Basics of CT

CT uses x-rays to obtain images.

Much of the same principles that you learned about x-rays still apply.

• Ionizing radiation (x-ray photons) passes through patient.
• X-rays reach detector to form an image.

CT Scanner

• Unlike plain films, radiation source and multiple detectors rotate around the patient.
• Computer collects data as patient moves through the scanner.
• Density for each pixel can be calculated.

Axial Plane

• Data is typically obtained in the axial plane.
• Cross sectional "slice" through the patient.
• Viewed as if you are looking at the patient from the foot of the bed.

Sagittal plane

• Data already obtained in the axial plane can be reconstructed in the sagittal plane.
• Viewed as if you are looking at the patient from the side.

Coronal plane

• The coronal plane can also be reconstructed from the original axial data set.
• Slices viewed as if you are in front of the patient, looking at them standing upright.

3D imaging

• In addition to the 3 planes of imaging (axial, sagittal, and coronal), CT data can also be used to create 3D images.
• Helpful for visualization of vasculature and bony structures (complex fractures and surgical planning).

(Need pictures of 3D bone and 3D vessels*)

Density

• Dense material absorb x-rays and prevent them from reaching the detector.
• Air – does not absorb many x-rays – most reach detector – appears dark.
• Bone – absorbs more x-rays – few reach detector – appears light on image.
• In CT, we are able to measure the amount of x-rays that are transmitted (pass through) to the detector to get a value corresponding to the density of the material.
• Tissue density is measured in Hounsfield units (HU).