Image
above: Shimadzu
Helical CT system. Note that the torus is relatively large (particularly
compared to a MRI system) and the adjustable height patient platform.
The rotating X-ray tube is contained within the machine.
Background
• Process of creating a computer reconstructed, cross-sectional
plane of any body part
• First used in 1971 in London
Principles
• Patient lies supine on CT table with knees slightly flexed
and feet flat on table
• Must lie still
• An x-ray tube rotates about patient to provide the image
• Radiation exiting the patient is detected and fed back into
the computer as raw data
• Computer calculates data according to a preselected algorithm,
an axial (transverse) image is formed
• Computer creates a CT number, based on the average intensity
of the radiation
Each image (slice) is displayed on a cathode ray tube
Categorization
• Scanners categorized by generation
• Refers to technological advancement
• 4 recognised generations
• Slip ring technology - xray system that continuously rotates
about patient
• Decreased scanning time to 5-10 mins
• 4th generation CT scanner : rotate-only
Comparisons
with other imaging techniques
• Demonstrates bony structures better than MRI
• Patients with metal in body can use CT
• Can scan patients quicker than MRI
• Less costly than MRI
Uses
• Excellent bone detail
• Difficult to image #'s - stress #'s, calc #'s
• Bone cysts and tumors
• Osteochondritis dessicans
• Tarsal Coalition
CT
- 3D images
• Computer can reconstruct a 3D image by combining all axial
images of a scan
• Useful in diagnosis of hindfoot conditions
• Standard xray views unable to show up the complexity of the
soft tissue and joint structure
Reading
the image
• Black, white and shades of grey
• Transverse and coronal sections
• Can alter slice thickness - usually b/w 2-5mm. Smaller the
slices, higher the radiation exposure however
Examples of CT image:
