24 Fundamentals of Pediatric Radiology
The last three decades have produced an enormous era of technical advance, data acquisition, and data transfer especially pertaining to imaging. This has resulted in significant changes in protocols for imaging of various symptom complexes. New imaging modalities such as computed tomography (CT), magnetic resonance (MR), and positron emission tomography (PET) have expanded our ability to diagnose previously hidden conditions and also our knowledge of these conditions.
The goal of this chapter is to highlight pediatric imaging and to review the imaging issues faced in routine pediatric practice. This chapter is not meant to be a comprehensive review of the subject because many well-selected radiology images and related discussions have been included in the other chapters. The first section includes an introduction to pediatric radiology and radiation safety, and conveys concise, up-to-date information on imaging modalities as they are applied to each body part. The second section describes the various CT and MR applications in neuroimaging. The last section covers key concepts on how and why nuclear medicine and PET procedures are performed.
Introduction
Children have special needs and different disease processes, and thus the diagnostic imaging approaches are also different. Pediatric imaging should be problem oriented. Communication between the referring physician and the pediatric radiologist is encouraged (e-Table 24-1). The essential components of a pediatric imaging facility are listed in Table 24-1.
Table 24-1 Essential Components of Pediatric Imaging
From Osborn LM, DeWitt TG, First LR, et al, editors: Pediatrics, Philadelphia, 2005, Mosby Elsevier.
e-Table 24-1 Role of the Generalist
From Osborn LM, DeWitt TG, First LR, et al, editors: Pediatrics, Philadelphia, 2005, Mosby Elsevier.
Imaging Modalities
Pediatric diagnostic imaging can be achieved by various modalities (Table 24-2). X-rays are used in conventional radiography, computed radiography (CR), fluoroscopy, angiography, and computed tomography (CT). Gamma rays are used in nuclear scintigraphy and positron emission tomography (PET). Ultrasonography uses inaudible sound waves ranging in frequency from 1 to 20 MHz to produce images, whereas magnetic resonance (MR) images are generated with a strong magnetic field and radiofrequency (RF) pulse. The digital age has provided us with picture archiving and communication systems (PACS). A PACS eliminates the use of films, permits rapid retrieval of images and remote viewing, and compacts storage.
Table 24-2 Advantages and Disadvantages of Imaging Modalities
Modality | Advantages | Disadvantages |
---|---|---|
X-ray film | Fast; relatively inexpensive; available | Uses radiation; poor soft tissue contrast; two-dimensional imaging only |
Fluoroscopy | Real-time imaging; relatively inexpensive; available; useful in operating room; can be portable | Uses radiation; no cross-sectional imaging |
CT | Readily available; excellent delineation of bones, soft tissues, and calcification; multiplanar and 3D reconstruction; minimally invasive (CT angiography); assists in interventions | Intermediate to high radiation dose; relatively expensive; IV contrast side effects (nephrotoxicity and anaphylaxis); weight limit |
MRI | Excellent soft tissue characterization; no ionizing radiation; multiplanar imaging; minimally invasive (MR angiography); functional imaging; assists in interventions | Less readily available; expensive; claustrophobia often a problem; lengthy exams; limited use in unstable patients; may need sedation/GA; metal artifact; contraindicated with cardiac pacemakers and some devices; gadolinium-induced nephrogenic systemic fibrosis (NSF) in patients with renal impairment; weight limit |
Ultrasound | Portable; inexpensive machine; real time; least expensive cross-sectional imaging modality; no radiation; differentiates cystic vs. solid masses; multiplanar imaging; Doppler evaluation of blood flow; assists in interventional procedures | Difficult with obese and immobile patients; highly operator dependent; bone and gas obscure anatomy |
Nuclear medicine | Readily available; functional/molecular imaging | Intermediate to high radiation dose; weak anatomic analysis; may need sedation; radioactive urine andbody fluids; expensive |
3D, three-dimensional; CT, computed tomography; GA, general anesthetic; IV, intravenous; MRI, magnetic resonance imaging.
