Computed Tomography (CT)
Angell Boston currently has a 16-slice Toshiba CT unit, which was installed in 2012. This has allowed us to perform all of the studies described below, and other procedures including CT-guided aspirates, biopsies, and injections.
Computed tomographic images are obtained using X-rays, similar to normal radiographs, but the X-rays are generated and detected in a ring-like structure (the gantry) that surrounds the patient. The detected information is displayed as an image by assigning each pixel of data to a shade of gray.1 The human eye can only resolve approximately 90 shades of gray, so we manipulate that, or “window and level,” each image to allow optimal evaluation of a given anatomic structure.
Multidetector CT: Increased speed
MDCT derives its name from the fact that there are multiple detectors available to receive the X-rays that have passed through the patient, giving us the ability to simultaneously obtain multiple cross-sectional images, compared with a single-detector unit. This speeds up the image acquisition process by a factor of the number of slices the machine can acquire.
Images easy to reconstruct in multiple planes
Another benefit, and an inherent property of MDCT, is that the image information is extremely easy to reconstruct in multiple planes, including 3-D reconstructions. Just as an orthogonal view of a radiograph is helpful in determining the location of a lesion within a patient, evaluation of a lesion in a different plane can be helpful in determining the extent of a lesion. Traditionally, CT images are evaluated in a transverse plane, as if the patient were sliced lengthwise like a loaf of bread. For example, reconstructing a transverse image of a large nasal tumor into a dorsal plane could help us more clearly evaluate the cribriform plate for destruction and tumor invasion into the patient’s brain. This additional information allows us to be more specific in our recommendations for you and your client.
3-D recon allows the surgeons to better plan their approach for major orthopedic procedures such as comminuted pelvic fractures, as the image can be flipped and rotated in any direction. CT evaluation of elbow dysplasia is becoming more commonplace and can help the surgeons to determine the extent of chondromalacia suspected below the surface of a fissured or fragmented medial coronoid.2 Given the rapid acquisition time, some orthopedic patients, if they are good anesthetic candidates, are able to be scanned under heavy sedation alone, and not requiring general anesthesia.
Computed tomographic angiography (CTA)
An iodinated, non-ionic contrast agent (ie, ioversol) is administered intravenously and can be employed to identify anomalous vessels or areas of increased vascularity (such as certain tumors). Peak arterial and venous contrast enhancement times can be determined by using a bolus-tracking system built into the CT machine. This can be useful in certain disease processes; for example, pancreatic insulinomas, which are typically very small and difficult to evaluate on ultrasound, tend to have intense arterial vascular uptake but rapid washout, and would therefore not likely be seen in a typical post-contrast sequence.3 Additionally, vascular anomalies such as persistent right aortic arches4; and portosystemic shunts5, 6 can be clearly highlighted using CTA, which can assist in surgical planning.
Recent research shows a significant difference in lesion detection using abdominal CT in dogs greater than 25kg compared with abdominal ultrasound, suggesting that abdominal CT is a good screening tool.7 The urinary system is also more readily observable with reconstructions of post-contrast images. Because the contrast material we use, iohexol, is filtered by the kidneys, the post-contrast images show the margins of the kidneys, the ureters and the contrast pooling in the urinary bladder.
Thoracic CT is ideal for evaluating a variety of disease processes. There is reported improved visibility of smaller pulmonary metastatic nodules using CT, compared with both digital and plain film radiographs, as well as improved diagnostic confidence when evaluating for thoracic metastatic neoplasia8 and CTA for evaluation of pulmonary thromboemboli (PTE), as can be seen with heartworm disease.9 However, these patients will require general anesthesia, and careful patient selection will be important should you choose to refer for thoracic CT.
1. D’Anjou, MA. Principles of Computed Tomography and Magnetic Resonance Imaging. In Thrall, DE, editor: Textbook of Veterinary Diagnostic Radiology, ed 6, St. Louis, 2012, Elsevier Saunders, p57.
2. Reichle, JK et al. Computed tomographic findings of dogs with cubital joint lameness. Vet Radiol Ultrasound 2000; 41(2):125–130.
3. Mai, W and AV Caceres. Dual-phase computed tomographic angiography in three dogs with pancreatic insulinoma. Vet Radiol Ultrasound 2008; 49(2):141–148.
4. Joly, H et al. Imaging diagnosis—CT angiography of a rare vascular ring anomaly in a dog. Vet Radiol Ultrasound 2008; 49(1):42–46.
5. Zwingenberger, AL et al. Helical computed tomographic angiography of canine portosystemic shunts. Vet Radiol Ultrasound 2005; 46(1):27–32.
6. Nelson NC and LL Nelson. Anatomy of extrahepatic portosystemic shunts in dogs as determined by computed tomography angiography. Vet Radiol Ultrasound 2011; 52(5):498–506.
7. Fields, EL et al. Comparison of abdominal computed tomography and abdominal ultrasound in sedated dogs. Vet Radiol Ultrasound 2012; 53(5):513–517.
8. Alexander K et al. A comparison of computed tomography, computed radiography, and film-screen radiography for the detection of pulmonary nodules. Vet Radiol Ultrasound 2012; 53(3):258–265.
9. Seiler GS et al. Computed tomographic changes associated with the prepatent and early patent phase of dirofilariasis in an experimentally infected dog. Vet Radiol Ultrasound 2010; 51(2):136–140.