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.
Materials and Methods
Animals
Six sexually intact male purpose-bred Beagles were used in the study. Median age of the dogs was 3 years (range, 2 to 4 years), and median body weight was 9.7 kg (range, 8.5 to 12 kg). All dogs were healthy, as determined on the basis of results for a physical examination, a CBC, biochemical and electrolyte analyses, thoracic radiography, abdominal ultrasonography, echocardiography, and blood gas analysis. None of the dogs had signs of respiratory tract problems. Drugs for heartworm prevention were administered every month, and all dogs had negative results when tested for heartworm infection.a The study protocol was approved by the Institutional Animal Care and Use Committee of Chonnam National University.
Anesthesia
A crossover study design was used. Food was withheld from dogs for 24 hours. Anesthesia was induced by IM injection of zolazepam hydrochloride-tiletamine hydrochlorideb (0.75 mg/kg) and medetomidine hydrochloridec (0.03 mg/kg). Dogs were positioned in sternal recumbency, and an endotracheal tube was placed in each dog. Anesthesia was maintained by additional injection of anesthetic agents, if needed. Dogs were allowed to have spontaneous respiration and breathed room air. Supplemental oxygen was supplied only during the induction of hyperventilation and for PEEP maintenance during CT. Heart rate was recorded during anesthesia and during CT procedures by use of a multiparameter patient monitor.d
CT
All lung CT images were acquired by use of a 16-row multidetector CT scannere in the caudocranial direction with the following settings: slice thickness, 1 mm; pitch, 0.7; rotation duration, 600 milliseconds; tube voltage, 120 kV; and tube current, 130 mA. All images were reconstructed by use of a reconstruction algorithm,f with a lung window, slice thickness of 1 mm, and increment of 1 mm to assess lung parenchyma. Amount of time for each scan was < 30 seconds. Immediately after anesthesia was induced, CT was performed with the dogs positioned in sternal recumbency at 2 PEEPs (0 and 15 mm Hg [approx 20 cm H2O]).
To investigate the effects of position, duration of recumbency, and PEEP on ground-glass opacity, each dog was arbitrarily placed in 1 of 4 positions (sternal, dorsal, left lateral, and right lateral recumbency) and held in that position for 30 or 60 minutes. Dogs then were repositioned in sternal recumbency, and lung CT scanning was performed at a PEEP of 0 mm Hg and repeated at a PEEP of 15 mm Hg. A breath-hold technique was performed by inducing apnea with manual hyperventilation. To minimize lung expansion, PEEP was maintained at < 10 mm Hg during hyperventilation. A 5-day interval between subsequent examinations was used to allow for sufficient recovery.
Arterial blood gas analysisg was performed immediately before the first CT scan of all dogs to evaluate the effects of position and holding time on oxygenation. Holding time was defined as the duration of recumbency from the beginning of anesthesia to the beginning of the CT scan after repositioning into sternal recumbency.
Image analysis
All CT images were evaluated at a workstation by use of a window width of 2,000 HU and a window level of −600 HU. Location, distribution, and degree of ground-glass opacity were subjectively evaluated on CT images obtained at a PEEP of 0 mm Hg (Appendix). Degree of ground-glass opacity was assessed on transverse CT images, which revealed the most ground-glass opacity.
Tracer ROIs were placed over the left and right lung lobes of all transverse CT images by use of a software program.h Mean density and volume of the lungs and volume of ground-glass opacities were measured by use of the software program.h Thresholds were set to −700 to −900 HU for normal lung tissue and −300 to −700 HU for ground-glass opacities to prevent including the wall of the pulmonary vessels or bronchioles (Figure 1).
Transverse CT images obtained from a healthy dog positioned in sternal recumbency immediately after induction of anesthesia that were used to measure volume and mean density of normal lung tissue (A) and ground-glass opacities (B and C). Tracer ROIs were placed in the left and right lung lobes by use of a software program. Thresholds were set to −700 and −900 HU for normal lung tissue and −300 and −700 HU for ground-glass opacities.