Child-friendly Atmosphere
A few simple techniques can help create a positive hospital experience for the child. Distraction in the waiting room may be beneficial for children of all ages, and can be achieved with posters, pictures, and toys. Effective distraction for toddlers includes interactive toys, pinwheels, blowing bubbles, and singing. School-age children enjoy blowing bubbles, TV/video games, books, counting, and deep breathing. Teenagers may prefer deep breathing, stress balls, TV/video games, books, and music.
The well-trained imaging staff can relieve the patient’s apprehension and decrease time and effort to obtain the optimal examination. Technologists should have a gentle demeanor and wear child-friendly, cheerful uniforms.
Child life specialists are specially trained to help children prepare for health care experiences and enable them to cope with imaging or invasive procedures. If aspects of a procedure are painful or uncomfortable, any child older than age 2 is prepared in advance with truthful information, using words that can be understood. The child life specialist is especially useful in some situations, such as (1) a child whose injuries have resulted from suspected child abuse; (2) a child admitted with accidental injuries (e.g., a motor vehicle accident); (3) a child newly diagnosed with chronic illness; (4) a child who recently experienced traumatic loss or has a chronic illness (developmental delay); (5) a child who exhibits oppositional behavior; (6) a child having difficulty coping with a necessary procedure, that is, crying, fighting or hiding; and (7) a child who needs preparation for an invasive procedure.
Environment
The imaging room’s environment is modified to reduce a child’s anxiety. Smaller equipment can be hidden or concealed with covers and images. Large equipment such as CT scanners can be decorated to give a sense of adventure (Fig. 24-1), and allow study acquisition without sedation. Other distraction techniques include lamps placed in the line of vision of the patient and displaying entertaining images on the ceiling (Fig. 24-2), or the release of piquant aromas in the imaging room (e.g., coconut). Movie goggles allow the child to watch a favorite movie while undergoing magnetic resonance imaging (MRI) or a nuclear medicine scan.
Positioning
The technologist usually allows parents to be present in the examining rooms and even to assist with some studies. This decreases the repeat rate and so decreases the radiation dose to the child. When a child lies supine, he or she may feel vulnerable. Comfort positions can give children a sense of control and help them feel more relaxed, that is, sitting on a chair or in a parent’s lap, hand-holding with a parent, or hugging. The child is encouraged to participate in producing the best possible examination by being permitted to make individual choices (e.g., placement of a toy or blanket, the position of a parent, the flavor of oral contrast material, or the selection of a bandage).
Language
The radiology team should always remember to use simple language for child-friendly explanations. For children of any age, the wrong choice of words can have a negative impact and potentially long-lasting effects. Avoid saying “dye/contrast”; say instead, “a drink/sticky water that helps the doctor see the inside of your belly clearer in your pictures.” Also, describe the imaging room environment in ways that can be related to the child’s home, for example, the noise of a CT scanner can be described to be like that of a “washing machine,” or a urinary catheter can be described as a “tiny straw that takes out pee-pee and puts in the x-ray water.” Young children have little sense of time; the radiology team should always prepare them to know when they will be done with their examination. For example, if a child is undergoing a voiding cystourethrogram (VCUG), avoid saying, “this will only take 15 minutes”; instead, say “you’ll be all finished and get up after you potty on the table for us.”
Effective Radiation Dose
Effective dose is expressed as an SI unit, the millisievert (mSv) (Table 24-3). A major benefit of the effective dose is that it permits all radiologic examinations that use ionizing radiation to be directly compared, using a simple common scale. Note that the effective radiation dose of one adult chest radiograph (0.1 mSv) is comparable to natural background radiation for 10 days (background radiation is 3 mSv/year in the United States; people living in Colorado or New Mexico receive about 1.5 mSv more per year than those living near sea level, that is, 4 to 5 mSv/year).
