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.
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.