Assessment of Small Airways with Computed Tomography: Mosaic Attenuation or Lung Density?
BACKGROUND
Computed tomography (CT) assessment of air trapping has been considered useful as a measure of small airway disease. Mean lung density (MLD) and the percentage of the lung field occupied by low attenuation area (LAA%) can be evaluated automatically, and their expiratory/inspiratory (E/I) ratios correlate with asthma severity and spirometry parameters. However, mosaic attenuation, another indicator of air trapping, has been assessed visually, and its functional relevance remains controversial.


OBJECTIVE
This retrospective study was conducted to correlate mosaic attenuation, which was assessed visually and automatically, and the E/I ratios of MLD and LAA% (defined as areas <-960 Hounsfield units) with clinical and physiological variables, including impulse oscillometry (IOS) indices.


METHODS
In 36 nonsmoking patients with stable asthma, the lungs were scanned at full inspiration and full expiration. Mosaic attenuation was measured visually and automatically, by counting areas with CT values higher than the surrounding areas. MLD and LAA% were measured using our validated method. Spirometry, IOS, exhaled NO and the sputum eosinophil count were evaluated.


RESULTS
The automatic results and visual scores of mosaic attenuation correlated well on expiratory scans (r = 0.894) and to a lesser degree on inspiratory scans (r = 0.629; p < 0.0001 for both). However, only the E/I ratios of MLD and LAA% correlated with forced expiratory volume in 1 s/forced vital capacity of spirometry and the IOS indices of resistance from 5 to 20 Hz and the integrated area of low-frequency reactance.


CONCLUSIONS
Our automatic method for analysis of mosaic attenuation is likely useful, but the results themselves may not be reflecting small airway involvement of asthma, unlike the E/I ratios of MLD and LAA%.
Department of Respiratory Medicine, Graduate School of Medicine, Kyoto University, Kyoto, Japan.
Tsuyoshi
T
Oguma
Akio
A
Niimi
Toyohiro
T
Hirai
Makiko
M
Jinnai
Hisako
H
Matsumoto
Isao
I
Ito
Masafumi
M
Yamaguchi
Hirofumi
H
Matsuoka
Kojiro
K
Otsuka
Tomoshi
T
Takeda
Hitoshi
H
Nakaji
Kazuo
K
Chin
Michiaki
M
Mishima
D000368:Aged; D001249:Asthma / Q000000981:diagnostic imaging* / Q000503:physiopathology; D001944:Breath Tests; D055745:Bronchioles; D045853:Exhalation; D005260:Female; D005541:Forced Expiratory Volume; D006801:Humans; D007091:Image Processing, Computer-Assisted; D001239:Inhalation; D008168:Lung / Q000000981:diagnostic imaging* / Q000503:physiopathology; D008297:Male; D008450:Maximal Midexpiratory Flow Rate; D008875:Middle Aged; D061330:Multidetector Computed Tomography; D009569:Nitric Oxide / Q000032:analysis; D012189:Retrospective Studies; D013147:Spirometry; D014797:Vital Capacity
D009569:Nitric Oxide