Table 24-3 Estimated Medical Radiation Doses for a 5-Year-Old Child
Imaging Area | Effective Dose (mSv) | Equivalent No. of CXRs |
---|---|---|
Three-view ankle | 0.0015 | 1/14th |
Two-view chest | 0.02 | 1 |
Anteroposterior and lateral abdomen | 0.05 | ![]() |
Tc-99 m radionuclide cystogram | 0.18 | 9 |
Tc-99 m radionuclide bone scan | 6.2 | 310 |
FDG PET scan | 15.3 | 765 |
Fluoroscopic cystogram | 0.33 | 16 |
Head CT | 4 | 200 |
Chest CT | 3 | 150 |
Abdomen CT | 5 | 250 |
CT, computed tomography; CXRs, chest x-rays; FDG PET, fluorodeoxyglucose positron emission tomography; Tc-99 m, technetium-99 m.
Data provided by R. Reiman, MD (Occupational and Environmental Safety Office, Radiation Safety Division [www.safety.duke.edu/RadSafety], written communication, 2006). From Brody AS, Frush DP, Huda W, et al: Radiation risk to children from computed tomography, Pediatrics 120:677-682, 2007.
Radiation Safety
The radiologist as “consultant” can triage imaging examinations to eliminate inappropriate referrals or to use procedures with less or no ionizing radiation. Imaging protocols must be as evidence-based as possible and the American College of Radiology (ACR) and the Society of Pediatric Radiology (SPR) guidelines should be implemented.
Americans were exposed to more than seven times as much ionizing radiation from diagnostic medical procedures in 2006 than they were in the early 1980s. The increase over the past quarter century puts the cumulative national medical exposures on a level with natural background radiation exposure (Fig. 24-3). The estimated cumulative individual dose from all sources in the early 1980s was 3.6 mSv and in 2006 was 6.2 mSv, almost double the previously reported value. The increase in medical exposure was the only significant change in the two estimates. The largest part of the increase in medical exposure was from CT scans, amounting to almost one half of the imaging exposure, and nuclear cardiac scans, amounting to one fourth of the current total (Fig. 24-4). In 2006 alone, more than 63 million CT scans were performed in the United States. Approximately 7 million CT scans were obtained in children in 2007. Children are at increased risk from radiation because of their greater sensitivity to radiation and a longer lifetime to manifest those changes. To be safe, we should act as if low doses of radiation cause harm using the ALARA (as low as reasonably achievable) principle routinely.

Figure 24-3 Exposure of the population of the United States to ionizing radiation in the early 1980s and in 2006, according to National Council on Radiation Protection and Measurements (NCRP) report no. 160 (Ionizing radiation exposure of the population of the United States).
(From Schauer DA, Linton OW: National Council on Radiation Protection and Measurements report shows substantial medical exposure increase, Radiology 253:293-296, 2009.)

Figure 24-4 Collective effective dose as a percentage for all exposure categories in 2006, according to National Council on Radiation Protection and Measurements (NCRP) report no. 160 (Ionizing radiation exposure of the population of the United States).
(From Schauer DA, Linton OW: National Council on Radiation Protection and Measurements report shows substantial medical exposure increase, Radiology 253:293-296, 2009.)
The exact radiation risk in CT examinations, and even whether a risk absolutely exists, are controversial topics. However, most scientific and medical organizations support the concept of the linear, no-threshold model for ionizing radiation risk of cancer induction, and believe that radiation even at low levels (doses below 100 mSv) may have a harmful effect. This assumption, however, overlooks cellular repair mechanisms. Some researchers estimate the increased risk that a young child might develop cancer related to an abdominal CT scan is in the magnitude of 1 : 4000. This is based on the most widely used estimate of risk of cancer from ionizing radiation at 5% per sievert (Sv), and the diagnostic imaging doses are in the millisievert (mSv) range (5 mSv for abdominal CT). One should also note that the background lifetime risk of fatal cancer is 20% to 25% (1 in 4 or 5). The benefits of CT are real and known, and the risks are tiny and unknown.