Citation: American Journal of Veterinary Research 78, 3; 10.2460/ajvr.78.3.279
Maximum density of ground-glass opacity was measured on transverse CT images, which had the most ground-glass opacity. An investigator (SK-L) manually drew a circular ROI with an area of 10 mm2 on each ground-glass opacity; the ROIs did not include vessels or bronchioles (Figure 2). Maximum densities of ground-glass opacities were measured 3 times, and the mean value was used for data analysis. Effects of lung inflation on ground-glass opacity were assessed on lung CT images obtained at a PEEP of 15 mm Hg.
Transverse CT image of the left cranial lung lobe obtained from a dog that was positioned in left lateral recumbency for 60 minutes and then repositioned into sternal recumbency for CT. The CT image was obtained at a PEEP of 0 mm Hg. The maximum HU value of a ground-glass opacity was measured by placing a circular ROI with an area of 10 mm2 (white circle) on the image.
Citation: American Journal of Veterinary Research 78, 3; 10.2460/ajvr.78.3.279
Statistical analysis
Data were reported as mean ± SD. The Mann-Whitney signed rank test and Kruskal-Wallis test were used to assess changes in volume, mean HU, and maximum HU of the normal lung tissue and ground-glass opacity as well as the blood gas values measured for the various positions and both holding times. Significance was set at values of P < 0.05. Statistical analysis was performed by use of a commercially available statistical program.i
Results
Ground-glass opacities were observed in CT images of 3 dogs scanned while positioned in sternal recumbency immediately after induction of anesthesia (2 in the ventral part of the left cranial lung lobe and 1 in the dorsal part of the left caudal lung lobe). Although these ground-glass opacities were focal and mild, they slightly decreased after the lungs were inflated at a PEEP of 15 mm Hg, but they did not completely disappear (Figure 3). These ground-glass opacities were excluded from further analysis.
Transverse CT images of the lungs of an anesthetized dog positioned in sternal recumbency. Images were obtained immediately after induction of anesthesia at a PEEP of 0 mm Hg (A) and 15 mm Hg (B). Mildly focal ground-glass opacities (arrow) are evident in the ventral portion of the left cranial lung lobe in the CT image obtained at a PEEP of 0 mm Hg. These opacities decreased but did not disappear after inflation of the lungs at a PEEP of 15 mm Hg.
Citation: American Journal of Veterinary Research 78, 3; 10.2460/ajvr.78.3.279
Ground-glass opacities were found in a variety of lung lobes for dogs held in various positions before CT. They were mainly observed in the part of the lung that had been dependent for the various positions before CT, and most were mild or moderate. Location and distribution of ground-glass opacities (Table 1) and degree of ground-glass opacities (Table 2) were summarized. When CT scanning was performed at a PEEP of 15 mm Hg, ground-glass opacity was not detected in most CT images. Thus, location, distribution, and degree of ground-glass opacity were not determined for CT images obtained at a PEEP of 15 mm Hg.
