CT of the Thorax: Current State of the Art in Diagnosing Parenchymal and Airway Diseases
Gabriela S. Seiler, Dr.med.vet., DECVDI, DACVR
With the increasing availability of computed tomography (CT) in veterinary medicine, the diagnostic imaging workup of patients with respiratory disease is shifting from radiography towards CT, similar to what has happened in human medicine in the past. Multislice CT scanners are being introduced in many veterinary clinics, and the increased speed and resolution of these scanners enable short duration anesthesia and sedation protocols to be used, making this method even more attractive for imaging of patients with respiratory disease. CT has been shown to have increased sensitivity and specificity for diagnosing respiratory disease. In the following, imaging protocols and CT features of common respiratory disorders will be described.
Thoracic CT imaging is complicated by the fact that respiratory motion creates artifacts that may prevent correct diagnosis. Helical imaging however allows fast enough scanning to image the thorax during a single breath hold in most cases. Positioning the patient in sternal recumbency is recommended, it allows for good aeration of the dorsal portion of the lungs. In some instances, for example if ventral lung disease or atelectasis is suspected on an initial scan, the patient has to be repositioned and rescanned in dorsal recumbency. In any case, lateral recumbency should be avoided prior to placing the patient in the gantry as it leads to a larger degree of atelectasis than ventral or dorsal recumbency. General anesthesia and intubation are necessary to hyperventilate the patient immediately prior to imaging to induce a pause in respiratory drive for the duration of the scan. Imaging the thorax in a caudocranial direction is helpful, should the patient start to breathe towards the end of the scan the thoracic wall excursions are smaller and there is no diaphragmatic motion present in the cranial thorax. Slice thickness is chosen to allow scanning during a single breath hold (up to 50-60 seconds). Once areas of pathology are identified, thinner slices (1mm) can be acquired focally using a high resolution imaging technique (high kVp, high mA small field of view, high resolution reconstruction algorithms). Tube rotation speed should be kept at a minimum to reduce the amount of motion artifact. Images are viewed in several windows to assess the lungs (approximate window width 2000 Hounsfield units (HU), window level -800 to -400 HU) and mediastinum (window width 180-300 HU, window level 50-100 HU).1 Reformatted images in dorsal and sagittal planes are usually very helpful. For imaging of the upper airways, special considerations apply. Endotracheal tube placement may obscure tracheal pathology, and sedation combined with fast (helical) scanning techniques may be necessary. Pulling back the endotracheal tube into the cervical trachea can be sufficient to evaluate lower cervical tracheal disease.
CT Findings in Airway Diseases
Helical CT of the trachea gives an excellent three dimensional impression of the tracheal lumen and wall. Aspirated foreign bodies can be differentiated from tracheal wall lesions such as parasitic granulomas (Oslerus osleri), neoplasia (lymphoma, adenocarcinomas in cats; osteochondromas, mast cell tumors, leiomyomas and chondrosarcomas in dogs), or tracheal polyps. CT is sometimes used to identify tracheal lacerations in patients with severe pneumomediastinum. Small lacerations may be difficult to clearly delineate. Evaluation of tracheal or stem bronchus collapse requires dynamic scanning during different respiratory phases. The use of CT for the diagnosis of tracheal collapse has not been fully explored, mostly because of the need of anesthesia which is associated with higher risk in these patients. However, since there is variation in shape and amount of rotation (especially of the cervical trachea), and the tracheal diameter during in- and expiration, exact measurement of the tracheal size using a dynamic, cross-sectional imaging method may be advantageous for selection of an appropriate tracheal stent size.2
CT is not commonly used to diagnose bronchial disease, but allows a much better assessment of bronchial walls and peribronchial interstitial tissue than radiographs due to the lack of superimposition. In a recent study, the sensitivity of thoracic radiographs to detect canine bronchitis ranged from 52% to 65%.3 Chronic bronchitis may lead to bronchiectasis which is readily identified using thoracic CT. A ratio of 2 or more of bronchial to adjacent arterial diameter is considered highly suggestive of bronchiectasis.4 CT is especially advantageous in cats with mucus deposits in the airways which can resemble pulmonary nodules radiographically, whereas they are identified as intraluminal bronchial soft tissue on CT images. In addition to thickened bronchial walls, multifocal interstitial infiltrates giving the pulmonary parenchyma a ground-glass appearance as well as peripheral linear parenchymal soft tissue bands representing areas of atelectasis and fibrosis may be identified on CT.5
Bronchial Foreign Bodies
Non-radiopaque bronchial foreign bodies are difficult to diagnose radiographically, and are usually only suspected once secondary focal pneumonia and bronchitis has developed. With CT imaging, anatomical association of interstitial to alveolar disease along an airway is identified. Additionally, focal intraluminal accumulation of bronchial secretions, focal bronchiectasis and secondary changes such as mild lymphadenopathy, pleural effusion or pneumothorax can be determined.6 In some cases the actual bronchial foreign body can be identified within the bronchial lumen.
