HRCT chest: expiration (technique)

Expiratory high-resolution CT (HRCT) imaging corresponds to an additional CT acquisition performed as part of the HRCT chest protocol. It represents a scan performed with the patient on supine and images obtained at the end-expiration.

It is a useful method for detecting small airways obstructive lung disease, in which the air remains trapped in the pulmonary lobules even after the expiration (air-trapping). This technique may also be applied in the assessment for tracheobronchomalacia, although a dedicated protocol with small ROI focused in the central airways is preferred.

NB: This article is intended to outline some general principles of protocol design. The specifics will vary depending on CT hardware and software, radiologists' and referrers' preference, institutional protocols, patient factors (e.g. allergy) and time constraints.

Indications

Ideally, an expiratory HRCT scan should be performed in all obstructive airway diseases. Some institutions make it routinely acquired in every initial HRCT assessment.

Common indications include:

• obliterative bronchiolitis
• cryptogenic organizing pneumonia
• bronchial asthma
• sarcoidosis
• hypersensitivity pneumonitis
• emphysema
• bronchiectasis

Technique

• patient position
  • supine with their arms above their head
• scout 
  • apices to diaphragm
• scan extent 
  • apices to diaphragm
• scan direction
  • caudocranial
• contrast injection considerations
  • no contrast 
• scan delay
  • minimal
• respiration phase
  • ​​scan performed on expiration

Practical points

The patient should be taught, also practice before, how to perform and hold a full expiration for a few seconds. If needed, time can be taken between axial slices to give the patient a break.

Findings

• expected findings in expiration include:
  • diffuse increase in lung attenuation
  • decrease in the cross-sectional lung area
  • reduction in the caliber of the airways
    • the trachea is usually taken as a reference to assess technique adequacy: it should change from a round/elliptical shape on inspiration to a crescent shape on expiration, with bowing of its posterior membranaceous wall
• air-trapping: pulmonary lobules with less than normal increase in attenuation and lack of volume reduction
• obliteration of the tracheal or main bronchi lumen on expiration is indicative of tracheobronchomalacia