CT Technique
How are CT Images Obtained?
Computed Tomograhy (CT) is based on similar principles to conventional X-Ray in that X-rays are passed through a patient and are detected on the opposite side by a film detector. Dense structures (bone) will attenuate or stop the X-Rays from passing through and less dense structures (air, fat) will allow the X-Rays to pass through. The difference between CT and X-Ray imaging is that in CT imaging the X-Ray tube rotates around the patient and this fan of information is absorbed by multiple detectors. Computers integrate the data and 2 dimensional images are created.
Reference 1
Picture of a CT Scanner
The patient lies on the table on their back with their head at the far end near the gantry and their feet at the near end. The table moves and images are acquired as the patient moves through the opening in the scanner (gantry). Images are acquired in the axial plane. Modern scanners acquire data continuously (Spiral or Helical CT). These axial images can then be reformatted in various additional planes.
How are Head CT's performed?
The technologist first obtains a "scout" radiograph, which is an X-Ray of the body region that he/she is scanning. The scout serves as a guide for the technologist which will let him/her know where to start and stop scanning. All CT images are acquired in the axial/transverse plane, like slicing a loaf of bread. If the slices are thin enough, different imaging planes may be reconstructed.
Different "windows" can be created to highlight specific structures. For example a standard head CT will include both brain windows and bone windows. Brain windows are useful to evaluate the parenchyma whereas bone windows are useful for evaluating fractures and the paranasal sinuses.
Scout Radiograph
"Brain" Windows
"Bone" Windows