Location and distribution of ground-glass opacities in lung CT images of 6 healthy dogs on the basis of position and holding time after induction of anesthesia before CT.*
| Sternal recumbency | Right lateral recumbency | Left lateral recumbency | Dorsal recumbency | ||||||
|---|---|---|---|---|---|---|---|---|---|
| Lung lobe | Part | 30 min | 60 min | 30 min | 60 min | 30 min | 60 min | 30 min | 60 min |
| Left cranial | Ventral | — | — | — | Diffuse (1) | Diffuse (3) | Focal (1) Diffuse (3) | — | — |
| Dorsal | — | Focal (3) | — | — Multifocal (1) | Focal (2) Multifocal (1) Diffuse (3) | Focal (2) Diffuse (3) | — | Focal (1) | |
| Left caudal | Ventral | — | — | — | — | — | — | — | — |
| Dorsal | — | — | — | — | Focal (2) | Focal (1) | Focal (4) | Focal (4) | |
| Right cranial | Ventral | — | — | Diffuse (2) | Diffuse (1) | — | — | — | — |
| Dorsal | — | — | Focal (1) | Focal (2) | — | — | — | — | |
| Diffuse (2) | Diffuse (1) | ||||||||
| Right middle | Ventral | — | — | — | — | — | — | — | — |
| Dorsal | — | — | — | — | — | — | — | — | |
| Right caudal | Ventral | — | — | Focal (1) | — | — | — | — | — |
| Dorsal | — | — | — | Focal (1) | — | — | — | — | |
| Accessory | Ventral | — | — | — | — | — | — | — | — |
| Dorsal | — | — | — | Focal (1) | — | — | — | — | |
Numbers in parentheses are the number of dogs with ground-glass opacity in that part of the lung lobe.
Dogs were anesthetized and then placed in 1 of 4 positions for 30 or 60 minutes before they were repositioned into sternal recumbency for CT.
— = Ground-glass opacity not detected.
Degree of ground-glass opacities in lung CT images of 6 healthy dogs on the basis of position and holding time after induction of anesthesia before CT.*
| Sternal recumbency | Right lateral recumbency | Left lateral recumbency | Dorsal recumbency | ||||||
|---|---|---|---|---|---|---|---|---|---|
| Lung lobe | Part | 30 min | 60 min | 30 min | 60 min | 30 min | 60 min | 30 min | 60 min |
| Left cranial | Ventral | — | — | — | Mild (1) | Mild (1) Moderate (1) Severe (1) | Mild (2) Severe (2) | — | — |
| Dorsal | — | Mild (3) | — | — | Mild (3) Moderate (2) Severe (1) | Mild (2) Moderate (2) Severe (2) | — | Mild (1) | |
| Left caudal | Ventral | — | — | — | — | — | — | — | — |
| Dorsal | — | — | — | — | Mild (2) | Mild (1) | Mild (2) Moderate (2) | Mild (2) Moderate (2) | |
| Right cranial | Ventral | — | — | Mild (1) Moderate (1) | Mild (1) | — | — | — | — |
| Dorsal | — | — | Mild (2) Moderate (1) | Mild (3) | — | — | — | — | |
| Right middle | Ventral | — | — | — | — | — | — | — | — |
| Dorsal | — | — | — | — | — | — | — | — | |
| Right caudal | Ventral | — | — | Mild (1) | — | — | — | — | — |
| Dorsal | — | — | — | Mild (1) | — | — | — | — | |
| Accessory | Ventral | — | — | — | — | — | — | — | — |
| Dorsal | — | — | — | Mild (1) | — | — | — | — | |
Degree of ground-glass opacity was assessed as follows: mild = ground-glass opacity with distinct pulmonary vessels and bronchial wall, moderate = ground-glass opacity with partially obscure pulmonary vessels and bronchial wall, and severe = ground-glass opacity obscures pulmonary vessels and bronchial wall.
See Table 1 for remainder of key.
Ground-glass opacity was minimized when dogs were positioned in sternal recumbency for 30 or 60 minutes before CT, compared with results for the 3 other positions before CT. Ground-glass opacities were not observed in dogs positioned in sternal recumbency for 30 minutes before CT, whereas mild focal ground-glass opacities were observed in 3 dogs that were positioned in sternal recumbency for 60 minutes before CT. In contrast to results for the other positions, for which ground-glass opacities were observed in the dependent part of the lung lobe, all ground-glass opacities seen in dogs positioned in sternal recumbency for 30 or 60 minutes before CT were in the nondependent dorsal portion of the left cranial lung lobe (Figure 4).
