Advanced respiratory tract imaging: Ride the wave (Proceedings)

Respiratory tract disease is both serious and extremely common. For 95% of all patients the only imaging modality available has been radiography. Radiography has the advantage of a strong historical basis, a good overview of everything within the thorax and inherently good contrast to identify the underlying structure involved. However, we are extremely limited in the characterization of mild or less classic abnormalities. Additionally, radiographs are quite poor for the evaluation of heart size in cats and may not be indicated in extremely dyspneic patients early in the course of therapy. During this election we will discuss more advanced respiratory imaging, especially computed tomography, and its role in both emergency and general veterinary practice.

Considerations to advanced conventional radiography are many. In our practice we routinely take 3 views for the evaluation of the most common diseases, including metastatic lesions. The 3rd view is the opposite lateral which allows us to see the middle lung field without superimposition. We often add a lateral neck section for dogs where laryngeal or tracheal disease are likely. This includes dogs with suspected trachea collapse (small breed dogs and miniatures) or cats would laryngeal masses. Additionally dogs with laryngeal paralysis or laryngeal collapse will benefit from a lateral neck projection. Laryngeal collapse is seen brachycephalic breeds. Laryngeal paralysis is seen in large breed dogs and certain miniature breeds. A 5th view is added when a dynamic large airway disease is suspected, such as collapse of either the trachea or larger bronchi. This is extremely common in our small breed dogs.

We routinely take horizontal beam VD radiographs in our trauma patients. These projections greatly improve our ability to detect small volumes of pneumothorax and better characterize the volume in more serious cases. Additionally, it allows us to evaluate for concurrent free fluid in the dependent portion of the chest. Examples will be given that display the improved characterization and detection of pleural disease with horizontal beam radiography.

Video fluoroscopy is very useful in cases of dynamic disease. This is especially true with the esophagus, which is beyond the scope of this lecture. Classically this modality has been used for the evaluation of collapsing trachea. However, given the prevalence of collapse of additional large airways including the principal and lobar bronchi, video fluoroscopy does not perform as well for those large airways. Multiple recent papers indicates that collapse of these large bronchi is more common than true tracheal collapse in dogs with chondromalacia.

Perhaps the most exciting new form of respiratory imaging is being performed with multiple slice CT. Multi-slice CT provides extremely rapid imaging and therefore opens the door to imaging patients on an emergency basis for evaluating functional lesions. CT has greatly improved spatial resolution and also allows for contrast enhancement of any space occupying lesion within the chest.

Combining this new technology with novel motion limiting devices provides the basis of the new wave of emergency respiratory imaging. The paradigm for the future should be imaging dogs that are awake, without any sedation or anesthesia if possible, to better demonstrate the lesions under disease states (rather than exaggerated or minimized by the effects of anesthesia). Many novel motion limiting devices have been proposed. Most allow for the administration of both oxygen and intravenous fluids during the imaging. The device should provide no additional artifacts or stress to the patient. It should be a device that could be additionally used in an emergency or intensive care setting for the administration of oxygen even without imaging being performed. It should be relatively inexpensive to manufacture and rugged for routine clinical use.

CT has greatly improved contrast resolution compared to radiology. Lung lesions are much better characterized on CT. Lung lesions can be detected even when surrounded by pleural lesions. The ability to perform contrast enhanced CT greatly improves on this contrast resolution. Angiographic CT also provides the ability to diagnose pure vascular lesions such as lung lobe torsion and pulmonary thromboembolic disease. Both of these lesions have been imaged and awake patients using modern systems and novel CT protocols.

Under certain emergency circumstances, echocardiography is relatively contraindicated due to the stress associated with restraint for the procedure. This is especially true in cats. By delaying echocardiography until the patient is more stable often results in unacceptable delays in appropriate therapy. CT has the unique capability of characterizing the size of the left atrium compared to the size of the aortic root on survey imaging. The importance of this cannot be exaggerated on an emergency basis. The implications are that a patient without an IV catheter and too dyspneic for echocardiography can be imaged without any handling or restraint within a motion limiting device and determination made for all the components necessary for the diagnosis of left–sided congestive heart failure; pulmonary edema and large left atrium. Additionally, this imaging can be performed in approximately 15 seconds without the need for any initial scanning to determine location of the body part of interest.

Finally, and perhaps most exciting, is the ability of CT to evaluate for large airway collapse. This takes us beyond the diagnosis of tracheal collapse into the region of principle and lobar collapse. A particularly difficult clinical situation has been in circumstances of the cardiac murmur and concurrent cough. There is a tendency to over diagnose this clinical scenario as being a cardiogenic cough due to pulmonary edema with the ensuing overtreatment with diuretics and other cardiac medications. Recent evidence indicates that costs are quite unlikely with pulmonary edema and that the most common cause is bronchial collapse due to chondromalacia. CT is uniquely able to make this diagnosis in an awake patient. Cases will be discussed that exemplify this clinical situation. There is some suggestion that bronchomalacia (caused by chondromalacia) may be genetically associated with another cartilaginous degenerative disease; and endocardiosis. Therefore concurrence of the lesion should be expected rather than thought an unlikely combination of two unassociated diseases.

In summary, traditional ready graphic projections are easily made to improve respiratory imaging and cats and dogs in general practice. Video fluoroscopy adds to the sensitivity and characterization of functional diseases, especially involving the esophagus. But it is with CT that we can make major improvements in respiratory tract imaging that involve both primary pulmonary lesions, large airway characterization and vascular lesions that are so common in dogs and cats.