How Many Chest X-Rays Are Enough? When Should I Order Imaging Studies, and Which Studies Should Be Done?

Every test has its risks as well as its benefits. I would only order a test if the results would change my clinical decision or management, and the same is true for x-ray studies. It is usually more common to order imaging studies during the initial evaluation of a patient with chronic respiratory symptoms, but once a diagnosis is established and treatment initiated, it is best to order studies much more selectively and based upon clinical changes or questions that come up. Personally, I think one chest x-ray is enough unless clinically indicated.

Chest x-rays are a great screening tool for respiratory disorders but they have their limitations. Some common indications for ordering a chest x-ray are 1) a first-time wheezing episode, 2) a positive tuberculosis skin test (PPD), 3) focal breath sounds are heard, 4) pneumonia is suspected, 5) increased work of breathing or low oxygen saturations, and/or 6) during the evaluation of a patient with a history of a chronic cough (Figure 46-1). These are just some general indications when to order a chest x-ray. I typically do not repeat a chest x-ray in a patient with documented asthma unless pneumonia or a new pathologic process is suspected. Although the increased risk of cancer is extremely low with a single chest x-ray, the risks are cumulative, and therefore even a chest x-ray should be ordered judiciously to minimize any long-term risks. Chest x-rays are helpful but are often insufficient as a sole diagnostic tool.

Figure 46-1. This is a chest radiograph from a 14-month-old girl with a 3-week history of cough, revealing a triangular density in the right hilar region and subtle air trapping (hyperlucency) in the right lower lobe. Rigid bronchoscopy with retrieval of a piece of glass from the right mainstem bronchus was performed and resulted in resolution of her symptoms. 

Chest computed tomography (CT) is an excellent tool to investigate the lung parenchyma because it allows for a cross-sectional image of the chest. Chest CT can be very helpful in diagnosing lung disorders, such as interstitial lung disease, pneumonia with or without empyema, bronchiolitis obliterans, bronchiectasis, lung masses, congenital lung disorders, and airway abnormalities. Chest CT with contrast should be ordered to visualize vascular structures to determine if a lung mass is vascularized or to visualize lymph nodes as in a patient suspected of having hilar adenopathy. Contrast is also helpful in determining if there is vascular compression of airways (Figure 46-2) or to determine if pulmonary lesions are calcified. While rare in pediatrics, a chest CT with contrast should be ordered when pulmonary embolism is suspected.

Figure 46-2. Contrast-enhanced chest computed tomography image revealing a double aortic arch in a 3-year-old boy undergoing evaluation for chronic cough and wheeze, which had been persistent since early infancy. As part of his initial evaluation, a chest radiograph and barium swallow were performed, and these revealed a right-sided aortic arch. An MRI can also be performed to define vascular anatomy. 

In addition to the cross-sectional views that a CT can provide, many centers are able to create 3D images, making visualization of the chest much more accessible to the non-radiologist. Three-dimensional images are a great way for the nonradiologist to visualize complicated anatomy. The CT can also be used not only diagnostically, but also therapeutically. CT-guided procedures can be helpful in the removal of loculated fluid in empyema or when performing a biopsy of a lesion. Overall, the chest CT is an easily accessible and very helpful diagnostic tool. However, a chest CT results in much higher doses of radiation exposure than a chest x-ray and therefore should be ordered judiciously as well.

Airway fluoroscopy can be a useful tool in diagnosing airway pathologies. The advantages of airway fluoroscopy are that it requires no sedation, is quick, and can assess airway patency in real time. Thus, it is often used to diagnose laryngo- or tracheomalacia because the airway may appear normal on exhalation but narrows on inspiration. Functional abnormalities such as malacia may be missed or under- or over-estimated on a single capture point as on CT but likely to be better evaluated on airway fluoroscopy because this test can visualize the airway dynamically.

Barium esophagram/upper gastrointestinal series (UGI) can be a useful tool to evaluate vascular structures in cases where a vascular ring is suspected. Similarly, if the esophagus is compromised due to the presence of a mediastinal mass (Figure 46-3), it will be detected as well. In addition, an UGI may provide some insight into the presence of gastroesophageal reflux, although other studies can also evaluate for this common condition. If there is a question about swallowing function, the barium swallow can be performed in the presence of a speech pathologist as well.

Figure 46-3. Barium esophagram image obtained in a 12-month-old boy evaluated for chronic wheezing, revealing esophageal deviation due to the presence of a large bronchogenic cyst. 

Flexible bronchoscopy can be a diagnostic as well as a therapeutic tool. It is often used to collect broncho-alveolar lavage (BAL) fluid to identify an organism during an infection. Pediatric pulmonologists perform flexible bronchoscopy as opposed to rigid bronchoscopy, which is usually performed by an otolaryngologist. The benefit of flexible bronchoscopy is the ability to reach the lower airways and the right upper lobe, which can sometimes be difficult to access with a rigid scope. Therefore, flexible bronchoscopy allows fluid to be obtained from a particular segment of the lung. In addition, the risk of pneumothorax and bleeding is much lower than that of a rigid scope due to the inherent malleability of a flexible bronchoscope.

Common studies performed on BAL samples include cell count, which may help identify certain disorders such as collagen vascular disease, or cancers, which alter the normal cellular ratio of neutrophils and lymphocytes. Macrophages can also be stained for lipid or hemosiderin. Lipid-laden macrophages are often present with chronic microaspiration, and hemosiderin is found in patients with hemoptysis or pulmonary bleeding. BAL fluid can also be gram-stained and cultured for bacteria, viruses, and fungi. Although cultures can always be sent, if the patient is receiving antibiotics, the probability of identifying a bacterial organism lowers with time.

Chest magnetic resonance imaging (MRI) is rarely ordered in pediatric respiratory disorders. Currently, the MRI of the chest does not image the lung parenchyma well. However, it is an excellent imaging tool to visualize vasculature and does not use any ionizing radiation as does CT and chest x-ray, but uses magnetic fields to create an image. While the advantages of MRI consist of no radiation, the time required for imaging is extensive and usually requires sedation.

Ultrasonography of the chest is a useful diagnostic tool that is not associated with any radiation. It can be used to distinguish chest wall masses from cystic lesions, to distinguish fluid collection from pneumonia on a “white out” on chest x-ray, and to follow effusions. In addition, ultrasound can be useful in detecting diaphragm abnormalities. Ultrasound of the diaphragm can measure diaphragm thickness during contraction and therefore may be a good tool to assess patients with diaphragmatic dysfunction.

Suggested Readings

Brenner DJ, Hall EJ. Computed tomography—an increasing source of radiation exposure. N Engl J Med. 2007;357: 2277-2284.

Frush DP, Donnelly LF, Chotas HG. Contemporary pediatric thoracic imaging. AJR Am J Roentgenol. 2000;175:
841-851.