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Computed tomography (Proceedings)

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Nov 01, 2010

The use of computed tomography (CT), though not readily available at every institution, is becoming a more widespread modality for use in the small animal patient. Computed tomography affords a rapid evaluation of skeletal images with a small slice thickness that can be as small as 0.625. Computed tomography in chondrodystrophic breeds of dogs is being performed instead of myelography due to the rapid acquisition and ability to see mineralized disc material. Due to the increased availability of multislice CT scanners, sedated CT scans are becoming possible due to the fast acquisition time. The purpose of this proceeding is to describe the physics and benefits of CT imaging in the small patient and the new uses that this modality provides.

Computed tomography is a method of image acquisition similar to radiography. An x-ray tube is used to generate photons that pass through a patient and is read by a detector (similar to digital radiography). Where CT differs is the fact that the x-ray tube spins around the object being imaged to provide a cross-section of each section of the object without superimposition of structures as occurs with conventional radiographs. This elimination of superimposition allows for enhanced ability to detect fracture fragments, improves the ability to localize lesions (such as dental disease and sinus lesions) and increases the ability to see smaller lesions, when compared to conventional radiography.

Computed tomographic technology is generally talked about in terms of what "generation" a scanner is considered. For example, a third generation scanner (also called a rotate-rotate scanner) has a series of detectors that spins with the x-ray tube. A fourth generation scanner (also called a rotate-stationary scanner) has a spinning x-ray tube and a complete ring of transducers that do not move with the x-ray tube. A sixth generation scanner is the most widely available today and this is also called a helical scanner. The difference is that a fourth generation scanner generally has a large number of detectors, which means better resolution, when compared to the third generation scanner. A helical scanner allows for continuous acquisition of an image while the table moves to decrease the scan time required.

The way an image is generated in a normal acquisition is that the x-ray tube completely circles the desired area and an image is generated. This provides the best possible resolution, but when three-dimensional reconstruction is performed, the image can look very irregular. This is because, like slices of bread, each slice will not perfectly align with the image cranial and caudal making a stepped appearance. During a helical acquisition, the table continues to move through the gantry allowing only 66% of a complete revolution around the object. The remaining 34% of the image is then extrapolated from the slice in front of the image and behind the image. Though this means that the image is not completely accurate, but it provides for a smoother transition between slices, smaller slice thickness and a more rapid acquisition.

Due to the use of x-ray technology, the use and interpretation of CT is more intuitive then other modalities like ultrasound or magnetic resonance imaging. Computed tomography works on the basis of attenuation of the x-ray beam. Due to this difference in attenuation, it is possible to see acute hemorrhage, mineralization, small fragments of bone secondary to a fragmented medial coronoid process, as well as the difference between fluid and soft tissue when contrast medium is given.

Computed tomography's main benefits are rapid acquisition and lack of superimposition. The average CT study can take approximately 2-3 minutes per acquisition. Multislice CT scanners are similar to single slice, except that the multislice scanners have multiple rows of detectors allowing more than one slice to be acquired at a time. Multislice scanners range from 2 slice to 256, or infinite, slice. The more detectors that are present, the faster the acquisition and the smaller the possible slice thickness. However, just like with radiographs, a small slice thickness means more energy is required, which would adversely increase the patient's radiation dose. A complete head CT of a dog using a multislice CT scanner generally will take 20 seconds.

Due to the rapid acquisition and the amount of image detail, CT is considered the first imaging modality to evaluate the nasal cavity, the elbows and the thorax for pulmonary metastatic disease. Computed tomography is also ideal for trauma patients or patients that are paralyzed and are not stable enough for longer procedures like MRI. With the ability to reconstruct images in multiple planes as well as generating three-dimensional images, CT can provide rapid guidance for surgeons as well as being used by oncology for radiation therapy planning.