Conservative estimations of potential risk (i.e., any required assumptions are made toward the direction of overestimating risk rather than underestimating it) show that the potential risk of dying from undergoing a CT examination is less than that of drowning or of a pedestrian dying from being struck by any form of ground transportation, both of which most Americans consider to be extremely unlikely events (e-Table 24-2); this provides a comparison of the statistical odds of dying from an abdominopelvic CT examination relative to other causes of death. It can be seen that the lifetime risk of a fatal cancer from all causes is 22.8%, and the lifetime potential risk of a fatal cancer from the radiation associated with a body CT examination is approximately 0.05%.
e-Table 24-2 Estimated Lifetime Risk of Death from Various Sources
Cause of Death | Estimated Number of Deaths per 1000 Individuals |
---|---|
Cancer | 228 |
Motor vehicle accident | 11.9 |
Radon in home | |
Average U.S. exposure | 3 |
High exposure (1%-3%) | 21 |
Arsenic in drinking water | |
2.5 µg/L (U.S. estimated average) | 1 |
50 µg/L (acceptable limit before 2006) | 13 |
Radiation-induced fatal cancer | |
Routine abdominopelvic CT, single phase, ∼10-mSv effective dose | 0.5 |
Annual dose limit for a radiation worker | |
10 mSv (recommended yearly average) | 0.5 |
50 mSv (limit in a single year) | 2.5 |
Pedestrian accident | 1.6 |
Drowning | 0.9 |
Bicycling | 0.2 |
Lightning strike | 0.013 |
From McCollough CH, Gimarães L, Fletcher JG: In defense of body CT, AJR Am J Roentgenol 193:28-39, 2009.
The ordering physician needs to ensure that a CT scan is justified, and the radiologist needs to optimize the scan. Because children are smaller than adults and need less radiation to create the same signal-to-noise ratios, the tube current (milliamperes, or mA) can be greatly reduced when imaging a small child. Other techniques include reducing the peak kilovoltage (kVp); using in-plane shielding for areas such as the eye, thyroid, and breasts; increasing beam pitch; and picking a CT manufacturer that has put effort into dose-reducing technology (e.g., adaptive statistical iterative reconstruction, or ASIR) (Fig. 24-5).
The “Image Gently” Campaign
The Alliance for Radiation Safety in Pediatric Imaging has created resources to address the relative risk of CT for children, including parent information pamphlets and a convenient medical imaging record card, similar to the familiar immunization card, for parents to track their child’s imaging history. This card may be distributed by pediatricians or downloaded by parents from the Image Gently website (e-Fig. 24-1). This card alerts families and their doctors to the frequency of patient imaging examinations. With relocation of families and the use of different hospital centers in the same community, this card may help decrease the number of repeat examinations performed.
Radiography
Introduction
Advantages of Radiography in Pediatric Imaging
The advantages of radiography in pediatric imaging include the following:
Physics
X-rays are a form of short electromagnetic radiation produced by energy conversion when fast-moving electrons from the cathode filament of the x-ray tube interact with the tungsten anode (target) (Fig. 24-6). The amplitude of the tube current (expressed as milliamperes, or mA) depends on the emission rate of electrons from the cathode, which is determined by the cathode temperature. The speed of the electrons as they are propelled from the cathode to the anode is determined by the x-ray tube potential (kilovoltage peak, or kVp). When an x-ray beam is directed toward the examined part of the body, an image is formed. The resultant image is a recording of internal body structures in which the black areas represent the least dense body structures that have allowed the x-rays to pass through (i.e., lungs) and the more dense structures (i.e., bone), which have absorbed the x-rays, appear white (e-Fig. 24-2).

