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.

Evaluation of Lungs

CT is valuable in evaluating not only the air spaces (which may be either opaque or hyperlucent) but also interstitial and vascular abnormalities and parenchymal nodules or masses in the lung. In addition, mediastinal, hilar, and chest wall abnormalities also can be assessed. However, soft tissue contrast resolution of CT is inferior to that of MRI. Furthermore, MRI is better than CT in evaluating intraspinal and paraspinal regions. However, expiratory CT is necessary to identify air trapping accurately.40,43

To obtain proper expiratory CT data, patient cooperation is required. In uncooperative infants and young children, lateral decubitus CT may be used as an alterative.⁴⁴ In this position, dependent and nondependent lungs show different lung volumes that mimic inspiration and expiration, respectively (Fig. 49-5, A). However, a potential drawback of the lateral decubitus CT technique is the need to change the position of the patient, which may awaken the patient, thus making interpretation difficult because of considerable respiratory misregistration between the dependent and nondependent lungs and exaggerated respiratory motion artifacts in the nondependent lung. Cine CT in the supine position can overcome some of the limitations of lateral decubitus CT (Fig. 49-5, B and C).⁴⁰ Recently, dual-energy chest CT has been used in pediatric patients to assess regional lung perfusion and ventilation.^45–51 By using 3D CT data, quantitative analysis to determine lung volume or density can be performed.

Evaluation of Airways

Central airways and their relationships to adjacent cardiovascular structures can be assessed with noncontrast black-blood MRI (Fig. 49-7).⁵⁸ Tracheobronchomalacia can be diagnosed with real-time dynamic airway MRI. However, it is preferable to perform airway evaluations with CT. The delineation of peripheral small airways on MRI is somewhat limited because of relatively low spatial resolution.

Evaluation of Lungs

Despite the inherent limitations of lung MRI that were previously mentioned, a study showed that lung nodules of a diameter larger than 3 to 4 mm can be detected with MRI with use of 1.5T systems.⁵⁹ Contrast-enhanced time-resolved magnetic resonance angiography offers not only anatomic details of pulmonary vessels but also hemodynamics of pulmonary circulation. Because contrast bolus timing is not needed and less motion artifact occurs because of high temporal resolution (e.g., <1 second for the entire lung), time-resolved MR angiography often replaces static high-resolution MR angiography in pediatric patients.⁶⁰ Lung perfusion MRI can be obtained with either the contrast-enhanced or non–contrast-enhanced technique (Fig. 49-8). Real-time dynamic chest MRI with the patient breathing freely may be used to demonstrate normal and abnormal respiratory dynamics.⁶¹ Regional ventilation may be assessed with oxygen-enhanced MRI or hyperpolarized gas (³helium or ¹²⁹xenon) MRI. Diffusion-weighted imaging using hyperpolarized gas enables calculation of the size of peripheral air spaces. Hyperpolarized xenon MRI has an additional capability of assessing diffusing capacity in the lung. A preliminary study recently reported a promising free-breathing proton MRI technique without any contrast agents that demonstrated both lung perfusion and lung tissue density in a single data acquisition by means of Fourier decomposition.⁶² Although MRI may provide anatomic and functional information of the lung without radiation, CT generally is preferred in pediatric patients, mainly because of its high spatial resolution.

Lung Perfusion and Ventilation Study

Lung perfusion imaging, although largely supplanted by CT techniques for the diagnosis of pulmonary embolus, remains an important technique for the evaluation of lung physiology and pathology. The incidence of pulmonary embolism in the pediatric population is lower than in the adult population, but occasionally perfusion imaging is used with patients who have significant risk factors to evaluate for an embolus when a contraindication to the administration of radiographic contrast material exists. Nuclear ventilation and perfusion imaging is much more commonly used to evaluate relative function in diseases in which knowledge of physiology helps to guide clinical care. Both relative ventilation and perfusion can be quantitated, which permits treatment planning and follow-up evaluation in a variety of clinical entities such as lung malformations resulting from congenital lobar emphysema and congenital pulmonary airway malformations (Fig. 49-9). The quantitative analysis of relative lung perfusion and ventilation also is useful in preoperative cases before pneumonectomy to evaluate for the estimated postsurgical lung capacity.⁶⁴

Patients with congenital heart disease frequently undergo ventilation and perfusion imaging to quantitate relative perfusion between the right and left lung at baseline and after catheter intervention (Fig. 49-10).

