Korean J Radiol.  2002 Mar;3(1):24-29. 10.3348/kjr.2002.3.1.24.

Dependent Lung Opacity at Thin-Section CT: Evaluation by Spirometrically-Gated CT of the Influence of Lung Volume

• Lee KN
• Yoon SK
• Sohn CH
• Choi PJ
• Webb WR

• KMID: 1758443
• DOI: http://doi.org/10.3348/kjr.2002.3.1.24

Abstract

OBJECTIVE
To evaluate the influence of lung volume on dependent lung opacity seen at thin-section CT.
MATERIALS AND METHODS
In thirteen healthy volunteers, thin-section CT scans were performed at three levels (upper, mid, and lower portion of the lung) and at different lung volumes (10, 30, 50, and 100% vital capacity), using spirometric gated CT. Using a three-point scale, two radiologists determined whether dependent opacity was present, and estimated its degree. Regional lung attenuation at a level 2 cm above the diaphragm was determined using semiautomatic segmentation, and the diameter of a branch of the right lower posterior basal segmental artery was measured at each different vital capacity.
RESULTS
At all three anatomic levels, dependent opacity occurred significantly more often at lower vital capacities (10, 30%) than at 100% vital capacity (p = 0.001). Visually estimated dependent opacity was significantly related to regional lung attenuation (p < 0.0001), which in dependent areas progressively increased as vital capacity decreased (p < 0.0001). The presence of dependent opacity and regional lung attenuation of a dependent area correlated significantly with increased diameter of a segmental arterial branch (r = 0.493 and p = 0.0002; r = 0.486 and p = 0.0003, respectively).
CONCLUSION
Visual estimation and CT measurements of dependent opacity obtained by semiautomatic segmentation are significantly influenced by lung volume and are related to vascular diameter.

Keyword

Lung, collapse; Lung, CT; Lung, density; Lung, function; Lung, ventilation

MeSH Terms

Adult
Female
Human
Lung/*radiography
Lung Volume Measurements
Male
Spirometry
Support, Non-U.S. Gov't
*Tomography, X-Ray Computed
Vital Capacity

Figure

• 

  Fig. 1 The region of interest for measurements of regional lung attenuation was determined as a zone of the right lung posterior to a horizontal line drawn 1 cm from the most dependent lung. For the measurement of regional lung attenuation, we selected the same area (3.6 ± 0.6 cm2) as the most dependent area of a crescent shape (arrows).

• 

  Fig. 2 Thirty-year-old man who showed dependent opacity (grade 2) at 10% vital capacity. A. Dependent opacity occurred at the level of the carina. Lung attenuation is greater in the posterior aspect of the upper lobes than in the superior segment of the lower lobes, located more posteriorly. Thus, dependent opacity is influenced not only by gravitational effects. B. Dependent opacity is seen at the level of the inferior pulmonary vein. C. Dependent opacity is prominent at a level 2 cm above the diaphragm.

• 

  Fig. 3 Comparison of dependent opacity and arterial diameter in a 29-year-old man A. Dependent opacity was obvious (grade 2) at 10% of vital capacity, and arterial diameter in the right posterior basal segment was 7.1 mm (artery, arrow). B. At 30% of vital capacity, dependent opacity was graded as subtle (grade 1) and arterial diameter measured 6.8 mm (arrow). C. Dependent opacity was not apparent at 50% of vital capacity (score 0), at which point arterial diameter was 6.6 mm (arrow). D. Dependent opacity was absent at 100% vital capacity, and at the same level, arterial diameter was 6.3 mm (arrow).

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