Computed Tomography (CT)

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

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

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

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

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

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

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

Density of image Measured in Hounsfield units (HU) Range from -1000 to +3000 Relates to amount attenuation -1,000 HU air 0 HU water 3,000 HU bone Which HU is radiopaque/radiolucent

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

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)

Image interpretation Can be easier to interpret than radiographs

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

Practicalities Monitoring conducted from outside room

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

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

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

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

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

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

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

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