The perfusion study is performed with technetium-99m (^99mTc) macroaggregated albumin (MAA). The dosage in pediatrics is based on patient weight (Table 49-2). In cases of congenital heart disease when differential lung perfusion is evaluated, a specially prepared radiopharmaceutical agent with a low concentration of particles of MAA is recommended. The tracer is always injected with the patient supine so a hydrostatic gradient is not present from lung apex to base. Imaging can be performed with the patient either upright or supine, but supine imaging typically is performed in children. If imaging is being performed to evaluate pulmonary physiology, typically only anterior and posterior planar views are acquired. The differential lung perfusion is then calculated by drawing a region of interest around each lung and calculating the geometric mean (i.e., the square root of the product of anterior and posterior views) of counts in each lung. If the perfusion scan is being performed to evaluate for a pulmonary embolism, additional anatomic information is recorded, with a standard of eight planar images (i.e., anterior, posterior, left and right lateral, right anterior oblique, left anterior oblique, right posterior oblique, and left posterior oblique views). Alternatively, single photon emission computed tomography (SPECT) imaging of the lungs can be performed. When evaluating for a possible pulmonary embolus, a correlation should be made with a chest radiograph obtained within 24 hours of the study.

The ventilation study can be performed either with a radioactive gas (xenon-133) that demonstrates air flow or a radio aerosol (^99mTc diethylenetriaminepentaacetic acid [DTPA]) that localizes in alveoli without significant large airway deposition. Xenon-133 gas has the principle photon energy of 81 keV, which is lower than ^99mTc and thus the ventilation images must be acquired first. The DTPA aerosol study uses the same ^99mTc label as MAA perfusion imaging, but much less aerosol tracer activity localizes in the lungs than is present with the intravenous MAA injection. Thus both xenon and aerosol ventilation imaging must be performed before the perfusion agent is administered.

Xenon-133 gas is administered by inhalation. The recommended dose is 10 to 20 mCi. The dynamic nature of the xenon study limits the projections that can be obtained, and thus imaging typically is acquired in the posterior view with a single-head camera. If a dual-head camera is used, simultaneous anterior and posterior imaging can be acquired. The presence of the anterior detector over the child’s face and thorax can cause the patient to become anxious, and thus in young children only the posterior image typically is acquired.

^99mTc DTPA aerosol is administered via a shielded nebulizer; the child inhales long enough to accumulate sufficient aerosol in the lung for imaging. The aerosol remains in the lungs long enough for imaging to be performed in multiple projections, matching those acquired with perfusion imaging. When SPECT perfusion imaging is performed, SPECT aerosol images also can be acquired for meaningful comparison.

Salivagram

Radionuclide evaluation for salivary aspiration (i.e., a salivagram) offers a safe and easily performed test with minimal radiation exposure to the patient. The sensitivity of the salivagram has been reported to range between 26% and 73%.65–67 The salivagram can be performed with the administration of a small drop (approximately 100 µL) of ^99mTc sulfur colloid (300 µCi [11.1 MBq]) that is placed in the oral cavity while the patient is lying in a supine position in the imaging bed. After administration of the radiopharmaceutical agent, the patient is allowed to swallow normally and posterior planar imaging of the mouth, chest, and upper abdomen is obtained dynamically for 60 minutes. Imaging is acquired using a low-energy, high-resolution or ultra–high-resolution collimator, with consecutive dynamic images recorded every 30 seconds for a total of 120 frames. A positive salivagram for aspiration shows the radiopharmaceutical agent entering the bronchi (Fig. 49-11).

