Conventional radiography

General description of procedure, equipment, technique

As currently performed, conventional chest radiography employs digital x-ray receptors that are embedded in radiographic cassettes. This digital technology has replaced the traditional film-screen systems used previously. Digital radiography units convert x-ray photons into an electrical charge in one of two ways: directly, using an x-ray photoconductor and thin-film transistors that convert the x-ray photons traversing the patient, or indirectly, using a scintillator that first converts the x-ray photons into light and then converts the light into an electrical charge by either a photodiode or a charged coupled device (CCD).

Most dedicated digital radiography systems installed in radiology departments are termed “direct radiography” units, or DR units. In these systems, a layer of amorphous selenium serves as the photoconductor and directly converts x-ray photon energy into an electrical charge that is “read out” by a thin film transistor immediately beneath the detector layer. In comparison, portable chest radiographic units are termed “computed radiography” units, or CR units; they use a photostimulablephosphor that replaces the traditional film-screen combination within the x-ray cassette.

The phosphor absorbs transmitted x-ray energy from the radiographic exposure. When the radiographic plate is placed in the processor unit, the phosphor’s absorbed energy is sensed by a high-energy laser. Light from the phosphor layer is collected by a photomultiplier and converted by an analog-to-digital converter into a digitized image for display. Computed radiography is typically employed in critical care or emergency room settings. On the other hand, most fixed digital systems that are currently in radiology departments use direct radiographic technology.

Standard frontal (Figure 1), lateral (Figure 2), and decubitus chest radiographs are useful in evaluating patients in a broad array of clinical settings.

Indications and patient selection

Common clinical indications for obtaining conventional chest radiographs include:

Preoperative assessment of cardiopulmonary disease
Detection of pneumonia or assessment of infection-related complications, including lung abscess, empyema, and acute respiratory distress syndrome (ARDS) (Figure 3)

Figure 3.

Portable chest radiograph demonstrates diffuse bilateral air-space opacities reflecting non-cardiogenic edema in patient with ARDS.
Screening of patients with positive PPD or gamma interferon blood tests to exclude active tuberculosis
Assessment of chest trauma to exclude pneumothorax, causes of mediastinal widening, parenchymal lung injury, and chest wall injury
Assessment of chest pain to exclude chest wall or intrathoracic mass, pneumothorax, mediastinal hemorrhage, pneumonia, and rib fracture(s)
Assessment of patients suspected of having diffuse infiltrative lung disease.

Portable chest radiography is useful in assessing immobile patients in the emergency room, the critical care unit, or the recovery room for:

Anatomic localization of medical devices used in patient monitoring, provision of intravascular access, and support of cardiovascular or respiratory functions
Detection of complications of tube, line, or catheter placement, or complications relate to various diagnostic and therapeutic procedures, including bronchoscopic biopsy, endoscopic biopsy, and esophageal dilatation
Detection of causes of acute clinical deterioration in hospitalized patients

Decubitus chest radiographs are helpful in selected cases for:

Detection of small pleural effusions (Figure 4 and Figure 5)

Figure 4.

Upright chest radiograph in a patient with a left pleural effusion shows blunting of the left lateral costophrenic sulcus with a meniscus indicative of a left pleural effusion.

Figure 5.

Left lateral decubitus radiograph confirms the presence of a free-flowing left pleural effusion.
Evaluation of a suspected pneumothorax in the non-dependent hemithorax of a patient who is unable to sit or stand for upright radiographs

Contraindications

Patients who are unable to sit or stand for radiographs in the radiology department or who are too unstable for transportation to the department should undergo portable radiography for assessment of chest abnormalities.

Details of how the procedure is performed

Standard frontal and lateral chest radiographs are performed in the radiology department with the patient sitting or standing. Virtually all chest radiology services utilize digital technology with photostimulable phosphors (computed radiography) or charge-coupled devices (direct digital radiography), as previously discussed. The frontal projection is typically performed with the x-ray beam traversing the chest from back to front (i.e., posteroanterior, or PA, projection). In PA projection, the patient’s anterior chest contacts the recording device, which is mounted on a wall unit.

The PA projection provides for a sharp image of the heart and mediastinum, without organ magnification. The arms are raised over the head, causing rotation of the scapulae forward and minimizing their superimposition on the lungs. A high tube kilovoltage (kVp) of 140 is used to penetrate the heart, ribs, mediastinum, and liver, allowing for assessment of mediastinal interfaces, retrocardiac regions, the thoracic spine, the posterior costophrenic sulci, and the lungs. The standard distance from the x-ray source to the recording device is 72 inches. Patients are instructed to take a deep breath and hold it for maximum lung expansion during the exposure.

The lateral radiograph is typically performed with the patient’s arms raised and the left chest against the recording device.

Portable radiographs are performed using a mobile unit and individual x-ray cassettes that produce digital images. The cassette processing unit is typically situated in the radiology department or critical care unit. As the images are generated by placing the cassette against the patient’s back, the projection is anteroposterior (AP). The lower kVp (80-100) output of portable units results in a somewhat higher-contrast image than that provided by fixed imaging units, and the image is more subject to motion artifacts because of the longer exposure times required to achieve radiographic density compared to that required by fixed imaging units. The distance between the x-ray source and the cassette (usually 40-48 inches). As a result of the AP projection utilized in portable images, greater magnification of the cardiomediastinal silhouette occurs, making assessment of cardiomegaly and mediastinal widening more challenging than it is when fixed units are used.

