Diagnostic Imaging of the Neonate

Introduction

Diagnostic imaging is integral to the evaluation of neonates with medical and surgical conditions. Understanding the advantages and shortcomings of available imaging modalities as well as the most recent practice patterns facilitates selection of appropriate imaging. This chapter begins with a brief introduction to the benefits and potential hazards of relevant imaging modalities; the remainder of the chapter reviews the imaging approach to common neonatal disease according to organ system. This discussion highlights normal anatomy and the spectrum of imaging appearances of common neonatal pathology.

Radiography, Fluoroscopy, and Computed Tomography

Because a large number of radiographs may be performed in critically ill neonates, attention has been drawn to the potential cancer risk from cumulative doses of ionizing radiation in medical imaging. The radiation dose of a portable chest radiograph is approximately 0.02 mSv; this is a very small fraction of the 3 mSv of annual natural background radiation to which the average individual is exposed. Commonly performed fluoroscopy procedures in the newborn, such as an upper gastrointestinal series, contrast enema, or voiding cystourethrogram, can be performed at doses between 0.5 and 2.0 mSv in infants. Although performed far less frequently, computed tomography (CT) is also performed at low radiation doses; most head and body CT scans can be performed at a dose less than 2 mSv. Current literature indicates that the risk of a future cancer from low dose radiation (below 100 mSv for serial exams) is “too low to be detectable and likely nonexistent.” In the absence of data confirming a causal relationship between low dose radiation from medical imaging and cancer, parents and providers should be reassured that the benefits of a medically necessary exam far exceed any potential future cancer risk. Even so, current recommendations endorse performing only medically necessary exams and limiting radiation exposure to doses that are “as low as reasonably achievable.” Such steps include using patient size–based tube current and tube voltage, attention to patient positioning, artifact removal, and beam collimation.

Infants requiring a CT scan of the brain or body are typically scanned without sedation, because scan times are usually less than 5-10 seconds. Iodine-based intravenous contrast may be administered to evaluate for cardiovascular abnormalities, neoplasm, or infectious disease. In the absence of known renal failure, neonates show no increased risk for renal toxicity after receiving iodinated intravenous contrast material. A distinct disadvantage of CT imaging is the lack of portability. Because CT equipment requires room shielding, a relatively large footprint, and is highly sensitive to small fluctuations in temperature and humidity, patients must travel to the radiology department's CT suite for imaging.

Ultrasound

Following radiography, sonography is the most common imaging examination performed in the neonate. Ultrasound is particularly suitable for newborns because of its low cost, accessibility, portability, lack of ionizing radiation, and absence of known health risks. There is no need for patient sedation, and technologists can easily capture both static and cine images that include characterization of flow dynamics with color and spectral Doppler. High-quality sonographic images are dependent upon the skill of the technologist in identifying optimal sonographic windows and in the appropriate selection of ultrasound probes of varying frequencies. Because sound waves travel poorly through air and calcified bone, sonographic evaluation of anatomy can be limited if obscured by gas-filled bowel, aerated lung, and overlying bones. Another disadvantage of ultrasound is the relatively low spatial resolution relative to CT and MRI.

MRI

While ultrasound remains the screening modality of choice in the prenatal period, magnetic resonance (MR) imaging has become a valuable adjunct to identify at-risk infants who may benefit from in utero intervention as well as to prepare for optimal perinatal management. In the postnatal period, MR imaging can provide vital detailed information that ultrasound and CT cannot—particularly in the diagnosis of traumatic or anoxic brain injury and congenital anomalies. Gadolinium-based intravenous contrast may be used without risk of renal toxicity in infants without known renal failure. Despite scan times that may exceed 30 minutes, the majority of infants under three months of age can be scanned without sedation using the “feed and swaddle” technique. The most significant barrier to MR use is the need to travel to the radiology department MR suite, frequently not in close proximity to the NICU. New dedicated infant-sized MR scanners designed to be housed in the NICU may offer a solution to the risks of transport and minimize the disruption of care for critically ill neonates.

Scintigraphy

Nuclear medicine contributes to the care of newborns with congenital anomalies, infection, and neoplasm. The radiopharmaceuticals used are nonallergenic and show no toxic effects. The radiation dose for most exams is between 0.6 mSv and 5.5 mSv, in the range of CT imaging. Although most studies require that the patient remain still for a relatively long period of time, most neonates can be imaged without sedation. As with CT and US, these exams require patient transport to the nuclear medicine suite of the radiology department.