Neuroendocrine Imaging Study

Neuroendocrine tumors can be imaged very effectively with use of iodine-123–labeled metaiodobenzylguanidine (¹²³I MIBG). The two tumors most commonly evaluated with this tracer are pheochromocytomas and neuroblastomas (Fig. 49-12). Although other tumors such as ganglioneuromas, carcinoid tumors, and medullary thyroid carcinoma also can be imaged with this tracer, they show a lower sensitivity of detection. MIBG is a norepinephrine analog, and its uptake in malignancy can be used both to evaluate a primary tumor and metastasis at presentation and to evaluate tumor response after treatment. It is important to review the patient’s medications before imaging because a number of pharmaceutical agents have the potential to block the tumor uptake of MIBG. These agents include preparations that contain ephedrine and its derivatives, along with neuroleptic drugs, tricyclic antidepressants, and central nervous system stimulants. The MIBG dose always contains a small amount of unlabeled radioiodine that will localize within the thyroid gland, thus increasing local radiation exposure. Patient radiation dosimetry can be reduced by administering nonradioactive iodine to block this uptake before the procedure. The increased circulating nonradioactive iodine reduces subsequent extraction of radioactive iodine and is easily achieved by administering a strong solution of potassium iodide (1 drop three times a day) beginning 1 day before the injection and continuing for 3 days after the tracer injection.

The recommended dosage of ¹²³I MIBG is 5.2 MBq/kg (0.14 mCi/kg). In cases of suspected pheochromocytoma, the tracer should be administered slowly, and the patient’s blood pressure should be monitored during and after tracer administration, because rare hypertensive reactions have been reported.

The ¹²³I MIBG scan is performed the day after the intravenous administration of tracer. Whole-body planar imaging is performed, including lateral skull images, and SPECT of the neck, thorax, abdomen, and pelvis is obtained.

Another radiopharmaceutical agent used to image neuroendocrine tumors is indium-111–labeled pentetreotide (Octreoscan), a somatostatin receptor imaging agent. Pentetreotide is useful in imaging a wide variety of neuroendocrine tumors that express a high density of somatostatin receptors, such as carcinoid tumors. The dosage is 100 µCi/kg (3.7 MBq/kg), with a minimun of 1 mCi (37 MBq), and a maximum of 6 mCi (222 MBq). Imaging is performed 24 hours after administration with planar and SPECT images of the torso.

Bone Scan

A bone scan is used frequently in the evaluation of the thoracic skeleton, such as in the evaluation of chest wall pain (Fig. 49-13). ^99mTc-labeled methylenediphosphonate is the most common radiopharmaceutical agent used for bone scans. This tracer is administered intravenously, and imaging is performed from 2 to 4 hours after administration, using a whole body sweep (anterior and posterior) or individual images of the area of interest. SPECT imaging also can be performed. This technique is particularly useful in the evaluation of the ribs and spine. When evaluating a patient for suspected osteomyelitis, initial blood flow images are recorded immediately after the administration of tracer, in addition to the standard delayed imaging.

Positron Emission Tomography

Positron emission tomography (PET) is useful in evaluating many primary and metastatic intrathoracic tumors in pediatric patients (Fig. 49-14). Pediatric tumors commonly evaluated with PET include Hodgkin lymphoma, non-Hodgkin lymphoma, rhabdomyosarcoma, osteosarcoma, Ewing sarcoma, neuroblastoma, and other less common malignancies.

The recommended dosage of fluorine 18-labeled deoxyglucose (FDG) is 150 µCi/kg (5.55 MBq) (minimum 500 µCi [18.5 MBq] and maximum 10 mCi [370 MBq]). Imaging is performed about 60 minutes after the injection to allow adequate accumulation of the tracer in the tumor. Images of the torso or of the entire body can be obtained according to the indication of the study. Several elements of patient preparation for the study are important. Because FDG is taken up competitively with glucose, the patient must fast (i.e., no solid food and nothing to drink except water) for at least 4 hours before the PET scan. The patient must not receive any form of glucose, including intravenous fluids containing glucose, for at least 4 hours before administration. The blood glucose level is measured routinely at the time of tracer administration and should be between 50 mg/dL and 200 mg/dL. Patients who are diabetic and/or dependent on insulin should take their diabetic medication and/or insulin 2 hours before the examination with a small amount of juice and dry toast or plain crackers. The patient should avoid any strenuous exercise 24 hours before the study because such exercise might lead to an increase in muscle uptake that hinders the interpretation of the study.

Another important aspect of pediatric PET studies is the high incidence of metabolically active brown fat activation compared with the adult population. The presence of brown fat increases the number of equivocal studies.⁶⁸ Warming patients at 24° C before and during the FDG uptake phase decreases the incidence of brown fat activation significantly.⁶⁹

In the evaluation of tumors, PET provides an advantage over anatomic imaging because it detects viable tissue. Use of PET in the evaluation of Hodgkin lymphoma has been shown to increase the detection of metastases when compared with CT alone.⁷⁰

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