1. Computed Tomography (CT)

2. CT In clinical use in humans since 1973
Used to image most body parts, particularly the head, thorax and abdomen
Becoming common in veterinary referral practices

3. Image Formation A narrow, fan-shaped x-ray beam is emitted from a tube
Moves around the patient
The x-rays that pass through the patient are counted by a series of small electronic detectors
Signals from these detectors are passed to a computer
The data is reconstructed

4. Image Formation A CT scanner is a high quality piece of equipment
Scanner includes
high-output x-ray tube
detector array
patient bed
Operation of the scanner is controlled by computer
Images are viewed as they are acquired on a monitor

5. Image Formation
During a scan, the x-ray tube normally revolves

6. Image Formation
The size of the area scanned and the thickness of tissue represented by each image can be selected by the radiographer

7. CT vs Radiographs Similarities
Produced by the absorption of x-rays by different density tissues

8. CT vs Radiographs Differences CT image represents a thin sections
CT can detect tissue differences that are small

9. Density of image Measured in Hounsfield units (HU)
Range from to +3000
Relates to amount attenuation
-1,000 HU air
0 HU water
3,000 HU bone
Which HU is radiopaque/radiolucent

10. Image interpretation Image viewed on monitor
Can display what range needed based on area examined
Air or bone images wide range
Soft tissues narrow range
View multiple images at once
Distinguish between soft tissue and fluids based on CT numbers
Blood from transudate

11. Image Interpretation
The principles of interpretation are the same as conventional radiography
Number, position, size, shape; while opacity becomes tissue density
Hyperdense (white)
Isodense
Hypodense (black)

12. Image interpretation
Can be easier to interpret than radiographs

13. Practicalities Accurate patient positioning
Eg: rotated positioning can complicate the interpretation of a CT just as it does in radiography
Although the exposure time for a CT scan may be as little as 30 seconds, the patient must remain perfectly still
Patients are usually anesthetized

14. Practicalities
Monitoring conducted from outside room

15. practicalities Contrast medium may be used
Accumulates in vascular, hemorrhagic or edematous lesions
Aids in defining lesion margins
Non-ionic contrast IV injection

16. Applications
Allows a more detailed exam on a wider range of structures than radiographs/ultrasound
When used
Alternative to a multiple view radiographic study
Following normal results of radiography and ultrasonography
More information on an abnormality found on other imaging
Guide biopsy
Prior to surgery or radiation – extent of lesion

17. applications When to choose Based on individual patient’s needs
Local factors
Cost
Convenience
Availability
CT is more $ than ultrasound but less $ than MRI

18. Areas commonly studied with CT
Head
Well suited for head studies due to high contrast provided by bone and air containing structures

19. Areas commonly studied with CT
Head
CT may be indicated in
Trauma
Exopthalmos
Chronic nasal discharge

20. Areas commonly studied with CT
Thorax
Used for imaging all structures except the heart,
CT images of the lungs are useful, often revealing lesions not visible on survey rads, or enabling assessment of the extent of a lesion

21. Areas commonly studied with CT
Abdomen
Not as common due to ultrasonography convenience and lower cost
Good for exam of the pancreas, mesentery, and pelvic canal

22. Areas commonly studied with CT
Musculoskeletal structures
Minimally displaced or incomplete fractures easier to see
Useful in examining the canine elbow (fragmented coronoid process)
Spine in cases of suspected intervertebral disk prolapse, fracture or neoplasia