e-Figure 24-2 An x-ray image. Shown is an anteroposterior chest radiograph of an infant, demonstrating the attenuation of the various body structures. The normal lungs are air-filled and therefore black. The heart (H) and liver (L) have absorbed some radiation and are slightly gray. The bones are dense, and are a lighter shade of gray to white. Note the sail sign of the normal thymus (T) and the rightward deviation of the distal trachea (arrow) by the normal left-sided aortic arch.
Computed radiography (CR) has replaced conventional film-based radiography. The acquired image is displayed instantly on the high-resolution monitor of the PACS. e-Table 24-3 lists common indications for plain radiography.
e-Table 24-3 Common Indications for Plain Radiography
Organ System | Indications |
---|---|
Head and neck | |
Chest | |
Abdomen | |
Musculoskeletal system | |
Miscellaneous |
From Osborn LM, DeWitt TG, First LR, et al, editors: Pediatrics. Philadelphia, 2005, Elsevier.
Radiography of the Airway
The anteroposterior (AP) and lateral views of the neck are useful in assessing the trachea, pharynx, retropharynx, epiglottis, tonsils, adenoids, and bony skeleton. Stridor is one of the most common indications for imaging the neck. Other indications include snoring, hoarseness, abnormal cry, neck mass, suspected foreign body, epistaxis, trauma, and caustic ingestion.
Lateral Soft Tissues of the Neck
The retropharyngeal soft tissues extend from the adenoids, which are visible by 3 to 6 months of age, to the origin of the esophagus at the level of C4 to C5. A useful ratio is the width of the retropharyngeal soft tissue to that of the C2 vertebral body. The ratio varies in inspiration from almost 1.0 before 1 year of age to 0.5 by 6 years of age. The soft tissue width should not exceed 50% of the accompanying vertebral body to C4 (Fig. 24-7). Expiratory tracheal buckling can create buckling of the trachea anteriorly, causing an apparent increase in retropharyngeal soft tissues and creating a “pseudo-retropharyngeal abscess” (Fig. 24-8). Pseudo-retropharyngeal abscess can be differentiated from a true abscess when the appropriate inspiratory film demonstrates supraglottic airway and hypopharyngeal distention with air (Fig. 24-9). Appropriate patient positioning is critical. The examination of the lateral view of the soft tissues of the neck must be performed in slight extension and during inspiration (Fig. 24-10). The most common cause of a pseudo-retropharyngeal abscess is a film taken during expiration or swallowing, or with an improperly positioned child.


Figure 24-7 A, Lateral radiograph of a normal airway. The cornua of the hyoid (arrowhead) point to the epiglottis. a, adenoids; t, tonsil. B, Diagrammatic representation of the normal anatomy of the upper airway.
(B, From Blickman JG, Parker BR, Barnes PD: Pediatric radiology: The requisites, ed 3, Philadelphia, 2009, Mosby Elsevier.)

Figure 24-8 Normal tracheal buckling. Chest radiographs of a 4-month-old infant demonstrate tracheal buckling, a normal occurrence when the film is exposed during flexion and/or expiration. A, In the frontal projection, normal tracheal buckling occurs rightward, away from the aortic arch. B, Anterior buckling is evident on the lateral projection. This normal anterior tracheal displacement frequently causes confusion because it simulates a retropharyngeal mass. Note that the airway should be visible on all normal chest films.

Figure 24-9 Effect of phase of respiration on the prevertebral soft tissue space. Arrow in A demonstrates the prevertebral soft tissue widening on expiration that disappears on inspiration (B).
(From Blickman JG, Parker BR, Barnes PD: Pediatric radiology: The requisites, ed 3, Philadelphia, 2009, Mosby Elsevier.)

Figure 24-10 False-positive identification of a retropharyngeal mass. A, An examination with poor head extension suggests a retropharyngeal mass. B, A repeat examination with better head extension and pressure applied to the anterior neck with a lead-gloved finger; the retropharyngeal soft tissues are normal. Intervention with a gloved finger is seldom necessary if the film is repeated in full extension and inspiration.
