Historically, airway foreign bodies have been a major cause of morbidity and mortality in the United States. Although foreign body aspiration most frequently occurs in children, it happens in adults as well. Foreign body aspiration is commonly referred to as a "café coronary" (elderly adults). [1, 2, 3, 4, 5]
See the radiographic images below.
In the United States, approximately 500-2000 deaths occur each year from foreign body aspiration.  Despite advances in radiologic techniques, the diagnosis of foreign body aspiration can be difficult, and bronchoscopy may be required. [3, 4]
When foreign body aspiration is suspected in a patient, screening radiographic studies employed include anteroposterior (AP) and lateral imaging of the soft tissues of the neck, inspiratory and expiratory posteroanterior (PA) chest radiographs (CXRs), and lateral CXRs. [7, 8] The potential is great for morbidity and mortality resulting from an aspirated foreign body; hence, if foreign body aspiration is suspected, the appropriate radiographic studies should be performed. [9, 10, 11] Radiopaque foreign bodies are easy to diagnose by using radiographs. With radiolucent foreign bodies, secondary radiographic signs, such as obstructive emphysema, atelectasis, pneumonia, and a mediastinal shift, help in diagnosing foreign body aspiration. 
Lateral decubitus chest radiography, fluoroscopy, or both may help in diagnosing foreign body aspiration in patients who are unable to cooperate with inspiratory and expiratory CXRs, such as young pediatric patients. [13, 14, 15, 16, 17, 18]
As a result of the limitations of radiographic studies in the diagnosis of aspirated foreign bodies, all patients in whom the clinical suspicion for aspirated foreign bodies is high should undergo bronchoscopy for definitive diagnosis and treatment. [19, 14, 20, 21, 22]
Initial radiographic studies should include AP and lateral views of the soft tissues of the neck, PA CXRs obtained during inspiration and expiration, and lateral CXRs. The depiction of radiopaque foreign bodies is straightforward. Obtaining 2 views of the foreign body helps in determining its location and excludes the presence of superimposed multiple foreign bodies. Most foreign bodies are radiolucent; therefore, indirect radiologic findings must often be obtained.  Radiolucent tracheal foreign bodies may show signs of an infraglottic opacity or of swelling from airway inflammation on PA and lateral neck radiographs. 
Plain radiographic results cannot exclude foreign body aspiration. If the clinical suspicion is high for foreign body aspiration, bronchoscopy should be performed for definitive diagnosis and treatment.
Patients with bronchial foreign bodies may have normal findings on CXRs; however, the affected lung may show hyperaeration (obstructive emphysema) and shifting of the mediastinum away from the affected lung on expiratory CXRs because of the ball-valve effect of the tracheal foreign body (see the images below). In such cases, the patients can inspire air past the foreign body but have difficulty exhaling.
In patients who are unable to cooperate for expiratory imaging (eg, young children), decubitus CXR or fluoroscopy may show hyperaeration and mediastinal shifting. Decubitus CXRs reveal failure of the affected lung to collapse, even if the patient is in the decubitus position (see the image below).
Images in patients with chronic bronchial foreign bodies may show atelectasis, with a mediastinal shift toward the foreign body and/or recurrent pneumonias in the affected lung segment (see the images below).
Svedström and colleagues studied the accuracy of CXRs in the diagnosis of tracheobronchial foreign bodies and concluded that the diagnostic accuracy, sensitivity, and specificity of CXRs were 67%, 68%, and 67%, respectively. According to the authors, these results show that CXRs alone are neither sensitive nor specific enough to exclude tracheobronchial foreign bodies. They found that of the 34 patients from whom a foreign body was removed, preoperative CXRs showed airtrapping in 50%, atelectasis in 12%, and signs of infection in 18%. Normal CXR findings were obtained in 24% of patients who had endoscopically verified airway foreign bodies. 
In their study of patients with laryngotracheal foreign bodies, Esclamado and colleagues reported that 92% of neck radiographs showed an infraglottic density or swelling and therefore suggested that PA and lateral neck radiographs should be part of the radiographic workup when foreign body aspiration is a concern.  In contrast, 58% of the patients in their study who had laryngotracheal foreign bodies had normal CXR findings.
As a result of its greater contrast resolution, computed tomography (CT) scanning has been used to demonstrate airway foreign bodies that are radiolucent on plain radiographs. [26, 27] Many authors recommend using narrow windows when imaging the thorax, to decrease the likelihood of missing a foreign body. [28, 29]
In addition to providing plain radiographic findings, such as hyperlucency, atelectasis, and lobar consolidation, CT scans can depict the foreign body within the lumen of the tracheobronchial tree and the 3-dimensional position of the foreign body within the thorax. [26, 30, 31]
Current state-of-the-art helical multidetector-row CT scanners may improve the sensitivity of radiologic evaluation in patients who are unable to cooperate for inspiration and expiration radiography. The use of low-dose mutidetector CT and virtual bronchoscopy may help to detect the foreign body. These scanners may be useful with young children and with adults suffering from an altered level of consciousness. 
If CT scans demonstrate signs of foreign body aspiration, the patient should undergo bronchoscopy for definitive diagnosis and treatment. No further radiologic study is indicated.
Any process that causes obstruction or narrowing of the airway lumen can produce signs similar to those of foreign body aspiration. Examples include neoplastic disease, granulomatous disease, bronchial stenosis, and a mucus plug.
Magnetic Resonance Imaging
Many authors have reported on the use of magnetic resonance imaging (MRI) in identifying aspirated peanuts. [12, 32, 33, 34] Using T1-weighted images, these authors demonstrated the presence of peanuts via the direct depiction of the high signal intensity emitted by their fat content surrounded by low-intensity lung tissue.
Imaizumi and colleagues reported that peanuts can be clearly distinguished from the surrounding areas of granulation and atelectasis because of their hyperintensity on T1-weighted images. 
The advantages of MRI include its noninvasive nature and the lack of radiation exposure from this modality. MRI also offers high-resolution multiplanar images of soft tissue. 
The disadvantages of MRI include its cost, the long data-acquisition time, the need for sedation in some patients, and the necessity to remove all metallic devices from patients. 
As with all imaging modalities, if clinical suspicion of an airway foreign body remains high, bronchoscopy should be performed for definitive diagnosis and treatment.
Leonidas and colleagues used perfusion lung scans to demonstrate areas of decreased ventilation that resulted from tracheobronchial foreign bodies.  (Decreased ventilation causes reflex vasoconstriction.) Perfusion defects, or ventilation-perfusion (V/Q) mismatches, are not specific for the presence of airway foreign bodies. Asthma, tuberculosis, emphysema, pneumonitis, and neoplasms may cause perfusion defects.  If clinical suspicion for foreign body aspiration is high, further evaluation with bronchoscopy is warranted.