Thoracic imaging (Proceedings)

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Thoracic imaging (Proceedings)

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Nov 01, 2010

This lecture is designed as an overview of the various modalities available for thoracic imaging, and their specific utility. Recognizing that thoracic radiography is the first diagnostic imaging step for dogs and cats with thoracic disease, there is a lot more to thoracic imaging than radiographs. In specific circumstances additional modalities which may be considered include ultrasound, computed tomography (CT), fluoroscopy and nuclear scintigraphy. A complete description of the technical aspects, and interpretation of these modalities is beyond the scope of the presentation, however the reader is provided with a brief synopsis below.

Thoracic radiography:
      - Standard orthogonal or three-view thoracic radiographs
      - When to consider DV vs. VD?
      - Systematic evaluation of the entire thorax:

Thoracic ultrasound
Indications
      - to evaluate the mediastinum in cases of suspected mediastinal mass
      - evaluation of masses at the periphery of the lung, or involving the chest wall
      - guidance for aspiration of masses in the mediastinum, lung periphery or chest wall
      - diagnosis of pneumothorax in the setting of thoracic trauma (i.e. TFAST)
Thoracic ultrasound is not useful in animals with centrally located lung masses or to image lung lesions in the presence of pneumothorax.

Advantages:
      - affordable
      - well tolerated (+/- sedation)
      - imaging yield may be improved in the presence of residual pleural effusion

Disadvantages:
      - relative inability to view within the lung due to air artifacts
      - only able to evaluate a relatively small window of the chest at once

Thoracic computed tomography
      - Detailed anatomic describes of the normal canine and feline thorax by conventional CT ( IV contrast) have been provided
      - Protocols for helical and high-resolution CT have also been developed for the canine lung

CT modalities available:
      - Conventional CT:
      - Helical CT: Advantages include....
      - High-resolution CT:
           - Heart
           - Great vessels (MPA, aorta and caudal vena cava)
           - Pulmonary parenchyma
           - Pulmonary vasculature
           - Pleural space
           - Chest wall
           - Extra-thoracic structures
           - Contiguous images obtained without interscan delay
           - Improved spatial resolution
           - Shorter examination time
           - Performed with thick collimation (slice thickness)
           - Poor spatial resolution
           - Long examination times, and hence motion artifact
           - Not particularly useful for thoracic imaging
           - Slice thickness of 1-2mm
           - Maximizes spatial resolution (by using a tightly collimated x-ray beam), high kV and mA, decreased field of view and special reconstruction algorithms)
           - Close to vascular structures
           - Not well identified with ultrasound (i.e. surrounded with air), fluoroscopy or radiography
           - Zone 1 = pleural region
           - Zone 2 = subpleural region (measuring in diameter 5% of the maximum lobar width)
           - Zone 3 = peribronchovascular region (i.e. remainder of the lung parenchyma)
           - Linear and reticular opacities
           - Nodules and nodular opacities
           - Increased lung opacity
           - Decreased lung opacity