interest so the clinical question can be specifically answered with the ultrasound evaluation.^20,21 Ultrasound evaluation of the lungs typically is performed with the patient in the supine or upright position. However, to improve the visualization in some selected situations, the lateral decubitus view or the supraclavicular and/or suprasternal notch view may be beneficial. Optimal views can be achieved by placing a pillow or blanket on the dependent side or behind the shoulder to help extend the neck of the patient.²⁰

The choice of ultrasound transducer depends primarily on three factors: the age of the patient, the size of the patient, and the location of the abnormality.20,21 Although curved or linear array transducers typically are used to evaluate peripheral lung opacity, transducers that are smaller in size, such as sector or vector transducers, may be necessary for imaging in infants or young children who have a small available acoustic window (such as between the ribs). High-frequency transducers (e.g., 7.5 to 15.0 MHz) are helpful in evaluating the chest in infants and young children who do not have a substantial amount of subcutaneous fat because they provide higher resolution ultrasound images with a limited ability for soft-tissue penetration.²⁰ Conversely, use of low-frequency transducers (e.g., <5 MHz) may be necessary when evaluating older children who have a large amount of subcutaneous fat because these transducers provide better soft tissue penetration, although the ultrasound image resolution is decreased.²⁰ Color Doppler ultrasound is useful for evaluating lung lesions with underlying vascular structures in cases of pulmonary sequestration or blood flow in cases involving a neoplasm.

Evaluation of Pleura

Ultrasound can be useful in differentiating pleural fluid from atelectasis and/or consolidation when the diagnosis is equivocal on the basis of conventional radiography. Furthermore, ultrasound is more sensitive and accurate than conventional radiography or CT for characterizing pleural fluid, which may be simple or complex (Fig. 49-2).²⁰ Ultrasound evaluation of patients in both the erect and lateral decubitus positions can be useful for differentiating between freely flowing and loculated pleural fluid. In addition, ultrasound can visualize the internal debris, septations, and pleural thickening often associated with parapneumonic collections. Such complex pleural effusion often requires a drainage procedure that can be facilitated with the guidance of ultrasound.

Evaluation of Airways

Anatomic details of the pediatric airways can be imaged with CT.38,39 However, small airways distal to the subsegmental level (i.e., airways <0.5 to 1.5 mm in diameter) often are invisible on CT because of their size. CT is less invasive than bronchoscopy in evaluating pediatric airways and provides cross-sectional imaging without superimposition. Various airway abnormalities, including fixed or dynamic obstruction, bronchiectasis, and wall thickening, can be diagnosed with CT. MRI may be used to evaluate large airways with comparable diagnostic accuracy, but MRI is clearly inferior to CT in evaluating small airways because of lower spatial resolution and a lower signal/noise ratio.