Chest

Respiratory distress is the most common indication for imaging in the newborn. The chest radiograph (CXR) plays an important role in the assessment of cardiopulmonary pathology in the neonate and young infant and is usually the initial examination of choice.

Portable chest radiographs are obtained in the anteroposterior (AP) view with the infant lying supine. The arms should be extended away from the body or above the head, and the thighs should be immobilized. With proper positioning, the radiograph should demonstrate symmetry of the clavicles and ribs and a midline appearance of the superior mediastinum. It is important to ensure proper patient positioning because a rotation-distorted image may obscure or mimic cardiac and pulmonary pathology. Further, leads and electrodes may also compromise image quality by obscuring regions of critical importance such as tube and line placement or small air leaks.

Additional imaging modalities can be used to evaluate specific neonatal chest abnormalities. Sonography is useful in the assessment of diaphragmatic motion, pleural fluid, and catheter complications. Cross-sectional imaging with CT or MRI is useful for surgical and medical treatment planning of more complex thoracic conditions. Specifically, CT is the study of choice to evaluate parenchymal and mediastinal lesions. Both CT and MR are useful in the evaluation of complex congenital heart conditions.

Evaluating a Normal Chest

Familiarity with the normal appearance of the newborn chest radiograph improves recognition of pathologic changes. Normal lungs appear primarily radiolucent and symmetric in volume. The pulmonary vessels are seen as branching, linear shadows that taper in size as they extend from the hilum to the lung periphery. Normal vessels decrease in size and number in the lateral half of the lung and are not visualized in the lung periphery. Normally collapsed, the pleural space is visualized only when it is distended from fluid, air, or pleural thickening. The heart borders should be distinct and the diaphragm should be outlined clearly against aerated lung. The normal cardiac diameter on an AP radiograph should be less than 60% of the thoracic diameter. The normal thymus is visible in most newborns. Extremely variable in size and shape, it is composed of two asymmetric lobes and may therefore have an asymmetric appearance in chest radiography ( Fig. 38.1 ). The “wavy” undulations of the lateral borders often silhouette a portion of the heart and can give the appearance of the cardiomegaly ( Fig. 38.2 ).

Respiratory Disease: Medically Treated

Respiratory Distress Syndrome

Respiratory distress syndrome (RDS) from surfactant deficiency is the most common cause of respiratory distress and a leading cause of morbidity in the premature infant. The prematurity of type II pneumocytes results in an inadequate production of surfactant. This triggers a cascade of responses leading to decreased alveolar distensibility, capillary leak edema, noncompliant lungs, and respiratory distress.

The typical radiographic appearance of RDS reflects generalized alveolar collapse and shows a finely granular or ground-glass pattern with diminished lung volumes. The severity of radiographic disease is variable and usually correlates with the severity of clinical disease. Mild radiographic disease is characterized by a finely granular pattern that allows visualization of normal vessels ( Fig. 38.3 ), whereas severe disease results in loss of defined heart borders and diaphragm ( Fig. 38.4 ). Peripheral air bronchograms may be seen with severe disease because of air in the bronchi being visualized against a background of alveolar collapse. The distribution of disease is usually diffuse and symmetric; however, patchy or asymmetric disease may be seen. The radiographic changes associated with RDS are often seen immediately after birth but can also develop over the first 6-12 hours of life. The radiographic abnormalities related to uncomplicated RDS should resolve by the time the neonate is 3-4 days old or sooner if surfactant therapy is given. Severe RDS on the initial radiograph in the first few hours of life has been proposed as predictive of continuous positive airway pressure failure in extremely low birth weight infants. New diffuse worsening of bilateral opacities in the lungs may be seen with pulmonary edema or pulmonary hemorrhage. Sudden increase in focal opacity usually indicates atelectasis.

Chest radiography can be used to help assess the effectiveness of surfactant replacement therapy in infants with RDS. Lung ultrasound has also been proposed as a technique to differentiate RDS from transient tachypnea and possible need for surfactant therapy. Typically, improvement in the appearance of the lungs is rapid and uniform after surfactant administration. In 80%-90% of neonates treated with surfactant, improvement occurs in one or both lungs. When there is a partial response, the improvement may be asymmetric or even restricted to one lung. Explanations for asymmetric radiographic improvement following surfactant therapy include (1) maldistribution of surfactant, (2) insufficient surfactant, and (3) regional differences in lung aeration before surfactant treatment. The absence of radiographic improvement after surfactant administration is a poor prognostic sign and suggests a diagnosis other than surfactant deficiency. Surfactant-related pulmonary hemorrhage may mimic RDS with focal or diffuse alveolar parenchymal disease.