CT Findings in Pulmonary Parenchymal Disease
Pulmonary Masses and Nodules
Assessment of pulmonary masses and nodules is a frequent indication for CT imaging, especially for surgical planning. The pulmonary origin of a radiographically visible mass can be determined as well as the exact location, involvement of other structures and presence of metastatic pulmonary nodules or bone lesions. Enlarged tracheobronchial lymph nodes (>5mm), with ring enhancement is consistent with lymph node metastasis, which is strongly associated with survival time and therefore an important prognostic finding.7 CT may also be able to identify pulmonary masses that are not radiographically evident or obscured by other disease processes such as alveolar disease. While the threshold for radiographic detection of pulmonary nodules is 7-9mm, CT is able to detect nodules as small as 1mm. In dogs with metastatic pulmonary disease, CT was able to detect pulmonary nodules in all dogs whereas radiographs confirmed metastatic disease only in 50% of patients.8 CT is therefore the imaging method of choice for detection of pulmonary metastatic disease.
Pulmonary Bullae and Blebs
Spontaneous pneumothorax is a consequence of ruptured pulmonary bullae or blebs, and CT may be helpful in identifying and localizing cavitary pulmonary lesions and to determine the extent of lung lobe involvement and aid in surgical planning. Pulmonary bullae are defined as air-filled cavities within the lung parenchyma whereas blebs are accumulations of air between the visceral pleura and pulmonary parenchyma. Ruptured pulmonary bullae are not easily identified with CT, and may be mistaken for areas of atelectasis especially if the lungs are collapsed. The pneumothorax should therefore be evacuated as completely as possible prior to scanning.9 Pulmonary interstitial emphysema is characterized by gas tracking along the large vessels and bronchi into the periphery of the interstitial space and may be the cause or the consequence of pneumomediastinum.10 Rapid expansion and compression of the thorax (for example during forceful coughing) may lead to shearing forces in the lung parenchyma along the larger bronchovascular structures with subsequent alveolar rupture.
Lobar Pulmonary Disease
Similar to the radiographic appearance, pneumonia is seen on CT as a lobar consolidation in a cranioventral location with peripheral interstitial and peri-bronchial infiltrates. If pneumonia is suspected in the ventral portions of the lung, the patient should be ventilated in dorsal recumbency for a few minutes before imaging to rule out positional atelectasis and confirm pulmonary pathology. In patients with suspected pneumonia, CT imaging is mainly used to rule out abscess formation and other disease processes such as bronchial foreign bodies in cases that are resistant to treatment.
CT is an excellent imaging method to assess suspected lung lobe torsion. In addition to avoiding superimposition with pleural effusion with the cross-sectional imaging, the bronchi can also be followed along their entire length. An abruptly ending, blunted, slowly tapering or twisted bronchus entering a consolidated lung lobe is a sign of lung lobe torsion. Other findings include enlargement of the affected lung lobe with a mediastinal shift to the contralateral side, areas of emphysema and consolidation within the torsed lung lobe, displacement and compression of adjacent lung lobes and pleural effusion (focal or generalized) as well as occasional pneumothorax.11 Pulmonary thromboembolism (PTE) may lead to focal or multifocal pulmonary consolidations, occasionally a typical wedge-shape is observed. CT angiography should be performed if PTE is suspected to look for vascular filling defects.12
Diffuse Pulmonary Disease
Canine idiopathic pulmonary fibrosis (IPF) is most commonly seen in middle-aged to older West highland white terriers and is a chronic progressive disease that eventually leads to fatal respiratory compromise. The predominant histopathologic change is alveolar septal fibrosis. CT imaging findings of dogs with IPF have been reported.13 In early stages, an interstitial infiltrate results in multifocal ground-glass opacity. As the disease progresses, subpleural and parenchymal bands as well as interstitial thickening develops, and finally in severe cases traction bronchiectasis and honeycombing are observed. Honeycombing is a descriptive term for alveolar breakdown and loss of supportive pulmonary structures, resulting in small cystic, air-filled cavities mostly along the periphery of the lung lobes. Pulmonary edema (both cardiogenic and non-cardiogenic) results in a dense interstitial ground-glass appearance. Diffuse neoplastic infiltrates may be radiographically occult. Alveolar septal metastasis with carcinoma is characterized by ground-glass opacity, subpleural interstitial thickening, subpleural emphysema, lobar consolidation and interspersed pulmonary nodules.14 Pulmonary lymphoma is characterized by interstitial ground-glass opacity, usually combined with thoracic lymphomegaly.
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