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 focussed 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.
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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
Purpose
The aim of this technique is to highlight areas of air trapping, since during expiration the rest of the lung parenchyma will show an increase in density (as the amount of air in the alveoli decreases). These areas of air trapping will appear radiolucent.
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