Complications can result from the high distending pressures of mechanical ventilation that may be required in the treatment of RDS. Alveolar rupture from overdistention results in pulmonary interstitial emphysema (PIE). The radiographic appearance of PIE includes small, rounded or linear lucencies representing interstitial air coursing along the bronchovascular sheaths ( Fig. 38.5 ). These meandering radiolucencies may be diffuse or localized. Larger focal air collections (pseudocysts) may also form in the interstitium of the lung. Pulmonary interstitial emphysema can dissect into the mediastinum or the pleural space, resulting in a pneumomediastinum or pneumothorax. Radiographic signs of pneumomediastinum include (1) lateral displacement of the mediastinal pleura; (2) continuous diaphragm sign; and (3) superior elevation of the thymus, which is referred to as the spinnaker sail sign or angel wings ( Fig. 38.6 ). Radiographic findings of pneumothorax include (1) increased lucency, (2) identification of the visceral pleural line ( Fig. 38.7 ), (3) increased sharpness of the adjacent mediastinal border or hemidiaphragm, and (4) visualization of a deep costophrenic sulcus. Pneumothoraces may be seen bilaterally ( Fig. 38.8 ). Lateral decubitus or cross-table lateral radiographs can be useful in the detection of small pneumothoraces. Large pneumothoraces can produce tension, resulting in contralateral shift of mediastinal structures and depression or eversion of the ipsilateral hemidiaphragm.

Neonatal Pneumonia

Most neonatal pneumonias are of bacterial origin, including streptococci, Staphylococcus aureus , and Escherichia coli . These infections may be acquired in utero, during delivery, or after birth. Infection typically disseminates widely throughout the lungs because of incomplete formation of the interlobar fissures at this age. First described with group B streptococcus pneumonia, diffuse granular or ground-glass opacities are often seen and are indistinguishable from RDS. Alternatively, coarse nodularity or a streaky, hazy appearance of the lungs may be seen. The presence of pleural fluid should raise the suspicion of bacterial infection, because effusions are uncommon in RDS or viral pneumonia.

Transient Tachypnea of the Newborn

Transient tachypnea of the newborn (TTN) is associated with retained lung fluid that may be associated with precipitous delivery or cesarean section. Typically a clinical diagnosis, TTN has a varied and nonspecific appearance. The radiograph may appear normal or show streaky, perihilar opacities and hyperinflation. Small pleural effusions may also be seen. A normal heart size with TTN helps distinguish retained fluid from pulmonary edema or heart failure. The radiographic and clinical findings of TTN have a benign course, usually resolving within the first 24-48 hours of life.

Meconium Aspiration Syndrome

The expulsion of meconium before birth is often related to fetal distress leading to a hypoxia-induced vagal response. It typically occurs in full-term or postmature infants. Fetal aspiration of meconium causes obstruction of small airways with associated atelectasis and air trapping. Radiographic findings of meconium aspiration syndrome are seen within the first few hours of birth and include coarse, patchy, or nodular opacities and segmental hyperinflation ( Fig. 38.9 ). The distribution of disease is bilateral and often asymmetric. Complications include chemical pneumonitis, surfactant inactivation, pulmonary hypertension, and air-leak phenomena such as pneumothorax and pneumomediastinum. Pleural effusion may be present.

Pulmonary Hemorrhage

Pulmonary hemorrhage in infants may result from hypoxia-induced capillary damage. In premature infants whose RDS is resolving and pulmonary vasorelaxation occurring, the resultant pulmonary overcirculation from a PDA may cause pulmonary hemorrhage. In addition to PDA, periventricular leukomalacia and seizures are known associations with pulmonary hemorrhage. In intubated patients, the diagnosis is usually established by detecting blood in the endotracheal tube. The radiographic appearance of pulmonary hemorrhage is variable and nonspecific: small amounts of hemorrhage may not be visible, but more extensive hemorrhage results in focal or diffuse ground-glass opacities. Findings may mimic pneumonia or pulmonary edema. The radiographic changes from a single episode of pulmonary hemorrhage are usually transient, resolving within 24-48 hours.

