10 - CT Scan Flashcards
CT scan description
- CT scan uses thinly collimated X-ray beams arranged circumferentially around the body
- Computer analysis of beams measured and a picture created
- Rapid acquisition and reconstruction of data
NOTE: I might put a CT scan on the exam and you will need to identify a structure
What is the benefit of newer CT machines?
Newer machines offer decreased radiation doses due to decreased acquisition time
The study time is decreased and you expose the patient to much less radiation
What is included in the CT scan equipment?
- CT scanning gantry
- Movable patient table
- X-ray tube within gantry that rotates
- X-ray generator
- Stationary detectors
- Data processing system
- Foot boards, wedges and tape (allow us to keep the patient in the position we want them in and keep them still)
May have to sedate the patient and give them pain meds so they can remain still during a CT scan for accuracy
Schematic of a CT machine
- Lower extremity is inside the tube (gantry) of the CT machine
- There is a rotating x-ray tube that has an anode and cathode to produce the radiation
- The radiation is fired toward the target
- On the other side of the target you have stationary detectors which are like the x-ray film
- If the radiation passes by the foot and does not interact with any soft tissues, it will expose the film and create black
- If the radiation passes through soft tissue, it will show up as gray
- If the radiation passes through bone, it will show up as white
Over exposed film looks black, under expose film looks white (not enough radiation got to the film)
Why is patient history important in CT scans?
- Diagnosis
- Presence of internal fixation
- Dressing or cast
- Sedation
Planes of CT scan
Axial (Transverse) and Coronal (Frontal) images taken
The computer can then create a 3 dimensional image or show the sagital plane
Positioning for CT scan
- Scout view (the “legend” on a map - tells you what you’re looking at)
- Comfortable
- Provide proper visualization
- Reproducible (for comparison)
Axial CT scan positioning
- Obtained with the feet perpendicular to the gantry table
- Knees are extended with feet together
- Foot board and tape used to maintain position
- Tube can be angled if proper positioning can not be achieved due to a cast or pain
Coronal CT scan positioning
- Knees are flexed with feet flat against the gantry table
- Foot boards and tape used to maintain position
- Tube can be angled and wedges used if proper positioning can not be achieved due to a cast or pain
CT scan slide thinckness
- The thickness of a slice varies
Most common slice thickness is 5 - 10 mm - Thinner slices such as 1.0 mm and 3.0 mm slices can be obtained for increased detail
- Thinner slices need to be ordered for reformatted or three dimensional images
If you’re asking for thinner slices than the facility protocol, you are exposing the patient to much more radiation
Other than thickness, what does the CT tech need to know?
- CT tech also needs to know what area the slices should begin and end
- May need 20 slices for a rearfoot exam with 5 mm thickness, and 40 slices for 1.5 mm
Black arrow
“Positive black arrow” means that there is a positive test result that the radiologist has picked up
CT scans in slides
Look at the following slides for CT scans (TEST QUESTIONS)
21 through 39
Radiation exposure with CT scan
MUCH higher than with an x-ray
- Radiation dose ranges from 1 to 3 rads
- Average is 2.6 rads for a CT examination of approximately 20 slices
- Radiation exposure for five conventional radiographs is approximately 0.1 rads
- Scanning time is usually 3 - 5 seconds per slice with total examination lasting approximately 30 minutes
- Older machines used to take 20 seconds per slice
Clinical use of CT scan
** THIS IS IMPORTANT **
- Superior depiction of skeletal anatomy, especially articular anatomy such as STJ
- Acute skeletal trauma or traumatic arthritis
- Osseous coalitions and infections
- Planning conservative or surgical treatment
- Neoplasms? Although CT scans have good osseus affinity, MRI is favored over CT for bone tumors