Transverse CT images of dogs placed in sternal recumbency (A), right lateral recumbency (B), left lateral recumbency (C), and dorsal recumbency (D) for 60 minutes and then positioned in sternal recumbency for CT. Mild ground-glass opacity (arrow) is evident in the dorsal portion of the left cranial lung lobe (A). Mild focal ground-glass opacity (arrow) is evident in the dorsal portion of the right cranial lung lobe (B). Severe diffuse ground-glass opacity (arrows) is evident in the left cranial lung lobe (C). Focal mild ground-glass opacity (arrow) is evident in the dorsal portion of the left caudal lung lobe (D).
Citation: American Journal of Veterinary Research 78, 3; 10.2460/ajvr.78.3.279
Focal and diffuse ground-glass opacities were observed in CT images obtained from dogs that had been placed in right lateral recumbency for 30 or 60 minutes before CT. Although most ground-glass opacities were mild, a moderate diffuse ground-glass opacity extending from the ventral to the dorsal portion of the right cranial lung lobe was observed on the CT image of a dog positioned in right lateral recumbency for 30 minutes before CT. Ground-glass opacities were mainly observed in the right cranial lung lobe. In particular, ground-glass opacities were observed in the dorsal portion of the right cranial lung lobe in CT images of 3 dogs that were positioned in right lateral recumbency for both 30 and 60 minutes. The dorsal portion of the right cranial lung lobe was the most common site of ground-glass opacity seen in CT images of dogs positioned in right lateral recumbency before CT (Figure 4).
Ground-glass opacities were observed most frequently and had the greatest severity when dogs were positioned in left lateral recumbency before CT. Ground-glass opacities were evident in the CT images of all dogs positioned in left lateral recumbency, regardless of holding time. These ground-glass opacities were mainly in the left cranial lung lobes, and they were distributed as focal, multifocal, and diffuse types in the dorsal portion of the left cranial lung lobes. A severe degree of ground-glass opacity was observed only in CT images of dogs positioned in left lateral recumbency before CT (Figure 4). Ground-glass opacities were also observed in the ventral portion of the left cranial lung lobe in CT images of 3 and 4 dogs that were held in left lateral recumbency for 30 minutes and 60 minutes, respectively, before CT.
All ground-glass opacities observed in dogs positioned in dorsal recumbency before CT were in the left lung lobes (mainly in the caudal lung lobes, particularly the dorsal portion of the caudal lung lobes), whereas all of the ground-glass opacities observed in dogs positioned in sternal recumbency before CT were in the cranial lung lobes. Mild-to-moderate focal ground-glass opacities were observed in the dorsal portion of the left caudal lung lobe in the CT images of 4 dogs (Figure 4).
Ground-glass opacities observed in CT images obtained immediately after induction of anesthesia differed from those observed after dogs were held in various positions for 30 or 60 minutes before CT. All ground-glass opacities observed after dogs had been held in the 4 positions disappeared or decreased at a PEEP of 15 mm Hg (Figure 5).
Transverse CT images of a dog positioned in left lateral recumbency for 60 minutes and then repositioned into sternal recumbency for CT, which was performed at a PEEP of 0 mm Hg (A) and a PEEP of 15 mm Hg (B). Notice there is a diffuse ground-glass opacity (arrows) in the left cranial lung lobe in the image obtained at a PEEP of 0 mm Hg (A), but the ground-glass opacity is no longer present in the image obtained at a PEEP of 15 mm Hg (B).
Citation: American Journal of Veterinary Research 78, 3; 10.2460/ajvr.78.3.279
Lung volume was estimated to investigate whether there was an association between the frequency of ground-glass opacity and lung volume change. Volume of the left lungs of dogs was significantly smaller in CT images obtained after placing dogs in left lateral recumbency than after placing dogs in other positions, except for that of dogs placed in dorsal recumbency for 60 minutes before CT. However, right lung volume and total lung volume did not significantly change with the positions and holding times, although right lung volume typically decreased and there was a mild right mediastinal shift after dogs were held in right lateral recumbency for 30 or 60 minutes before CT (Table 3). Volume of ground-glass opacity was significantly larger in the left lung lobes of dogs held in left lateral recumbency than when dogs were held in the other positions before CT, except for the images obtained for dogs positioned in right lateral recumbency for 30 minutes and dorsal recumbency for 60 minutes before CT (Table 4). Volume of the ground-glass opacities found in the right lung lobes did not change on the basis of position and holding time before CT.
