Computed tomography (CT) is an outgrowth of space-age technology - the union between modern x-ray technology and computer science. The critical role of CT in pediatrie neuroradiology has been established.1 Although there is considerable evidence to support the worth of body CT in adults, there are a limited number of articles describing extracranial CT in children.2·3 The role of chest CT in adults is continually evolving.4,5
We will present some preliminary ideas on the potential application of CT for studying pediatrie chest disease. It seems obvious that a technique that can demonstrate anatomy with such great clarity should have considerable value in the chest. Our personal experience with approximately 50 chest CT examinations in infants and children leads us to be optimistic about its diagnostic potential.
We will describe the techniques, inherent problems, and indications for CT of the pediatrie thorax. Several illustrative examples will, we hope, support our enthusiasm for this new imaging modality.
Rapid scanners (with less than five-second scan time) are necessary for adequate chest CT. Sedation is usually not required in children four years of age or older. If sedation is required for a younger child or for a prolonged study, such as thoracic CT myelography, we give intramuscular Nembutal2 approximately 30 minutes before the study.
Contiguous 1-cm. sections are obtained from the thoracic inlet to the subdiaphragmatic region. It is particularly important that the entire thorax be imaged in contiguity in patients with possible parenchymal metastatic disease. Scanning levels may be marked after a preliminary locating radiograph with a radiopaque marker in place.6 This is most important when studying spine or chest-wall abnormalities. An "enhanced" scan after the intravenous injection of 3 cc. / kg. of 60 percent meglumine diatrizoate may be performed if the lesion is thought to be vascular or closely related to vascular structures.
The image field of view is varied from 20 cm. in the neonate to 40 cm. in the adolescent, depending on the size of the patient. By convention, CT images are displayed as if the viewer were looking from the patient's feet towards the patient's head. Since patients are usually scanned in the supine position, the right side of the patient will be to the viewer's left. The CT operator may alter the window level, window width, electrical magnification, and color display (high-density structures appearing white or black). For example, bony abnormalities are best visualized with a high positive level (bone density), wide window width (high latitude), and reverse-color display so that bone is black while soft tissue is white. Magnification may be helpful if the lesion is small. A mediastinal mass is best visualized with a low positive level (softtissue density) and a moderately narrow (mediumlatitude) window width. Chest CT often requires imaging for lung parenchyma, mediastinum, and bony thorax (Figure 1), with separate displays of the same reconstructed-image data.
UNIQUE PROBLEMS IN PEDIATRIC CT
Patient motion, as well as respiratory and cardiac motion, tends to degrade the CT image. Artifacts caused by patient motion, however, are decreased in the newer rapid scanners, and sedation can be used if necessary.
The small size of the neonatal chest requires that a smaller CT field of view be used than is the case with older patients, with a resulting poorer quality of image. The quality of CT scans in infants and children should be improved in the future, thanks to improvements in computer programs and in image resolution now being planned.
There is a paucity of peri visceral fat in children. This tends to decrease organ resolution. The thymus is frequently seen on chest CT in children. This causes considerable difficulty in delineating anterior mediastinal lesions. Moreover, it may be impossible to determine if the thymus is normal but prominent or pathologically enlarged.
The use of CT, like that of any other imaging modality, is dependent on diagnostic benefit vs. patient risk. Chest CT does require sedation, immobilization, and alteration of environment. Intravenous contrast material is used less frequently in chest CT than in abdominal or craniocerebral CT. Finally, a skin dose of approximately 2-3 rads is employed in a CT examination of the chest. This dose is comparable with that for an upper-gastrointestinal series, an intravenous pyelogram, or fulllung conventional tomography.
INDICATIONS FOR CT IN CHILDREN
Computed tomography provides a precise anatomic section of the chest. This modality frequently demonstrates abnormalities that are not apparent on conventional chest radiographs. For example, separate densities can be distinguished by CT far better than by conventional radiography (system sensitivity). The transverse display, moreover, permits various areas of the chest to be shown free of patterns caused by the overlying mediastinum, diaphragm, pulmonary vessels, or bony thorax.