(From Hilton SvW, Edwards DK III: Practical pediatric radiology, ed 3, Philadelphia, 2006, Saunders Elsevier.)
The lateral view of the neck is optimal for evaluating the supraglottic airway (see Fig. 24-7). The lower border of the nasopharynx is the hard palate, soft palate, and uvula. The oropharynx (below the hard and soft palate) leads to the air spaces at the base of the tongue, which are the valleculae. Immediately behind the valleculae is the epiglottis. The hyoid bone is inferior and anterior to the valleculae. The oropharynx also merges posteriorly with the nasopharynx to form the hypopharynx. The tonsils are seen in the lateral walls of the hypopharynx. Anteriorly, the hypopharynx leads to the larynx and becomes the esophagus. The pyriform sinuses are the most lateral and inferior margins and provide a landmark for the level of the vocal cords.
Anteroposterior Film of the Neck
The frontal radiograph is best for evaluation of tracheal position. Normally, the trachea is slightly deviated to the right by the aortic arch (deviation to the left is always abnormal). A normal thymus will not affect the trachea. Expiration causes buckling of the trachea to the right (see Fig. 24-8). Note that the airway is a dynamic system and changes in caliber and position so that an isolated, single film may be quite misleading. Nonetheless, an abnormal configuration of the airway should be pursued in light of the clinical history.
Chest Radiograph
Interpretation of the Chest Film: 1. The Radiologist’s Circle
A systematic approach to the radiographic evaluation is crucial for anyone dealing with children. Comparison with previous imaging studies is mandatory and is facilitated by the use of a PACS. A chest film is always examined for information about the heart and lungs, but radiologists look first at the nonpulmonary areas, that is, the abdomen, bones, soft tissues, and airway, to be sure that they do not miss any abnormality. Only then should one progress to the mediastinum. A good habit to develop is to imagine a circle on the film so as to dispense with all the noncardiopulmonary areas. Begin at the corners, where the patient information is. Check the name, date, and especially the left and right markers. An easy way to complete the circle is to progress from the name tag to the markers to the ABCS of the film: A, abdomen; B, bones; C, chest (airway, mediastinum, lungs, and diaphragm); and S, soft tissues. Carefully observe the easily missed areas: under the diaphragm, through the heart, paraspinal lines, lung apices, shoulders, and soft tissues of the neck.
On every chest film, read the abdominal portion as you would read an abdominal film. Evaluate the abdomen (regardless of how little of it can be seen) on every chest film, and note whether the stomach bubble is on the left and the liver on the right. Is it an erect film? If so, examine it specifically for calcifications, gallstones, or pancreatic calcification.
Determine the presence of bowel distention, air–fluid levels, and free intraperitoneal air. The heart and liver are transparent organs; one can see opacities or bronchial markings projecting over their shadows. Then look at bones and soft tissues; one can often see portions of the arms, shoulders, ribs, sternum, and mandible, as well as cervical, thoracic, and lumbar vertebrae. Be alert for fractures (Fig. 24-11), congenital abnormalities (e.g., absent clavicles), bone destruction, or other signs of disease. Examine the soft tissues of the neck, thorax, and abdomen to detect any swelling, foreign body, calcifications, and so on. The soft tissues may reveal multiple artifacts, such as hair braids, buttons, bandages, electrocardiogram (ECG) electrodes, or redundant skin folds. Soft tissue swelling or subcutaneous calcifications can be clues to systemic disease.

Figure 24-11 Supine chest radiograph of a 3-day-old newborn with surfactant deficiency. A radiologist interpreting this chest film systematically, that is, by examining the ABCS (A, abdomen; B, bones; C, chest [airway, mediastinum, lungs, and diaphragm]; and S, soft tissues), shouldn’t miss the fracture involving the right humerus at the edge of the image. Note that the patient is rotated to the left with the heart appearing prominent, likely due to rotation.
