Computed tomography is increasingly being used to assess lung status when abnormalities associated with clinical signs are not observed radiographically.1 Computed tomography has greater contrast resolution and a better dynamic range than does conventional radiography, and it allows evaluation of the lungs without superimposition of surrounding structures.2 Furthermore, CT can be used to quantify tissue density (number of HU) relative to the attenuation of water.3,4 Mean ± SD density for normal lung tissue observed in CT images is −745 ± 53 HU in humans,2 and density of lung tissue ranges from −900 to −700 HU in dogs during quiet respiration.5 Mean lung attenuation was −846 HU in healthy dogs positioned in sternal recumbency with a PEEP of 15 mm Hg.6
Lung density is determined by 3 components: lung tissue, blood, and gas.3,7 During typical physiologic conditions, the relative proportion of these components continuously changes to maintain lung density. When the balance of these components is disrupted by a disease, lung density is subsequently increased or decreased.
Ground-glass opacity in CT images is defined as a hazy increased opacity of the lungs, with preservation of the bronchial and vascular margins.8 This phenomenon is less opaque than for consolidation, in which the bronchial wall and vascular margins are obscured.8 Ground-glass opacity is a common but nonspecific finding on lung CT images.9 Ground-glass opacity can be caused by various pathological and nonpathological conditions, such as partial filling of the alveolar space, increased blood perfusion, thickening of the interstitium or alveolar walls, and reduction of air in the alveolar space.10 In humans, various diseases that cause ground-glass opacity have been extensively investigated, and a differential diagnosis can be made on the basis of the pattern and distribution of the ground-glass opacity.9–12
Computed tomography images obtained during the expiratory phase or CT images of the dependent portion of a lung lobe can reveal ground-glass opacities in the absence of pathological causes. Both expiration and the dependent portion of a lung lobe reduce air in the alveolar spaces and increase the number of alveolar walls per pixel, which thereby increases attenuation of a lung.10 Respiratory and positional ground-glass opacities can be prevented or minimized in humans by use of deep inspiration or by placing patients in the prone position. However, ground-glass opacities are frequently identified on lung CT images of dogs because dogs cannot maintain a spontaneous inspiratory state during CT, and general anesthesia is necessary for CT. Despite its common occurrence in CT images of animals, studies about the pathological conditions and histologic changes related to ground-glass opacities are lacking.5 Consequently, ground-glass opacity seen on a CT image of a dog is simply considered a nonspecific finding that might be observed with various diseases or an incidental finding associated with expiration and the dependent portion of a lung lobe, particularly when clinical signs do not exist. Positional or respiratory ground-glass opacity is reportedly a reversible finding, and repositioning an animal or inducing hyperventilation is recommended to remove the opacity.9 However, investigating the effect of position and holding time on the lung CT images of healthy dogs may allow clinicians to distinguish between ground-glass opacities caused by a physiologic condition and those resulting from a pathological condition. Thus, the study reported here was performed to evaluate the location, distribution, and degree of ground-glass opacities observed on lung CT images of healthy dogs placed in various positions for different holding times and PEEPs before CT. A second objective was to evaluate the ideal position of recumbency for a dog before a thoracic CT examination that would minimize incidental ground-glass opacities. In addition, the association of ground-glass opacification with lung volume and function was assessed.
Supported by the Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT and Future Planning (grant No. 2015R1A2A2A01003313).
Positive end-expiratory pressure
Region of interest
SNAP 4Dx test, IDEXX Laboratories, Westbrook, Me.
Zoletil, Virbac, Carros, France.
Domitor, Orion Corp, Espoo, Finland.
V12, Votem, Chuncheon-Si, Korea.
Siemens Emotion 16, Siemens AG, Forchheim, Germany.
Siemens B70s kernel, Siemens AG, Forchheim, Germany.
Stat Profile pHox Ultra, NOVA Biomedical, Waltham, Mass.
Syngo volume evaluation, Siemens AG, Forchheim, Germany.
IBM SPSS statistics 20, IBM Corp, Armonk, NY.
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Evaluation of the location, distribution, and degree of ground-glass opacity assessed in CT images obtained from healthy dogs at a PEEP of 0 mm Hg.
|Location||12 parts||Ventral or dorsal part of 6 lung lobes (left cranial, left caudal, right cranial, right middle, right caudal, and accessory)|
|Distribution||4 types||Focal = Ground-glass opacity limited to 1 of the 12 parts and ground-glass opacity not observed in over 30 consecutive transverse CT images reconstructed with a thickness of 1 mm.|
|Multifocal = ≥ 2 focal ground-glass opacities in 1 lung lobe|
|Diffuse = Ground-glass opacity extended into ≥ 2 of the 12 parts or focal ground-glass opacity observed over 30 consecutive transverse CT images reconstructed with thickness of 1 mm.|
|Lobar = Ground-glass opacity observed throughout 1 lung lobe|
|Degree||3 degrees||Mild = Ground-glass opacity with distinct pulmonary vessels and bronchial wall|
|Moderate = Ground-glass opacity partially obscures pulmonary vessels and bronchial wall|
|Severe = Ground-glass opacity obscures pulmonary vessels and bronchial wall|