Clinical Indications for CT
1. Nasal/sinus disease- helps differentiate neoplasia from rhinitis and guides biopsy.
2. Musculoskeletal soft tissue tumors- accurately shows the tumor margins for surgical and radiation planning.
3. Middle/inner ear evaluation- determines the need for bulla osteotomy.
4. Skull trauma- increased sensitivity for fracture identification and brain hemorrhage.
5. Elbows- allows diagnosis of MCP fragments.
6. Evaluate for lumbosacral disc herniation/ degeneration
7. Thoracic met checks- more sensitive than radiographs.
8. Evaluate mediastinal and lung masses- enhances surgical planning by evaluating the extent of vascular invasion and screens for hilar metastasis.
9. Evaluate for pituitary macroadenomas- present in ~10% of pituitary dependent hyperadrenocorticism cases.
10. Brain scans- allows visualization of most brain tumors, but much less sensitive than MRI for tumors, inflammatory, and vascular conditions.
11. Ureter Evaluation- diagnosis of ectopic ureters and ureteral stones.
Computed Tomography Applications in Veterinary Medicine
Craig Long, DVM, DACVR
Computed tomography (CT) was introduced into clinical practice in Britain in the early 1970s, and was developed initially as a brain scanner but other applications were quickly realized. The applications in veterinary medicine are continually being explored, and accessible CT imaging centers are emerging. In many cases, CT imaging can provide valuable information that cannot be obtained with ultrasound and radiography. It is important for veterinarians to know the indications for CT imaging so this modality can be incorporated into a case when financially feasible.
Advantages of computed tomography
Conventional radiographs depict a three dimensional object as a two dimensional image. Their main limitation is that overlying tissues are superimposed on the image. Computed tomography overcomes this problem by scanning thin slices of the body with a narrow x-ray beam which rotates around the body. Another limitation of the conventional radiograph is its inability to distinguish between two tissues with similar density, such as soft tissue and fluid. Computed tomography can differentiate between tissues of similar density because of the narrow x ray beam and the use of "windowing" (see below).
How it works
The information acquired by CT is stored on a computer as digital raw data and an image can be displayed on a video monitor or printed on to x ray film. The image is made up of a matrix of thousands of tiny squares or pixels (65000 pixels in a conventional image). Each pixel has a CT number (measured in Hounsfield units) attributed to it. The CT number is a measure of how much of the initial x-ray beam is absorbed by the tissues at each point in the body. This varies according to the density of the tissues. The denser the tissue is, the higher the CT number, ranging from -1000 HU (air) to 1000 HU (bone). Soft tissues average 0 CT units.
To image an area of the body in which many of the tissues have a similar density - for example, the mediastinum or abdomen - a narrow range of CT numbers is selected. These can be spread out over the available gray scale so that two tissues with only a little difference in density will be ascribed separate shades and can therefore be differentiated. For example, a window width of 500 is often used for imaging the mediastinum, with a window level of 39. The level refers to the CT number at the center of the selected window. Thus, in this case, all pixels within the range - 211 to 289 will be displayed. Most of the lungs (largely air) will have CT numbers below - 211 and will therefore appear completely black on the final image (fig 1a).
If you want to see all the lungs you need a much wider window and higher level (fig 1b). The value of windowing is that the raw data from a single scan can be displayed in different ways to give useful images of a wide range of tissues.
Indications for computed tomography
Nasal and sinus
CT can usually discriminate between most classes of nasal disease. Bacterial rhinitis will generally have diffuse exudate between turbinates and a small amount of sinus fluid. Fungal rhinitis will have the same findings as rhinitis, but in addition, regions of turbinate lysis and sloughing will be seen. Neoplasia will usually present as a focal, destructive mass. For all forms of nasal disease, CT provides more information than radiography for prognosis and treatment planning, and it also guides biopsy procedures. Because of its greater bone imaging capabilities, CT is preferred over MRI for nasal disease.
In general, CT is very useful to provide the following information about masses:
1. What is the origin of the mass?
2. Where are the margins of the mass?
3. Is the mass invading important structures?
4. Is underlying bone destruction present?
5. Is surgical resection possible?
If these questions are answered and a biopsy is obtained, an accurate prognosis can be given. Studies have shown that the gross margin of tumors often extends beyond palpable limits. By accurately mapping the margins of a mass, surgery and radiation therapy have a greater chance of success. MRI is also very accurate at depicting mass margins.
CT can depict the anatomy of the ear with great detail. The main indication for imaging the ear with CT is to document that there is middle or inner ear involvement and to determine which surgical procedure is indicated (ear canal ablation, bulla osteotomy, etc.). In cases of otitis media, fluid is visible in the osseous bulla. If chronic, the osseous bulla becomes thick and sclerotic. Rarely, tumors of the ear are encountered and are often differentiated from otitis by destruction of the bulla.
CT is relatively insensitive at detecting soft tissue injuries. Ultrasound and MRI have much greater soft tissue imaging capabilities and are preferred. CT is useful for confirming medial coronoid process fractures in the elbow. These fractures are very difficult to visualize radiographically, but are easily detected with CT.
For imaging of the head, CT has great advantage over radiographs. Small fractures can be difficult to see on radiographs, but are easy to visualize with CT. This is especially true for fractures involving the TM joints or calvarium. In the first 24 hours after skull trauma, CT can detect subarachnoid and brain hemorrhage. After this time, MRI is more useful for this purpose.
CT excretory urography is useful for confirming and localizing the presence of ureteroliths or other causes of ureteral obstruction. It can also be utilized to evaluate for ectopic ureters and is preferred over standard radiographic techniques at this time, though studies need to be done to determine if it has greater accuracy.
CT of the chest has been revolutionized by two new developments in scanning. These are the ability to scan quickly with the new spiral scanners and the ability to acquire very thin slices (high resolution CT). With spiral CT scanners, the entire chest can be scanned during a single breath. This also means that small intrapulmonary abnormalities such as a pulmonary metastasis will not be missed because of variations in inspiratory effort.
Contrast enhanced CT of the chest is usually used for staging lung cancer and for assessing masses in the mediastinum. High resolution CT is used to image the lung parenchyma. The thin slices (1.5 mm) improve the resolution, making it possible to diagnose and assess the extent of diseases. Pulmonary diseases can be characterized with greater clarity than radiographs, but at this time the added information does not add great value to prioritizing differentials or changing the diagnostic plan.
Ultrasound is the preferred modality to assess the abdomen in animals. As a diagnostic tool, CT provides no advantage for the diagnosis of most conditions (there are a few exceptions). The pelvis inlet is one place in the abdomen that is difficult to visualize with ultrasound, so CT is useful to stage tumors in this area.
In general, MRI is the preferred modality to assess problems of the spinal chord. With CT imaging, contrast in the subarchnoid space (such as with myelography) is necessary to outline the spinal chord and visualize lesions around the chord. The exception to this generalization is with the lumbosacral region. A large amount of epidural fat around the nerve roots in this region facilitates visualization of lesions at this site without contrast. Tumors involving the vertebral bodies can also be well characterized with CT.
As with the spine, MRI is preferred over CT for detecting brain diseases. MR has much greater contrast resolution for soft tissues and has higher sensitivity for detecting smaller neoplastic, inflammatory, and vascular lesions. The tradeoff is a higher expense and longer image acquisition times (~30-40 minutes for MRI vs. 5 minutes for CT). With contrast enhancement, CT can detect most lesions that are larger than 1 in size. CT is very sensitive at detecting pituitary macroadenomas since the pituitary gland does not have a bloodbrain barrier and will readily contrast enhance.