Interpretation of the Chest Film: 2. Technical Factors
Degree of Inspiration: Lung Volumes
On an adequate film obtained during deep inspiration, nine posterior ribs and five anterior ribs should be seen above the diaphragm. If the child is too young to cooperate, expose on full inspiration, during normal respiration. Babies’ and toddlers’ breathing is abdominal: watch for an expanded abdomen in the AP position. The child/baby must not be crying during exposure. If so, inspiration will be too deep, and the overinflated lungs may create a misleading appearance that can be mistaken for pathology. The differences in appearance on inspiration and expiration are more marked than in adults. With a good inspiratory effort on the frontal view, less than one third of the heart projects below the dome of the diaphragm; the domes of the diaphragm are rounded (if very domed, the film is expiratory). If the child has taken a shallow breath, the heart may appear enlarged; the vessels may coalesce to give a false impression of an opacity, especially in the region of the bases and hila. On the lateral view, obliquely oriented hemidiaphragms are seen in good or possibly increased lung volume (if horizontally oriented, the film is expiratory). The vertebral bodies become blacker as we progress from superior to inferior on the lateral view.
Position of the Patient
Conventional radiographs of the chest are frequently produced with a portable machine and with the younger patient (less than 2 years of age) placed supine. Upright films can be obtained after age 2; until 3 or 4 years old the patient is usually sitting for an AP projection. Children aged 5 years and over can stand for a posteroanterior (PA) projection. Proper immobilization and positioning are mandatory. For radiation protection purposes, the primary beam must be collimated within the area of the cassette, and pediatric lead rubber aprons, obtainable in several sizes, should be used for gonadal protection. Frontal views are often the only ones necessary, but lateral views can be obtained as indicated.
When the x-ray passes through the patient from back to front (a PA projection), the heart is closer to the film and is less magnified. Conversely, if the x-ray beam enters the front of the patient’s chest, passes through the back and onto the film (an AP projection), the magnified heart and great vessels may give the impression of cardiomegaly. This is a common problem with portable chest films, which are taken in the AP projection. Also, the closer the tube to the film, the more the magnification. Routinely, portable films are exposed 40 inches (1 m) from the tube, adding to the magnification. This is compared to 6 feet (1.8 m) used in the erect patient, which causes less magnification.
When the patient is supine, the vascular supply to the upper and lower lobes of the lungs is equal because gravity has no effect. When sitting or standing, gravity plays a significant role, and the upper lobe vessels are less distended than the lower lobe vessels and consequently smaller (one third to two thirds size). One can determine that a film was produced with the patient in the erect position by looking at the air–fluid level in the stomach and by comparing the relative sizes of the upper and lower pulmonary vasculature.
Determining Rotation
If the patient is well centered on the frontal view: (1) the medial aspects of the clavicles are symmetrical in relation to the midline; (2) the anterior ribs are equidistant from ipsilateral pedicles; (3) the position of the carina approximates the right pedicles; and (4) the two lungs are symmetrical in density. The signs of rotation include asymmetrical clavicles, a difference in lung aeration, heart projected over one hemithorax and not the other, and asymmetrical ribs when relating the anterior rib to the pedicles (see Fig. 24-11). On the lateral view, the ribs are not seen posteriorly in the straight (unrotated) patient. If the patient is slightly rotated, the ribs are shown on each side posterior to the spine.
Adequacy of Exposure
Adequacy of exposure can be assessed on the frontal film by examining the vertebral column behind the heart. The exposure is correct when we can see (1) the detailed spine and pedicles behind the heart, and (2) the pulmonary vessels in the peripheral lung. If we can see only the spine but not the pulmonary vessels, the film is too dark (overexposed).
Mediastinum
The mediastinum is composed of the thymus, trachea, heart, great vessels, esophagus, lymph nodes, and neural elements. The mediastinum is divided on the lateral radiograph into (1) the anterior portion, including the space in front of the heart and great vessels; (2) the middle portion, that is, the space between the anterior and posterior mediastinal components, including the heart, airway, esophagus, and lymph nodes; and (3) the posterior portion, including everything behind a line connecting the mid-portion aspects of the vertebrae, including the vertebrae, neural elements, and paraspinal lymph tissue. In some classifications the posterior mediastinum begins with the anterior aspect of the vertebral body.
