CT of awake cats: What is possible? (Proceedings)

Computed tomography is becoming more and more readily available to general practices, either as a local referral practice or in-house in larger practices. Typically this procedure is performed only on the most complex cases and only under general anesthesia. Recent developments have allowed faster, safer and higher quality imaging to all come together in a clinical setting. This session will discuss these developments and a series of case examples of nasal, laryngeal and intra-thoracic disease in cats imaged without sedation or anesthesia.

CT technology

The original computed tomographic systems were single slice sequential imaging machines. The gantry rotated about the patient in a few seconds then the patient couch moved forward a distance dictating the slice thickness and the gantry rotated again. These sequential systems were very slow. Often the imaging of a cat thorax required 30+ minutes for data acquisition and computer analysis for final display of the images. The bad old days of CT.

All newer systems provide helical imaging capability. With helical (=spiral") imaging the patient moves continuously, without pause for computer analysis, and the entire batch of data is then analyzed after the scanning is completed. This provides faster imaging then sequential systems. However with single detector CT systems the scanning is still limited by rotation time (up to 1 second) times the thickness of each slice imaged (preferable <1mm) times the length of the body part scanned. So a 20cm long thorax might still require minutes to scan (i.e. 1 sec x 1 mm x 200mm = 200 seconds sec, = 3+ minutes). We can't stop the motion under these circumstances and patients are still anesthetized with single slice helical systems.

Newer CT system are multidetector; 2, 4, 8, 16, 64, etc. With these systems we significantly decrease the time of imaging by 1) collecting many slices of data concurrently, and 2) faster rotation time (<0.5 seconds) resulting in dramatic shorter imaging times (0.5sec/slice x 1.0mm slice thickness x 200mm/16slices/rotation = 6 sec). Most of the chest imaging in cats can be performed in 5-8 seconds with a 16 slice helical CT protocol. Scanning is best performed with sub-millimeter slice thickness with overlap of the imaging sets. This will produce isotropic imaging.

Isotropic imaging means that the reconstructed images have the same image resolutiomn as the original imaging plane. This makes 3-D, MIP and MPR imaging possible. These technologies will be discussed at greater length in the last lecture today.

The device (VetMouseTrap)

Cats will not sit quietly on the CT couch. Even with very fast imaging, cats move. A device was needed to minimize patient motion for this brief period. This device also needed to provide a nurturing environment for dyspneic, anemic, shocky or otherwise compromised feline patients. The criteria for such a device included the following:

In 2009 research began on the design of devices that would restrict a feline patient without causing stress to a sick patient. The final design is a bivalved plexiglass tube, with clear plexiglass end-plates and symmetrical end-plate ports for oxygen and i.v. catheters. The device limits cat movement without stressing sick patients. We have run over 100 cats through the CT using the device and only 2 cats were removed because of behavior problems. Both these cats were farm cats and otherwise healthy. No clinically sick cat has ever demonstrated stress being in the device.

Clinical experimental protocol

We began the project looking at "normal" cats in the device. In this phase we were designing CT protocols (pitch, slice thickness, kVp, mAs) and recruited faculty's and student's own personal pets. We quickly determined that many (up to 50%) of these "normal" cats were quite abnormal. The most common abnormality was bronchial wall thickening consistent with a diagnosis of subclinical asthma. Maybe the term "clinically normal" would better describe these cats. Many of these cats looked normal radiographically.

We then began a clinical project determining the utility of CT imaging with the new device in emergency patients with clinical signs of respiratory disease; increased respiratory rate, noise or effort, cough, or open-mouthed breathing. This project included disease throughout the respiratory tract including nasal, pharyngeal, laryngeal, tracheal and intra-thoracic locations.

The goal was to compare to CT imaging with radiographs and determine:

        a. if the device was clinically useful (provide oxygen and venous therapy effectively)

        b. where CT fit into the overall clinical work-up

        c. the safety of this form of imaging

        d. the speed of imaging

        e. the relative sensitivity and specificity for certain diseases

Clinical experience

CT was performed earlier in the work-up than radiographs. This is because the CT was deemed less stressful that radiography. CT was performed with supplemental oxygen, as necessary, and without any handling of the patient. Radiographs are often quite stressful to patients during the handling and positioning process.

No complications were experienced with the CT imaging. No cat has worsening of the clinical signs, needed to be removed from the device or developed any complications. The CT was performed in 6-12 seconds, on average. A small number of cases had repeated CT studies because of patient motion or because of the need for post-i.v. contrast CT imaging.

CT was superior to radiographs for the diagnosis of most upper airway, pulmonary, bronchial, pleural, mediastinal and body wall diseases. The diagnosis of heart size, difficult radiogrphically, was equally difficult on CT imaging, unless augmented with i.v. contrast enhancement. With contrast enhancement, assessment of the left atrium luminal diameter and left ventricular wall thickness could be estimated. Statistical evaluation of the results comparing radiographs to CT is currently underway.

Clinical examples

Cases will be discussed comparing the radiographic appearance with CT imaging. Radiographs will be evaluated first and discussed followed by CT image evaluation.