Mean ± SD lung volume (cm2) in lung CT images of 6 healthy dogs on the basis of position and holding time after induction of anesthesia before CT.*
| Sternal recumbency | Left lateral recumbency | Right lateral recumbency | Dorsal recumbency | ||||||
|---|---|---|---|---|---|---|---|---|---|
| Lung part | Immediately after induction of anesthesia | 30 min | 60 min | 30 min | 60 min | 30 min | 60 min | 30 min | 60 min |
| Left lung | 215 ± 24 | 215 ± 20 | 231 ± 24 | 189 ± 19 | 196 ± 19 | 232 ± 32 | 241 ± 30 | 214 ± 18 | 217 ± 24 |
| Right lung | 302 ± 38 | 305 ± 37 | 336 ± 28 | 308 ± 24.5 | 313 ± 24 | 284 ± 60 | 299 ± 50 | 298 ± 29 | 319 ± 24 |
| Total | 517 ± 61 | 520 ± 55 | 567 ± 50 | 497 ± 41 | 509 ± 40 | 516 ± 88 | 539 ± 79 | 512 ± 46 | 500 ± 94 |
See Table 1 for key.
Mean ± SD ground-glass opacity volume (cm2) in lung CT images of 6 healthy dogs on the basis of position and holding time after induction of anesthesia before CT.*
| Sternal recumbency | Left lateral recumbency | Right lateral recumbency | Dorsal recumbency | ||||||
|---|---|---|---|---|---|---|---|---|---|
| Lung part | Immediately after induction of anesthesia | 30 min | 60 min | 30 min | 60 min | 30 min | 60 min | 30 min | 60 min |
| Left lung | 49 ± 10 | 47 ± 11 | 46 ± 10 | 52 ± 12 | 52 ± 15 | 45 ± 7 | 46 ± 9 | 47 ± 9 | 45 ± 9 |
| Right lung | 66 ± 13 | 65 ± 14 | 61 ± 7 | 64 ± 8 | 65 ± 11 | 64 ± 23 | 69 ± 10 | 65 ± 12 | 63 ± 11 |
See Table 1 for key.
Maximum density of ground-glass opacities was higher when dogs were held in left lateral recumbency before CT, compared with values for other positions, but this difference was not significant. Furthermore, mean ± SD density of ground-glass opacity did not change on the basis of position and holding time before CT (no ground-glass opacities and −543 ± 55 HU after sternal recumbency for 30 and 60 minutes, respectively; −299 ± 108 HU and −280 ± 75 HU after left lateral recumbency for 30 and 60 minutes, respectively; −406 ± 75 HU and −495 ± 107 HU after right lateral recumbency for 30 and 60 minutes, respectively; and −493 ± 82 HU and −529 ± 126 HU after dorsal recumbency for 30 and 60 minutes, respectively).
Blood gas analyses were performed on all samples. All results were within reference limits (data not shown).
Discussion
In general, ground-glass opacities were mild regardless of position or holding time; the only exception was when dogs were held in left lateral recumbency. Ground-glass opacities were observed in specific locations on the basis of position, and they mainly were found in the dependent portions of the lung lobes for each recumbency position and were reversible with inflation of the lungs.
