Department of Veterinary Clinical Sciences, Purdue University, West Lafayette, IN, USA
Computed tomography is a diagnostic imaging modality that is widely used because it can provide axial sectional or slice-oriented imaging of the patient. This helps to detect abnormalities in areas where there is a lot of superimposition on plain radiography such as the nasal turbinate, skull, and spine. Two additional advantages of performing a CT study are its capability of performing multi-planar and three-dimensional image reconstruction. This helps in precise determination of anatomical location of abnormalities detected. This is especially helpful in computed tomography angiography and the evaluation of complex fractures.
A CT unit is comprised of a gantry, couch (patient table), hardware equipment and an operator console. The gantry houses an x-ray tube, collimators and the x-ray photon detectors. The x-ray tube will rotate around the patient table while the incident photon from the x-ray tube will be recorded by the detectors. The x-ray photon will then be transformed into an electrical signal and then converted into a digital image. The patient table normally will move according with the movement of the gantry. For large animals, a special custom-designated table is needed.
Once the raw data is acquired, the images can be displayed using different reconstruction algorithms. Different window settings, defined as window width and level, can be used to view the study. The window width (W) determines the contrast, and the window level (L) determines the grey scale of the images. Usually the CT images are displayed using several different window and level settings for each study.
Computed tomography measures and computes the spatial distribution of the linear attenuation coefficient in each pixel and is displayed relative to the attenuation of water. These values are called CT numbers or Hounsfield units (HU). The normalized HU ranges from about -1000 to +3000. Typically, the HU of water is 0, air is -1000, lung is -845, fat is -100, brain is +30 to +40, muscle is +50 to +70, and bone and mineral are +100 to +1000.
One of the most common applications of CT is the investigation of nasal disease. This is normally performed when there are chronic clinical signs and a suspicion of nasal neoplasia. Computed tomography will show detailed anatomical changes such as bone and turbinate lysis, presence of soft tissue attenuating material and the pattern of contrast enhancement. Computed tomography will often be able to differentiate inflammatory rhinitis, fungal rhinitis and nasal neoplasia due to their distinctive features. Two important aspects to evaluate are ruling out invasion of the brain (especially by the nasal neoplasia) and the presence of tooth root abscess. Computed tomography will also be able to assess the mandibular and retropharyngeal lymph nodes to rule out metastasis of the nasal neoplasia.
Two major objectives of the CT evaluation of the lungs are to screen the patients for metastatic disease1 and to define the location of an abnormality seen on the radiography. Computed tomography is more sensitive than radiography for the detection of small pulmonary nodules. In patients with a continuous pneumothorax, CT examination is performed to locate a possible lung bulla that leads to this condition.
In recent years, CT examination of intervertebral disc disease (IVDD) has been used widely to replace the use of myelography.2 It has also become the first choice of imaging modalities to investigate IVDD when the cost and availability of MRI prohibit the use of MRI. When hyperattenuating mineralized disc material (readily detectable on CT examination) is found in conjunction with the history and clinical signs, a diagnosis of IVDD is easily made. Normally radiography of the spine is performed before the CT examination to rule out any obvious changes such as lysis of the bones.
For the abdominal cavity, a CT examination is normally performed after an abdominal ultrasound to investigate the margins and resectability of an abdominal mass or to confirm suspected ectopic ureters. Investigation of the invasiveness of an adrenal tumor to the surrounding blood vessels is also an important indicator for an abdominal CT.3 The investigation of portosystemic shunts using dynamic contrast study has become more common with the availability of a multislice CT machine.4
In conclusion, CT can be used to investigate various organ systems and diseases. It is mostly performed after radiography and ultrasonography due to its cost and the need of general anesthesia and/or deep sedation.
References
1. Armburst LJ, Biller DS, Bamford A, Chun R, Garrett LD, Sanderson MW. Comparison of three-view thoracic radiography and computed tomography for detection of pulmonary nodules in dogs with neoplasia. J Am Vet Med Assoc. 2012;240:1088–1094.
2. Bibevski JD, Baye RM, Henrickson TD, Axlund TW. A prospective evaluation of CT in acutely paraparetic chondrodystrophic dogs. J Am Anim Hosp Assoc. 2013;49:363–369.
3. Schultz RM, Wisner ER, Johnson EG, MacLeod JS. Contrast enhanced computed tomography as a preoperative indicator of vascular invasion from adrenal masses in dogs. Vet Radiol Ultrasound. 2009;50:625–629.
4. Nelson NC, Nelson LL. Anatomy of extrahepatic portosystemic shunts in dogs as determined by computed tomography angiography. Vet Radiol Ultrasound. 2011;52:498–506.