A list of indications for extracranial CT has recently been prepared by the Society for Computed Body Tomography.5 These generalized guidelines must be modified for the pediatrie chest. We have outlined our current "indications" for chest CT in children at Duke University Medical Center in Table 1 . Computed tomography accurately delineates the anatomic location and extent of an abnormality. The attenuation coefficient (density) of a lesion may also be determined.
Lesions of the chest wall and pleura are difficult to examine on chest roentgenograms, since they may not be tangential to the x-ray beam. Fluoroscopy, xeroradiography, and conventional tomography cannot separate the various components of the chest wall. Computed tomography is ideal, since such lesions are shown in profile (Figure 2). The subcutaneous tissues, muscle, bone, and pleura can be identified (Figures 1 and 2). Associated pleural effusion or parenchymal nodules may also be recognized.
Figure 1. Chondromatous osteogenic sarcoma. Same CT section with density level and window-width settings for mediastinum (top left), bone (top right), and pulmonary parenchyma (bottom left). Note large, calcified, extrapleural mass arising from the posterior aspect of the right ninth rib (black arrows). There are several metastatic lung nodules (open arrows).
Examination of the mediastinum is one of the most rewarding uses of thoracic CT. Imaging of the mediastinum in profile allows the separation of a mass from confusing overlap of normal structures. The site of origin or density of an abnormality may allow a specific diagnosis. The extent of disease is clearly seen, and follow-up scans may be valuable in observing the response of disease to therapy.
Anterior mediastinum. Computed tomography can detect thymoma or thymic hyperplasia in selected adult patients with myasthenia gravis when plain chest films are normal or suspect.5 This type of investigation seems to be less rewarding in children7 but still may be indicated in adolescents, particularly if immediate thymectomy is not planned. Early experience indicates that CT is valuable in detecting chest-wall extension of lymphomas.8 Appreciation of this extension results in significant alteration of radiation treatment techniques.
POTENTIAL INDICATIONS FOR COMPUTED TOMOGRAPHY OF THE CHEST IN CHILDREN
Figure 2. Recurrent Ewing's sarcoma. Left Soft-tissue extrapleural mass (open arrows) has recurred at the site of previous rib resection (black arrows). Right: Color reversal with electrical magnification shows mass and operative changes.
Middle mediastinum. Computed tomography can differentiate cystic, fatty, and solid masses in the middle mediastinum. However, the most common middle mediastinal mass in children, bronchopulmonary foregut malformation, may be seen as either cystic or solid by CT. Rarely, CT may aid in differentiating an enlarged pulmonary artery from a solid hilar mass when conventional tomography is equivocal. We still prefer 55-degree oblique conventional tomography to CT for the assessing of hilar abnormalities.
Figure 3. Ganglioneurobiastoma without extradural extension. Top left: Two conventional CT scans showing large calcified left posterior mediastinal mass. Black arrow (scan top right) points to aorta displaced anteriorly and to the right. In two views, bottom row, computed tomographic metrizamide myetogram demonstrate water-soluble contrast material within the nondisplaced subarachnoid space. Scan at bottom right is a magnified CTMM. The spinal cord (open arrow) is in normal position, and the neurocentral synchondroses (black arrows) are normal.
Figure 4. Neuroblastoma with extradural extension. This CTMM shows that the left intervertebral foramen (arrow) is widened. Both spinal cord and subarachnoid space are displaced from left to right by extradural tumor extension. (Courtesy of Derek C. HarwoodNash, M.D., Toronto.)
Posterior mediastinum. Computed tomography is particularly valuable in the preoperative study of posterior mediastinal masses. Approximately 95 percent of the posterior mediastinal masses found in children are neurogenic (neuroblastoma, ganglioneuroma, ganglioneuroblastoma). Computed tomography, in conjunction with injection of metrizamide into the subarachnoid space (CTMM), permits imaging of the mediastinal mass, adjacent mediastinal structures, pulmonary parenchyma, spine, and spinal contents.6 Subtle calcification not visualized on conventional radiographs may be readily identified by CT (Figure 3). Paravertebral chest masses in children are notorious for extradural extension . 9 And CTMM is the only study required in the preoperative study of such masses, since it can exclude (Figure 3) or confirm (Figure 4) extradural extension. It is our belief that patients with documented extradural extension of tumor should have decompressive Iaminectomy before thoracotomy for total removal of the mediastinal mass.9,10 This laminectomy decreases the potential for neurologic morbidity related to bleeding from residual extradural tumor.