For dogs placed in lateral recumbencies, focal as well as multifocal and diffuse ground-glass opacities were observed on both the left and right sides. For both lateral recumbencies, ground-glass opacities were distributed in the dependent portions of the lungs, particularly in the cranial lung lobes. This may have been because many structures that can press against a dependent lung lobe, such as the heart and cranial mediastinum, are located in the cranial portion of the thorax. Furthermore, the cranial lung lobes may have been more sensitive to the position of the heart than the caudal lung lobes because of the narrow space between the heart and thoracic wall. Furthermore, ground-glass opacities were observed more often in the dorsal part than in the ventral part of the lung lobes, but this finding could not be explained in the present study.
Positioning of dogs in left lateral recumbency before CT resulted in ground-glass opacities on lung CT images, regardless of the holding time. Dogs positioned in left lateral recumbency had CT images with ground-glass opacities more frequently and with more severity, compared with results for the other positions. Lung volume was smaller and volume and density of ground-glass opacities were higher for dogs positioned in left lateral recumbency than for the other positions. Ground-glass opacities may have been more severe on images of the left lung when dogs were positioned in left lateral recumbency than on images of the right lung when dogs were positioned in right lateral recumbency because the volume of the left lung is typically smaller than that of the right lung.13 Moreover, the left lung may be more sensitive to the position of the heart because the heart is normally located slightly to the left side in healthy dogs.4
Positioning of dogs in dorsal recumbency resulted in ground-glass opacities that appeared as focal lesions. They were all observed in the dependent (dorsal) part of the left caudal lung lobe, which is consistent with a previous report14 in which some collapse of the dorsocaudal aspect of the caudal lung lobes was evident in ventrodorsal radiographs. Ground-glass opacification was observed only in the left caudal lung lobe in the present study, but in a previous study15 that involved the use of CT, lung attenuation for dogs in dorsal recumbency was symmetrically increased. The effect of gravity on abdominal organs (particularly the stomach) may induce the dorsal part of the caudal lung lobes to collapse in dogs in dorsal recumbency, which can have a greater influence on the left caudal lung lobe than on the right lung lobe.
For the 4 positions investigated in the present study, sternal recumbency resulted in negligible ground-glass opacities, which is consistent with the findings of a previous study14 in which the least amount of recumbency-associated atelectasis was evident in dorsoventral radiographic views. This result can be explained by the fact that the positions of the thoracic structures of dogs in sternal recumbency are almost the same as those of dogs in a standing position. Interestingly, evaluation of CT images obtained after placing dogs in sternal recumbency did not reveal ground-glass opacities in the dependent portion of the left cranial lung lobe; rather, the ground-glass opacities were found in the nondependent dorsal part of the left cranial lung lobe. This study did not provide an explanation for this finding.
The location, distribution, and degree of ground-glass opacities were similar, regardless of holding time, except that ground-glass opacities were not observed in CT images of dogs held in sternal recumbency for 30 minutes but were observed in CT images of 3 dogs held in the same position for 60 minutes. Similarly, volume and density of the lung and ground-glass opacity were not different between the holding times. This result might have been because the maximum decrease in lung capacity appears to occur within the first few minutes after induction of anesthesia.16
Although the presence of atelectasis in anesthetized dogs is associated with impaired oxygenation, decreased compliance, increased pulmonary vascular resistance, and lung injury,17 results for blood gas analysis were within reference limits for all dogs in the study reported here. Similarly, total lung volume was consistently maintained, even when left lung volume was significantly decreased for left lateral recumbency. This might have been because the enlarged right lung compensated for the left lung. These results are not in agreement with findings of a previous study13 in which investigators found that volume of the nondependent lung did not change when healthy dogs were placed in lateral recumbency.
Although ground-glass opacities seen in a dependent lung lobe were not associated with lung capacity and total lung volume, they can mimic or mask pathological changes of the lungs. Therefore, prevention and reversal of atelectasis are important. In the study reported here, all ground-glass opacities in cases of atelectasis were reversible, although the degree of reduction of ground glass opacities differed for each dog depending on position and holding time. In other words, ground-glass opacities that do not disappear in an inflated lung may not be attributable to atelectasis, and it is possible they are pathological lesions, although histologic examination would be necessary for confirmation.
