Chest imaging includes use of
Nuclear scanning, including positron emission tomography (PET) scanning
There are no absolute contraindications to undergoing noninvasive imaging procedures except for MRI.
The presence of metallic objects in the patient's eye or brain precludes MRI. The presence of a permanent pacemaker or internal cardioverter-defibrillator is a relative contraindication (see MRI Safety). Additionally, gadolinium, when used as a contrast agent for MRI, increases risk of nephrogenic systemic fibrosis in patients with stage 4 or 5 chronic kidney disease or those receiving dialysis. Gadolinium may be harmful to a fetus and is generally avoided in pregnancy.
(See also Principles of Radiologic Imaging.)
Conventional Chest X-Ray Techniques
Conventional x-ray techniques that are used to image the chest and surrounding structures include
Plain x-rays
Fluoroscopy
Chest x-ray
Plain chest x-rays provide images of structures in and around the thorax and are most useful for identifying abnormalities in the heart, lung parenchyma, pleura, chest wall, diaphragm, mediastinum, and hilum. They are usually the initial test done to evaluate the lungs.
The standard chest x-ray is taken from back to front (posteroanterior view) to minimize x-ray scatter that could artifactually enlarge the cardiac silhouette and from the side of the thorax (lateral view).
Lordotic or oblique views can be obtained to evaluate pulmonary nodules or to clarify abnormalities that may be due to superimposed structures, although chest CT provides more information and has largely superseded these views.
Lateral decubitus views may be used to distinguish free-flowing from loculated pleural effusion, but chest CT or ultrasonography can provide more information.
End-expiratory views can be used to detect small pneumothoraces.
Chest x-rays taken with portable machines (usually anteroposterior views) are almost always suboptimal and should be used only when patients are too ill to be transported to the radiology department.
Screening chest x-rays are often done but are almost never indicated; one exception is in asymptomatic patients with positive tuberculin skin test results, in whom a single posteroanterior chest x-ray without a lateral view is used to make decisions regarding additional diagnostic studies and/or treatment for pulmonary tuberculosis.
Chest fluoroscopy
Chest fluoroscopy is the use of a continuous x-ray beam to image movement. It is useful for detecting unilateral diaphragmatic paralysis. During a sniff test, in which the patient is instructed to forcibly inhale through the nose (or sniff), a paralyzed hemidiaphragm moves cranially (paradoxically) while the unaffected hemidiaphragm moves caudally.
Several ancillary procedures such as bronchoscopy can be done with fluoroscopic guidance.
Chest Computed Tomography (CT)
CT defines intrathoracic structures and abnormalities more clearly than does a chest x-ray.
Chest CT is normally done at full inspiration. Aeration of the lungs during imaging provides the best views of the lung parenchyma, airways, and vasculature, and of abnormal findings such as masses, infiltrates, or fibrosis.
Images obtained with the patient in the prone position can help differentiate dependent atelectasis (which changes with changes in body position) from lung disorders that cause ground-glass attenuation in the dependent posterior parts of the lungs, which persists despite changes in patient position (eg, fibrosis due to idiopathic pulmonary fibrosis, asbestosis, or systemic sclerosis).
CT angiography
CT angiography uses a bolus of an IV radiopaque contrast agent to highlight the pulmonary arteries, which is useful in diagnosis of pulmonary embolism.
Contrast agent load is comparable to that with conventional angiography, but the test is quicker and less invasive. CT angiography provides sufficient accuracy for the detection of pulmonary emboli, so it has largely replaced conventional pulmonary angiography and, except in patients unable to tolerate contrast agents, ventilation/perfusion (V/Q) scanning.
Magnetic Resonance Imaging of the Chest
MRI has a relatively limited role in pulmonary imaging but is preferred over CT in specific circumstances, such as assessment of
Superior sulcus tumors
Possible cysts
Lesions that abut the chest wall
MRI/MRA of the chest can also be used to diagnose aortic dissection.
In patients with suspected pulmonary embolism in whom IV contrast agents cannot be used, MRI can sometimes identify large proximal emboli but usually is limited in this disorder.
Advantages include absence of radiation exposure, excellent visualization of vascular structures, lack of artifact due to bone, and excellent soft-tissue contrast.
Disadvantages include respiratory and cardiac motion, the time it takes to do the procedure, the expense of MRI, and the occasional presence of contraindications, which include many implanted devices and certain metallic foreign bodies. Gadolinium contrast may be harmful to the fetus, so use of contrast is usually avoided in pregnancy.