Thymus
The thymus may make interpreting pediatric chest radiographs difficult. It can simulate cardiac enlargement, lobar collapse, pulmonary infiltrates, and mediastinal masses. The thymus constitutes the major portion of the mediastinal silhouette in a normal newborn. It may extend from the lung apex to the diaphragmatic surfaces; be insinuated into the minor fissure on the right, giving a “sail sign” (see e-Fig. 24-2); and may be bilaterally symmetrical or predominantly one-sided (Fig. 24-12). The normal thymus is a “soft” organ situated in the anterior mediastinum and never “pushes” on the airway or any other intrathoracic structure. The thymus appears smaller as the child becomes older, but the thymus weighs most in adolescents. It is prominent in some children until 4 to 5 years of age, and may persist beyond 5 years, confounding interpretation. Thymic remnants can remain in adults and will be gradually replaced by fat. In an unwell child the thymus can decrease in size and is often not seen. The contour of the thymus is “wavy” because it insinuates itself between the anterior ribs (Fig. 24-13; and see Fig. 24-12).

Figure 24-12 Coronal MRI of the chest (short tau inversion recovery [STIR] sequence) demonstrates a left-sided normal thymus in a 7-month-old infant. An anterior cut shows the “wavy thymus sign” (arrows) as the thymus insinuates itself between the anterior ribs (r).
Effect of Age on the Normal Appearance of the Heart
The shape of the heart on plain radiographs changes with the patient’s age. The heart in younger individuals appears more globular in shape, making analysis of specific chamber abnormality difficult. The newborn right heart chambers are larger than the left, and before closure of the patent ductus arteriosus, right-sided cardiac output is greater than left-sided output. This makes identification of the aortic arch difficult or impossible. The right atrial contour in the frontal view and the right ventricular contour in the lateral view will appear abnormally enlarged in these patients. Furthermore, the transverse diameter of the heart is increased, thus increasing the normal cardiothoracic ratio. The thymic shadow regresses by the end of the first year of life, and the heart appears to rotate and descend into the chest. The typical “normal” appearance of the heart does not begin to become apparent until 6 to 8 years of life. However, through adolescence, the apparent size of the main pulmonary artery segment remains increased. Through the teens and early twenties, the size of the main pulmonary artery and base of heart continue to decrease, and the size of the aortic arch increases in caliber, so that by the mid-twenties, the appearance of a “normal” heart may be characterized (Fig. 24-14).

Figure 24-14 A, Posteroanterior radiograph of a 30-year-old woman is shown. The lateral border of the left-sided aortic arch (Ao) and proximal descending aorta (short arrows) are clearly seen. The main (MP) and proximal left (L) pulmonary artery are seen cephalad to the left bronchus (long arrow). The left atrial appendage section (small arrowhead) is inferior to the crossing of the left bronchus and cephalad to the left ventricular contour (curved arrows) of the left heart border. The superior vena cava is not seen on this examination. However, the ascending aorta (small open arrow) is seen barely over the hilar right pulmonary artery (R). The right atrial contour (large arrowheads) extends just to the right of the spine. B, Spin-echo MRI obtained from a different 30-year-old woman is shown. The left heart border–forming structures are the aortic arch (Ao), which is to the left of the trachea (T), the distal main pulmonary artery (PA), left atrial appendage (aa), and anterolateral portion of the left ventricle (LV). The transverse right pulmonary artery (RP) passes over the left atrium (LA). Notice that the Ao and PA are equal in caliber. The mitral valve (small arrows) and coronary sinus (large arrow) are shown.
(From Boxt LM: Plain-film examination of the normal heart, Semin Roentgenol 34:169-180, 1999.)

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