COMPUTED TOMOGRAPHY (CT)

CT uses x-rays to produce an image. The tomographic acquisition means that superimposition of structures is eradicated. 

The eradication of superimposition is the biggest benefit of CT. Soft tissues structures can be distinguished slightly more on CT than on radiography because there is reduced superimposition, however limitations still exist as the physics of the x-ray beam remains the same. The use of IV iodinated contrast medium helps optimize contrast between soft tissues and comparison between pre and post contrast images helps determine changes within soft tissue structures. 



The patient lies within the CT gantry and data is acquired as the tube and image detector rotate around the patient. Using back projection and Fourier transformation, the data acquired undergoes mathematical manipulation to result in the end image displayed. Different algorithms or kernels are used to bias the images towards the type of information required. This means that appropriate reconstruction algorithm selection is mandatory pre processing for a successful study and to prevent inaccurate assessment. A high frequency reconstruction algorithms is used for pulmonary issue and bone evaluation while a medium reconstruction algorithms is used for soft tissues. In addition to appropriate algorithm selection, images undergo post processing manipulation on the end workstation when the operator adjusts window width (WW) and window level (WL). This optimizes the images but relies on the correct WW and WL being applied to the correct reconstruction. 



CT has revolutionized evalaution of, in particular, the skull and thorax in marine mammal medicine. Some facilities now have their own CT scanners on site, however its more usual for animals to be transported to a local veterinary or human facility scanner. Weight and size limitations exist and discussion with facilities prior to scan day are needed. Typical gantry dimensions (width and height) are 60-70cm and typical table weight limitations are around 300lb. Tables able to hold greater weight do exist but are most frequently located in veterinary schools. 



CT scan time is relatively short however anesthesia is needed for live animal imaging (with the potential exception of cetaceans) to prevent motion during and between scans.