Ultrasonography of the Chest
Ultrasonography is often used to facilitate procedures such as thoracentesis and central venous catheter insertion.
Ultrasonography is also very useful for evaluating the presence and size of pleural effusions. It is commonly used at the bedside to guide thoracentesis. Bedside/point-of-care ultrasonography can be used to diagnose pneumothoraces and is becoming more widely used as an extension of the physical examination. Evidence suggests that lung ultrasonography is more sensitive and specific than plain chest x-ray for the diagnosis of pleural effusions, pneumonia, and pneumothorax, and can be helpful in diagnosis of pulmonary edema (1, 2, 3).
Endobronchial ultrasonography (EBUS) is increasingly being used in conjunction with flexible fiberoptic bronchoscopy to help localize masses and enlarged lymph nodes. Diagnostic yield of transbronchial lymph node aspiration is higher using EBUS than conventional unguided techniques.
Ultrasonography references
1. Ebrahimi A, Yousefifard M, Mohammad Kazemi H, et al. Diagnostic Accuracy of Chest Ultrasonography versus Chest Radiography for Identification of Pneumothorax: A Systematic Review and Meta-Analysis. Tanaffos 2014;13(4):29-40.
2. Ye X, Xiao H, Chen B, Zhang S. Accuracy of Lung Ultrasonography versus Chest Radiography for the Diagnosis of Adult Community-Acquired Pneumonia: Review of the Literature and Meta-Analysis. PLoS One 2015;10(6):e0130066. doi:10.1371/journal.pone.0130066
3. Lichtenstein D, Goldstein I, Mourgeon E, Cluzel P, Grenier P, Rouby JJ. Comparative diagnostic performances of auscultation, chest radiography, and lung ultrasonography in acute respiratory distress syndrome. Anesthesiology 2004;100(1):9-15. doi:10.1097/00000542-200401000-00006
Nuclear Lung Scanning
Nuclear scanning techniques used to image the chest include
Ventilation/perfusion (V/Q) scanning
Positron emission tomography (PET)
V/Q scanning
V/Q scanning uses inhaled radionuclides to detect ventilation and IV radionuclides to detect perfusion. Areas of ventilation without perfusion, perfusion without ventilation, or matched increases and decreases in both can be detected with 6 to 8 views of the lungs.
V/Q scanning is most commonly used for diagnosing pulmonary embolism, but it has largely been replaced by CT angiography. However, V/Q scanning is still indicated in the diagnostic evaluation for chronic thromboembolic pulmonary hypertension.
Split-function ventilation scanning, in which the degree of ventilation is quantified for each lobe, is used to predict the effect of placement of endobronchial valves and the effect of lobar or lung resection on pulmonary function.
Positron emission tomography (PET) of the lungs
PET uses radioactively labeled glucose (fluorodeoxyglucose) to measure metabolic activity in tissues. It is used in pulmonary disorders to determine
Whether lung nodules or mediastinal lymph nodes harbor tumor (metabolic staging)
Whether cancer is recurrent in previously irradiated, scarred areas of the lung
PET is superior to CT for mediastinal staging because PET can identify tumor in normal-sized lymph nodes and at extrathoracic sites, thereby decreasing the need for invasive procedures such as mediastinoscopy and needle biopsy.
Current spatial resolution of PET is 7 to 8 mm; thus, the test is not useful for lesions < 1 cm.
PET reveals metastatic disease in up to 20% of patients in whom it would not otherwise be suspected (1). The sensitivity of PET (80 to 95%) is comparable to that of histologic tissue examination (2). False-positive results can occur with inflammatory lesions, such as granulomas. Slowly growing tumors (eg, bronchoalveolar carcinoma, carcinoid tumor, some metastatic cancers) may cause false-negative results.
Combined CT-PET scanners are commonly used for lung cancer diagnosis and staging.
Nuclear lung scanning references
1. Prévost A, Papathanassiou D, Jovenin N, et al. Comparaison entre TEP(-FDG) et scanner dans le bilan d'extension du cancer bronchopulmonaire, conséquences pour l'opérabilité chez 94 patients [Comparison between PET(-FDG) and computed tomography in the staging of lung cancer. Consequences for operability in 94 patients]. Rev Pneumol Clin 2009;65(6):341-349. doi:10.1016/j.pneumo.2009.08.008
2. Gould MK, Kuschner WG, Rydzak CE, et al. Test performance of positron emission tomography and computed tomography for mediastinal staging in patients with non-small-cell lung cancer: a meta-analysis. Ann Intern Med 2003;139(11):879-892. doi:10.7326/0003-4819-139-11-200311180-00013