Bronchopulmonary Dysplasia

Chronic lung disease in the premature infant, known as bronchopulmonary dysplasia (BPD), is most often seen in very low birth weight infants. Bronchopulmonary dysplasia is also seen in higher birth weight infants following prolonged mechanical ventilation for conditions including neonatal pneumonia, meconium aspiration, and congenital cardiac disorders. The definitions of BPD are evolving and include both clinical and physiologic criteria. Pathologically, BPD is considered to be part of the group of alveolar growth abnormalities and can be recognized on imaging. Because BPD is the result of injury and repair of the immature developing lung, the pathologic and radiologic findings are affected by changes in therapy and the degree of prematurity of the infant. Bronchopulmonary dysplasia was originally described as airway injury, obstruction, inflammation, and parenchymal fibrosis. Chest radiograph findings of the later stages of BPD included hyperinflated lungs, asymmetric, coarse, patchy opacities, and cystic emphysematous changes.

The widespread adoption of antenatal glucocorticoid administration, postnatal surfactant therapy, and refinement of assisted ventilation have decreased lung injury from oxygen toxicity and barotrauma. Bronchopulmonary dysplasia is now rarely seen among infants of gestational age greater than 30 weeks or birth weight over 1200 grams. Modern therapies have also allowed an increased survival of very low birth weight infants. Despite the absence of prior severe RDS or barotrauma, these less mature infants often have a more insidious development of BPD, known as the “new” BPD. Arrested lung development leading to decreased alveolar and microvascular growth is thought to contribute to this condition. Radiographic and CT findings range from near normal to disordered lung architecture with hyperlucent areas, linear and subpleural opacities, and bullae typical of chronic lung disease ( Fig. 38.10 ) (see Chapter 69 ).

Respiratory Disease: Surgically Treated

Congenital Diaphragmatic Hernia

Congenital diaphragmatic hernia (CDH) is a complex life-threatening lesion caused by defective fusion of the pleuroperitoneal membranes during embryologic development. Patent pleuroperitoneal canals located posterolaterally are known as the foramina of Bochdalek. Bowel and solid organs may herniate through the foramen into the hemithorax, most commonly on the left side. At birth, the herniated bowel loops may be fluid-filled, making radiographic diagnosis difficult. Eventually, gas-filled bowel loops are seen in the thorax with a paucity of bowel loops in the abdomen ( Fig. 38.11 ). The ipsilateral lung is almost universally hypoplastic, and there is usually contralateral shift of the mediastinum, resulting in contralateral lung hypoplasia. However, partial aeration of the ipsilateral lung with a smaller hernia may also occur and may be difficult to distinguish from complete eventration ( Fig. 38.12 ). In such cases, ultrasound can be used to evaluate for continuity of the hemidiaphragm. Prenatal sonography and fetal MRI ( Fig. 38.13 ) allow early diagnosis and may predict neonatal survival by evaluating the degree of pulmonary hypoplasia and defining associated anomalies.

Congenital Pulmonary Airway Malformations

Congenital pulmonary airway malformations (CPAMs), previously known as congenital cystic adenomatoid malformations (CCAMs), are a group of congenital, hamartomatous cystic, and noncystic lung masses characterized by overgrowth of the primary bronchioles and a proximal communication with a defective bronchial tree. The new terminology has been recommended because not all lesions are cystic and only one is adenomatoid. Stocker updated the classification system to include five types (0-4) based on cyst size and similarity to segments of the developing bronchial tree and air spaces: Type 0 is acinar dysplasia of tracheal or bronchial origin and is incompatible with life. Type 1 , the most common, has a single or multiple large cysts (>2 cm) of bronchial or bronchiolar origin; type 2 has a single or multiple small cysts (≤2 cm) of bronchiolar origin; type 3 is predominantly solid with microcysts (<0.5 cm) of bronchiolar-alveolar duct origin; and type 4 , characterized by large air-filled cysts, has a distal acinar origin and is indistinguishable from pleuropulmonary blastoma on imaging. Congenital pulmonary airway malformation can also be seen in association with other foregut anomalies, most commonly pulmonary sequestration.

Prenatal sonography and fetal MRI classifies CPAMs mainly based on the presence of macrocysts or microcysts. At birth, chest radiography and CT may show a range of findings from large single or multiple air-filled cystic structures to solid lesions that resemble consolidation ( Fig. 38.14 A and B ). Because of the association of CPAMs with sequestration, precise vascular mapping is essential. Ultrasound may be useful in identifying an abnormal vascular systemic supply typical of sequestration (see Fig. 38.14 C ). However, CT angiography with 2D and 3D reconstructions provides the most accurate assessment of the lung parenchymal and vascular anatomy of these lesions.

