When and why would you use CT

Advantages

• CT provides superior bone imaging with excellent contrast resolution.
  Changes as small as 0.5% in contrast are easily detected with CT.
  On the contrary, radiographs require at least a 5% difference in x-ray attenuation before two adjacent tissues can be distinguished. Hence, radiographs are ten times worse than CT in regards to contrast.
• CT study acquisition time is shorter than other cross sectional imaging modalities. In some instances, CT studies are faster than conventional radiography. Examples include CT bulla or nasal studies vs. skull radiographs, and three views of the thorax vs. a CT thoracic study, evaluating for pulmonary metastases.
• The cost of CT is less than other cross sectional imaging modalities and in some instances, may be equal or less in cost than certain radiographic studies.
• CT is capable of 2D and 3D computer reconstruction in sagittal, dorsal and oblique planes allowing for evaluation of a lesion through multiple plans. This yields a more complete study and aids in defining the type, exact positioning and extent of the lesion. This information is paramount for surgical planning, and protocol development (chemotherapeutic, radiation therapy, etc.)

To a limited extent, CT is capable of differentiating between certain soft tissue attenuating disease processes such as soft tissue tumors from cysts.

Disadvantages

• CT images possess adequate spatial resolution (the ability to tell two adjacent structure apart). However, the resolution is much less than radiographs.
• Soft tissue resolution is inferior to MRI and superior to conventional radiography.

Image acquisition can only be performed in a single plane, usually axial. With smaller animals, multiple planes may be possible; however, this is time consuming and increases potential exposure rates to staff. The quality of the 2D and 3D reconstructions are limited to the quality of the scan (number of slices acquired, slice thickness, etc), and tend to be pixilated.

When should I use CT?

Axial Skeleton
Bulla studies. CT is the modality of choice for differentiating infectious, inflammatory and neoplastic processes involving the ear. It can clearly delineate between otitis externa, otitis media, and neoplasia. It defines the extent of bone involvement, i.e. reactive ostitis, osteolysis.

Nasal and Sinus Studies.
CT is superior to radiographs in defining destructive from non-destructive rhinitis, sinusitis and neoplasia, as well as the extent of involvement, and location. Also useful in the localization of foreign bodies and tooth root abscesses.

Brain (Benign and non-neoplastic)
Pituitary dependent hyperadenocorticism. CT is helpful in differentiating between macro and microadenomas. This information is important in determining treatment. Macroadenomas respond well to radiation therapy. Most microadenomas are addressed medically.

Skull
Hemorrhage. CT is the modality of choice for recent intracranial hemorrhage. There is a linear relationship between the amount of hemoglobin (Fe content) and x-ray beam attenuation. The hemoglobin content is highest within 24 hours of a bleed. Potential causes include trauma, DIC, neoplasia, anticoagulants (rodenticide toxicity), infectious (fungal granuloma formation with vascular erosion).

Trauma/Fractures.
CT is excellent at defining the type and extent of fractures, as well as the extent of adjacent soft tissue involvement. This is important in determining prognosis, management (medical vs. surgical), and surgical planning.

Appendicular Skeleton
Elbows Dysplasia. CT is superior to radiographs in defining lesions of the elbow. A CT scan is warranted in all young dogs with a history of persistent lameness localized to the elbow, especially if radiographs are normal. At most institutions, an elbow CT study is part of the routine protocol in all animals meeting this criteria. Lesions commonly diagnosed include fragmented medial coronoid process, ununitied anconeal process, OCD, DJD, joint incongruency and joint surface defects

Osteochondritis/Osteochondrosis Dissecans (OCD).
MRI is excellent in defining the location of the lesion, extent of bone and soft tissue involvement and, if present, the severity of degenerative joint disease, all of which are important in determining prognosis. Radiographs will help determine whether a CT or MRI study is warranted. General guidelines: CT is used with bone changes, MRI when they are absent.

Trauma.
CT is superior to radiographs in defining small, non-displaced fractures (i.e. fissure fractures, cortical stress fractures), especially in the distal limbs (carpus, tarsus). In certain instances of acute trauma, initial radiographs may be unrewarding and would require 7-10 days before the fracture was visualized. In this situation, because of it’s superior contrast resolution and cross sectional capabilities, a CT of the effected bone would be warranted. CT also provided useful information regarding soft tissue/muscle trauma and is considered a viable option in debilitated, geriatric and potential anesthetic risk patients because of the rapid scan time or when cost maybe an issue.

AbdomenPortosystemic Shunts.
MRI and contrast CT is a non-invasive, sensitive and specific means of identifying, localizing and determining the number of both intra and extrahepatic portosystemic shunts. MRI studies are actually faster than CT if limited to the abdomen. However, concurrent assessment of the brain is helpful, especially if the shunt is large. The extent of intracranial changes can influence the prognosis. Overall, both modalities can reduce surgical time while adding useful information.

Pancreatitis.
The use of contrast CT in evaluating for pancreatitis is very sensitive in defining the effected region, the viability of the effected region, and whether surgery is indicated. CT is sensitive in differentiating between necrotizing and non-necrotizing pancreatitis. CT and ultrasound combined are extremely sensitive in determining between surgical and non-surgical pancreatitis.

Ectopic Ureters.
Contrast CT is excellent for differentiating between unilateral vs. bilateral, intramural vs. extramural, and location of termination. This information is helpful in determining surgical approach; which in return reduces surgical time.

Mid abdominal masses.
Based on tissue kinetics and the vasculature supply of different lesions, contrast CT is becoming increasingly accurate at differentiating between neoplastic, infectious and inflammatory processes. CT is also helpful in defining location, extent of involvement and surgical vs. non-surgical masses. Contrast wash-in and wash-out rate, intensity, and persistence are helpful in differentiating between neoplasia, abscessation, cyst and granuloma

Thorax
Pulmonary Metastases. Comparing CT to conventional radiography, a recent human study revealed the following:

• 4x more powerful than routine chest radiographs
• Doubles the rate of early lung cancer detection
• In certain cancer types, CT improves the 5 year survival rate from 48 to 78%
• By the time pulmonary involvement is discovered on radiographs, so advanced, have 12-15% chance of survival
• CT is considered the gold standard when evaluating for pulmonary metastases.
• Other applications: Pulmonary thromboembolic disease (PTE), chylothorax, pulmonary contusions, fractures, bronchial or upper airway involvement, and chronic lower airway disease.

CT and MRI Availability
AIC cross-sectional imaging schedule is flexible and serve commonly available Monday through Friday evening. Our studies are performed at human diagnostic imaging facility utilizing state of the art equipment, which yields better image quality and shorter scan times. Shorter acquisition times mean less time spent under general anesthesia and therefore, less risk to patient