Aspiration Pneumonia Imaging 

Aspiration is defined as entry of a foreign substance, solid or liquid, into the respiratory tract or inhalation of fumes and vapors. Aspiration pneumonia is an infectious process caused by aspirated oropharyngeal flora. Aspiration pneumonitis, which is caused by a direct chemical insult due to the aspirated material, is technically a different entity, but is often referred to as aspiration pneumonia. (See the images below.)

The clinical history is important in diagnosing aspiration pneumonia. The nature of the aspirated material, the quantity of aspirated material, and the time course of the event influence the size and distribution of the lung parenchymal abnormality. The most common predisposing factors for aspiration in adults are alcoholism, stroke and other neuromuscular disorders, seizures, and loss of consciousness.

Preferred examination

Chest radiography is readily available and inexpensive and is by far the most commonly used imaging test to evaluate aspiration pneumonia.^{[1, 2, 3, 4]} Traditionally, posteroanterior (PA) and lateral chest radiographs have been recommended for imaging aspiration pneumonia and its complications. However, because many patients are not able to cooperate for PA and lateral imaging, anteroposterior (AP) portable images have been more commonly used for diagnosis.

Computed tomography (CT) scanning is the best method for diagnosing aspiration pneumonia, an abscess, or an empyema. CT scanning precisely delineates the location of the lobar or segmental opacity. A foreign body in the tracheobronchial tree and associated atelectasis or consolidation can be defined with relative ease on CT scans. Aspiration of specific material such as fat or contrast material can sometimes be determined by measuring the tissue attenuation on CT scans. Esophageal abnormalities may also be seen on CT images without the need for contrast material. Necrosis, cavity formation, and empyema are all complications of aspiration pneumonia that are seen better and earlier with CT scanning than with plain radiography.

The patient's swallowing mechanism can be studied by using fluoroscopy with a contrast agent. This is a real-time evaluation of the swallowing process that is often performed in conjunction with speech therapy.

Magnetic resonance imaging (MRI) is more sensitive than plain radiography. To date, no large study has been performed to compare MRI with CT scanning for the evaluation of aspiration.

Limitations of techniques

Radiography remains the most practical first-line imaging study for patients with suspected aspiration pneumonia. Chest radiographs usually adequately demonstrate lung consolidation, atelectasis, and abscess formation. However, CT scanning is more sensitive and specific than radiography.^{[5, 6]}

Many factors affect the appearance on initial studies, including the patient's hydration status, his or her ability to mount an adequate inflammatory response, and the nature and amount of aspirate. Days may pass before aspiration is visible on imaging studies.

AP portable chest images may demonstrate bilateral opacities in the middle or lower lung zones. On PA and lateral images, the opacities may be localized to the posterior segments of upper lobes or to the superior segments of lower lobes. Alternatively, the radiographic abnormalities may be more extensively distributed. (See the images below.)

Degree of confidence

Traditionally, most physicians have depended on plain radiographs, which have moderate to good specificity and sensitivity. However, CT scanning has greater sensitivity and specificity and should be used to more promptly diagnose aspiration pneumonia, to determine its cause, and to detect its complications earlier.

False positives/negatives

False-negative findings are associated with subtle or early findings in a clinical course. False-positive findings usually occur when the clinical history is unclear. The findings in aspiration pneumonia are not specific; pulmonary edema, pneumonias from other causes, and neoplasm are in the differential diagnosis.

CT scanning is superior to plain radiography for defining the nature, extent, and complications of aspiration, demonstrating abnormal lung opacities earlier and in more detail than can plain chest radiographs.^[7] An intratracheal or intrabronchial foreign body can be identified on CT scans, as can any associated atelectasis/consolidation or effect of partial obstruction such as focal overaeration. In select cases, specific aspirates such as fat or opaque material can be identified and even measured on CT images.

Aspirated low-density organic material (such as mineral oil) in the tracheobronchial tree or alveolar spaces, which cannot be diagnosed on plain radiographs, can be demonstrated and sometimes measured on CT scans. Opaque aspirates are also well demonstrated on CT scans, as shown in the images below.

CT or ultrasonographic guidance is useful for localization of abnormalities for biopsy or aspiration/drainage.

Complications from aspiration pneumonia (eg, abscess formation, lung necrosis, empyema) are well depicted on CT scans. Long-term complications, such as obliterative bronchiolitis, are diagnosed best with high-resolution CT (HRCT).

HRCT findings in patients with exogenous lipoid pneumonia may include air-space consolidations with fatty or nonspecific but low attenuation values, areas of ground-glass opacities, septal lines, and centrilobular interstitial thickening.^{[8, 9, 10, 11]} HRCT may also demonstrate a crazy-paving pattern, either isolated or surrounding a pulmonary consolidation.

Multidetector CT (MDCT) has proved to be effective in the evaluation of pneumonia from aspirated foreign bodies or liquid^[12] In patients with suspected foreign body aspiration, virtual bronchoscopy, in conjunction with MDCT, can delineate the precise location of an obstructing foreign body, thereby facilitating conventional bronchoscopy; it can also obviate bronchoscopy by providing an alternative diagnosis.^[13]

CT scanning also can define anatomic abnormalities in the head, neck, and chest areas. These findings may be helpful in detailing the underlying causes of aspiration such as fistulas or tumors in the pharynx, larynx, or esophagus. CT scans may also reveal esophageal strictures, including achalasia.

Degree of confidence

CT scanning is considered to provide a higher degree of confidence than a single AP, PA, or lateral plain radiograph. However, some of the same diseases that mimic radiographic findings of aspiration also can confound the diagnostic interpretation of CT scans.

Few large studies of MRI dedicated to aspiration diseases have been performed. However, results of published case studies appear to confirm the accuracy of MRI for imaging such conditions as acute inflammation, granuloma, and fibrosis. MRI performs well in defining the nature of the aspirate and the body's reactions to the aspirate. Some authors have found that MRI is superior to CT scanning in the diagnosis of lipoid aspirations.

False positives/negatives

The sensitivity of MRI is expected to be high, with few false-negative results, although, as with CT scanning, false-positive results due to pathologic processes with features mimicking those of aspiration pneumonia should always be considered.

A radionuclide salivagram can demonstrate the aspiration of saliva.^[14] Salivagrams can document salivary aspiration as the source of recurrent pneumonia, often in children with neurologic impairment.^[15]

Exogenous lipoid pneumonia mimicking malignancy on positron emission tomography (PET) scan has been reported.^[16]