Fundamentals of Pediatric Radiology

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.


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 and
body 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.


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.

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 image
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 [], 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.

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).




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).

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
Musculoskeletal system

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.

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.

The lateral film is important in the assessment of (1) the encroachment of adenoidal tissue on the nasopharyngeal airway; (2) retropharyngeal swelling/abscess (air in the retropharyngeal space); (3) the degree of hypopharyngeal airway distention as a measure for airway encroachment (croup); and (4) identification of a radiopaque foreign body. Calcification in respiratory cartilage, although very rare in children, is pathologic; it is seen in chondrodysplasia punctata and relapsing polychondritis. The hyoid bone may be ossified at birth.

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.

Interpretation of the Chest Film: 2. Technical Factors

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.


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).

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).

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Jul 11, 2016 | Posted by in PEDIATRICS | Comments Off on Fundamentals of Pediatric Radiology

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