Figure 5. Foreign body (pumpkin seed) in trachea. Views show linear density in trachea (arrows) on conventional CT (left) and with magnification (right). The seed is almost perpendicular to the normal plane of the carina. (Courtesy of Paul E. Berger, M.D., Buffalo.)
Airway patency can be visualized by CT. The transverse sections permit clear demonstration of trachea and major bronchi. Extrinsic compression of the trachea is readily apparent on CT, when plain films and conventional tomography may give normal findings. Computed tomography also may be useful in confirming the presence of an aspirated foreign body (Figure 5).
Recent studies11,12 have shown that parenchymal metastatic nodules can be identified on CT when they cannot be demonstrated by conventional chest x-ray or tomography (Figure 6). There are at least three reasons for this increased detection of pulmonary nodules by CT. As previously discussed, the CT system has an intrinsically greater sensitivity than conventional radiography. Second, the CT display format depicts nodules as white densities against a dark-gray background rather than as a lighter-gray density on a darker-gray background, as in conventional chest radiography or tomography. Finally, the transverse orientation of the CT section allows the depiction of nodules without the superimposition of heart, mediastinum, vessels, diaphragm, or bony thorax. Lesions of the lung apex, posterior cardiophrenic sulci, retrosternal area, retrocardiac region, and subpleural area are well visualized by CT (Figure 6).
While CT is able to detect more pulmonary nodules in patients with metastatic disease than any other imaging modality, this high sensitivity is accompanied by a decrease in specificity since - in adults - as many as 60 percent of the additional nodules detected by CT may prove to be benign lesions at resection.12 Although a comparable radiologic-pathologic correlative study has not been performed for pediatrie patients, our experience indicates that benign granulomas and pleural-based normal lymph nodes are not shown by CT in children. Thus, CT is both more sensitive and more specific for pulmonary metastatic disease in infants and children.
Figure 6. Metastatic osteogenic sarcoma. Chest radiograph shows only a single nodule (arrow) in right lung.
Figure 7. Computed tomograms ot same patient shown in Rgure 6. Chest CT at level of carina (scan at left) and T-8 (right) shows several bilateral metastatic lesions (arrows).
The pulmonary parenchyma is a common site for metastatic disease in tumors of children. Sarcomas, particularly osteogenic sarcoma and Wilms* tumor, have a propensity for metastasis to the lungs. The detection of these metastatic lesions may lead to additional treatment - surgery, chemotherapy, or irradiation. Computed tomography is the most accurate method for determining whether a patient with a known primary tumor has a solitary metastatic nodule or several metastatic nodules. This is important if resection of a nodule is planned (Figures 6 and 7). Therefore, CT has a critical role in the diagnosis and management of infants, children, and adolescents with known or suspected pulmonary metastatic disease. It provides valuable information that cannot be obtained by any other noninvasive procedure.
Figure 8. Cystic adenomatoid malformation. Chest radiograph at left shows questionable lucency at left base. But chest CT (right) demonstrates mass just above left hemidiaphragm (arrows), containing air-filled lucencies.
Density determinations possible with CT may be diagnostic of certain parenchymal abnormalities (Figure 8). Computed tomography is able to demonstrate minimal differences in tissue densities and record these differences both pictorially and numerically. Such information is certain to expand our knowledge of various disease entities. Preliminary work has established an average range of densities for different portions of the lung parenchyma in normal subjects of various ages.4 In the future, a change in pulmonary parenchymal density may be used as an indication of early lung disease.
Chest CT is a feasible imaging modality for use in children. It provides detailed information about the chest wall, bony thorax, mediastinum, and lungs. It is particularly valuable in the assessment of metastatic disease and posterior mediastinal masses.
The proper uses and indications for chest CT are only now being delineated. We are confident that this modality will become increasingly important in the diagnosis and management of pediatrie chest disease.
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POTENTIAL INDICATIONS FOR COMPUTED TOMOGRAPHY OF THE CHEST IN CHILDREN