Congenital Lobar Overinflation

Congenital lobar overinflation (CLO) or emphysema is a condition characterized by progressive overinflation of one or more pulmonary lobes. This may be caused by intrinsic bronchial narrowing from weak or absent bronchial cartilage or may be caused by extrinsic bronchial narrowing from mass effect of adjacent structures. The collapsed bronchus can result in one-way valve obstruction causing air trapping and progressive distention of the distal airways in the affected lobe. Congenital lobar overinflation most commonly affects the left upper lobe, followed by the right middle lobe and the right upper lobe. At birth, the involved lobe may be radiographically opaque because of retained lung fluid. When the fluid clears, imaging demonstrates progressively increased volume, hyperlucency, and attenuated vascular markings of the involved lobe with compression of the remaining ipsilateral lung and mediastinal shift ( Fig. 38.15 ). CT more precisely characterizes these findings and can distinguish multilobar involvement. CT is also helpful in excluding causes of extrinsic bronchial compression such as vascular anomalies or mediastinal masses (see Chapter 66 ).

Esophageal Atresia and Tracheoesophageal Fistula

The pairing of the terms esophageal atresia and tracheoesophageal fistula (TEF) describes a disorder in formation and separation of the primitive foregut and esophagus. A spectrum of malformations is noted, ranging from esophageal atresia (with or without a proximal or distal tracheoesophageal fistula) to a tracheoesophageal fistula without esophageal atresia. The most common type, involving proximal esophageal atresia with a distal tracheoesophageal fistula, accounts for more than 80% of cases. With this type, a blind-ending, air-filled proximal esophageal pouch is noted on a chest radiograph ( Fig. 38.16 ). The presence of a distal fistula is supported by air in the gastrointestinal tract. An esophagram can be performed to evaluate for a fistulous tract. Tracheoesophageal fistula is associated with multisystem abnormalities in approximately one-third of cases, including vertebral, cardiac, renal, and limb anomalies as well as other gastrointestinal tract atresias. All infants with a tracheoesophageal abnormality should undergo additional imaging evaluation to assess for associated anomalies.

Heart

Cardiac disease in infants is usually congenital in origin. Chest radiography rarely leads to a specific diagnosis and is primarily used to exclude pulmonary conditions as a cause of respiratory distress. Echocardiography, the primary initial imaging modality, may incompletely define extracardiac vasculature and associated airway involvement. CT angiography (CTA) and cardiac MRI (CMRI) complement echocardiography by illustrating these important structures as well as providing functional information.

Great variability in cardiac size is found in congenital heart anomalies. Cardiomegaly is present in neonates with large left-to-right shunts, Ebstein anomaly, hypoplastic left heart syndrome, and cardiomyopathy. Alternatively, cardiomegaly may be seen transiently in the absence of cardiac disease with hypoglycemia, hypocalcemia, severe anemia, or in infants of diabetic mothers. Enlargement of specific cardiac chambers cannot be accurately assessed by chest radiography.

In cyanotic congenital heart disease, the caliber of the pulmonary arteries is reduced, the hila appear small, and the lungs may appear more lucent. However, it is difficult to differentiate normal from small pulmonary vessels by chest radiography. Persistent pulmonary hypertension may mimic cyanotic congenital heart disease, both clinically and radiographically, because of cardiomegaly and pulmonary oligemia.

Radiography is useful in diagnosing increased pulmonary arterial vascularity associated with large left-to-right shunts seen in infants with patent ductus arteriosus, ventricular septal defect, atrial septal defect, and endocardial cushion defect. Increased pulmonary arterial vascularity becomes visible on a CXR when the ratio of left-to-right shunt is greater than 3 : 1. Increased branching linear shadows will be seen in the perihilar region, and vascular markings will be seen in the periphery of the lung fields. The pulmonary arterial vascularity usually appears normal with shunts of a lesser degree (see Chapter 72 ).

CT is particularly useful in evaluation of aortic arch anomalies (coarctation of the aorta, interrupted aortic arch, and vascular ring or sling) ( Fig. 38.17 ), pulmonary artery anomalies (pulmonary artery stenosis and aorticopulmonary collaterals), anomalies of pulmonary venous return (anomalous pulmonary venous return and pulmonary vein stenosis), coronary artery anomalies, extracardiac anatomy (pericardial masses, airway compression and parenchymal disease), and postoperative evaluation after surgery. Indications for CMRI include: cardiac chamber volume, mass, and functional analysis; cardiac morphologic analysis in complex congenital heart disease; soft tissue characterization of cardiac masses and tumors; hemodynamic analysis of pulmonary to systemic blood flow, ejection fraction, and valvular stenosis and regurgitation; and postoperative assessment ( Fig. 38.18 ).