Although CT can be performed with multidetector CT scanners in awake or sedated animals, animals typically are anesthetized to prevent motion and to enable clinicians to obtain images of sufficient quality. Several minutes may elapse between the induction of anesthesia and CT scanning because of the need to stabilize the anesthetic plane and perform other examinations or treatments, depending on a patient's situation. Thus, a holding time before CT for each position was set at 30 and 60 minutes in the present study. To minimize the effects of hyperventilation, apnea was induced during CT by use of manual hyperventilation with a lower PEEP of 10 mm Hg. In addition, injectable anesthetics were used instead of inhalation anesthetics to prevent effects from the supply of oxygen on blood gas analysis and CT images.
The present study had several limitations. First, only adult Beagles were included. The magnitude of atelectasis in the dependent lung lobe is directly related to the body mass of the patient.14 In a study18 of humans, the gravity-dependent anteroposterior density gradient was affected by anteroposterior chest diameters in lung CT images. Because mean body weight and chest conformation differ among breeds, results for the study reported here may not be relevant for all dog breeds. Second, CT was performed in dogs anesthetized with injectable agents; therefore, the depth of anesthesia changed slightly over time, which may have resulted in a change in the breathing depth of the dogs. Third, the effects of position and holding time on ground-glass opacities were evaluated only in healthy dogs. Patients with lung disease might have other degrees of atelectasis as a result of pronounced impairment of gas exchange, and hyperinflation may enable the lungs to resist collapse or may cause resorption of atelectasis in patients with chronic obstructive lung disease.17 Finally, supplemental oxygen was not administered during anesthesia; however, clinics commonly provide oxygen during inhalation anesthesia for CT. High oxygen concentrations have been associated with atelectasis formation,17 and this was not considered in the present study.
In the study reported here, ground-glass opacities on lung CT images were affected more by a dog's position after induction of anesthesia than by the holding time or PEEP. Left lateral recumbency resulted in more frequent and more severe ground-glass opacities, but diffuse ground-glass opacities were also found after right lateral recumbency. Ground-glass opacities were negligible on lung CT images obtained after dogs were positioned in sternal recumbency. Ground-glass opacities that developed as a result of atelectasis from the positioning of dogs for 30 or 60 minutes before CT decreased or disappeared after the lungs were inflated with a PEEP of 15 mmHg. This result may help to distinguish incidental ground-glass opacities attributable to atelectasis from those attributable to pathological changes. Ground-glass opacities were observed in specific locations, depending on position of the dogs. This was mainly observed in dependent portions of the lungs. However, dogs held in sternal recumbency had ground-glass opacities in the dorsal part of the left cranial lung lobe, dogs held in dorsal recumbency had ground-glass opacities in the dorsal part of the left caudal lung lobe, and dogs held in left lateral and right lateral recumbency had ground-glass opacities in the left cranial lung lobe and right cranial lung lobe, respectively. These particular locations might have been affected by adjacent organs and the anatomic conformation of the chest.
Dogs should be positioned in sternal recumbency after anesthesia to minimize the presence of ground-glass opacities. When clinicians or researchers need to perform several procedures between induction of anesthesia and CT, which results in an increased holding time for a dog in a specific position, left lateral recumbency should be avoided. However, even for dogs held in left lateral recumbency, ground-glass opacities can be reduced or eliminated in lung CT images by inflation of the lungs. Thus, intubation and positive-pressure ventilation are recommended for helping to differentiate atelectasis from pathological lesions, particularly when ground-glass opacities are observed in a dog anesthetized with injectable anesthetics.
Acknowledgments
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).
ABBREVIATIONS
| HU | Hounsfield unit |
| PEEP | Positive end-expiratory pressure |
| ROI | Region of interest |
Footnotes
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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Appendix
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.
| Factor | Classification | Criteria |
|---|---|---|
| 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 |