Decubitus radiographs can be performed in the radiology department or by using portable equipment. When being evaluated for suspected pleural effusion (Figure 4), the patient is positioned with the affected side down (Figure 5); when the patient is being assessed for pneumothorax, the patient is positioned with the affected side up.

Once a chest radiographic exposure has been performed, the image may be sent directly to the picture archive and communication system (PACS), as is the case using direct radiography (DR). Alternatively, the latent image, which is stored in the photostimuable phosphor (when using computed radiography or CR) is read by a laser and converted into electrical energy; a pixel or varying gray scale is rendered on the image, which is sent to the PACS for system-wide distribution and analysis.

Interpretation of results

Interpretation of chest radiographs requires a basic understanding of the technological limitations of the study and knowledge of the radiographic appearance of the chest in healthy states and in diseased states. Employing a routine survey of the chest, including assessment of the technical adequacy of the examination in light of the specific clinical indication for which the study was performed, is important.

Assessment of chest radiographs: Consider technical factors:

Is the entire chest included on the image?
Is there motion artifact (most easily assessed by noting whether the ribs and intrathoracic structures are blurry)?
Is there rotation (usually obvious if there is severe rotation, but when subtle, the finding is best assessed by determining whether there is alignment between a vertical line drawn centrally between the calvicular heads marking the anterior chest wall and the spinous processes marking the midine of the posterior chest wall)?
Are the lung volumes adequate (a subjective assessment, but the observation is obvious when volumes are markedly reduced)?
Is there adequate x-ray beam penetration of the mediastinum? (The vertebral bodies and intervertebral disc spaces should be visible through the heart, as should the lower lobe lung vessels.)

Assessment of the chest. A thorough, systematic approach works best:

Tubes, lines, catheters, monitoring devices, and foreign bodies should be noted and described. Are they appropriately positioned? Do they have their normal appearance?
The chest wall should be evaluated. Notes should be made regarding asymmetry of tissues, absence of breast shadows, air in soft tissues, and rib, spine, and shoulder abnormalities.
The heart should be assessed. Important considerations include heart size, position, and contour abnormalities, any of which may suggest chamber enlargement. Coronary artery, myocardial, and pericardial calcifications, and pericardial effusions can sometimes be detected radiographically.
Attention should be focused on mediastinal density, width, and contours. The normal lung–mediastinal interfaces and mediastinal width–should be evaluated. Abnormal air in the mediastinum, seen as vertically oriented lucencies outlining cardiomediastinal structures, should be noted. An evaluation of the trachea and central bronchi with regard to diameter, position, and filling defects is an important part of the mediastinal survey.
The lungs’ fields should be systematically examined. The vascular pattern should be assessed for increased vascularity, vascular redistribution, and oligemia. Abnormal air-space, interstitial, or focal lung opacities, including nodules or masses, should be detected. Abnormal regions of decreased attenuation may represent bullae, cysts, or diffuse hyperlucency, as seen in emphysema or hyperinflation that is due to asthma.
The hila should be inspected. Hilar position (normally, the left is higher than the right), size, contour, and density should be assessed.
The pleural surfaces should be inspected. Pleural fluid, air, thickening (focal or diffuse), pleural masses, and pleural calcification warrant description.

Examination of the upper abdomen is a critical part of the evaluation. The area should be assessed for free air, hepatosplenomegaly, or abnormal calcifications (e.g., gallstones or renal calculi).

Performance characteristics of the procedure (applies only to diagnostic procedures)

The American College of Radiology has developed criteria for the appropriate use of routine chest radiographs for patient monitoring, evaluation following selected procedures, and documentation of disease presence and disease course. The primary metric utilized is the value of the routine chest radiograph in detecting significant abnormalities that affect patient management.

An expert panel used the Delphi method to determine the best available evidence for a variety of indications for chest radiography. Each indication was rated for appropriateness using a scale of 1 to 9:

1-3 = usually not appropriate
4-6 = may be appropriate
7-9 = usually appropriate

Following is an abbreviated version of the recommendations for portable chest radiography:

Indication: Monitoring stable patient:

Routine admission (excluding pre-CABG or acute upper GI bleeding) = 9
Clinical indications only (clinical worsening only) = 8
Routine monitoring = 2

Indication: Respiratory failure. Patient receiving mechanical ventilation:

Daily = 9
Clinical indications only = 9

Indication: Compromised respiratory function. Patient with endotracheal tube:

After catheter or tube insertion = 9
Clinical indications only = 9
Follow-up = 2

Indication: Central venous pressure catheter (CVP) insertion:

After catheter or tube insertion = 9
Clinical indications only = 8
Follow-up= 2

Indication: Cardiopulmonary compromise. Swan-Ganz catheter insertion:

After catheter or tube insertion = 9
Clinical indications only = 8
Follow-up= 2

Indication: Potential cardiopulmonary compromise. Nasogastric (NG) tube insertion:

After catheter or tube insertion (feeding NG tube) = 9
After catheter or tube insertion (non-feeding NG tube) = 6
Follow-up= 2

Respiratory compromise. Chest tube insertion:

After catheter or tube insertion = 8
Clinical indications only = 8
Follow-up= 2

Outcomes (applies only to therapeutic procedures)

N/A

Alternative and/or additional procedures to consider

Alternative diagnostic imaging procedures include sonography and computed tomography. Indications for these alternative imaging procedures are discussed in other presentations.

Complications and their management

There are no significant complications.

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