Diagnostic Task
Detect collections of blood
Detect brain edema or ischemia
Identify shift in the normal locations of the brain structures including in the cephalad or caudal directions Evaluate the location of shunt hardware and the size of the ventricles Evaluate the size of the sulci and relative changes in symmetry
Detect abnormal collections
Detect calcifications in the brain and related structures
Evaluate for fractures or other osseous abnormalities of the calvarium (skull) Detect any intracranial air Detect abnormal densities
Key Elements
Scan may be performed axially/sequentially. It also may be performed helically in scanners with this capability (see below for discussion of pros and cons of axial versus helical) Contrast enhancement (if indicated by radiologist) Patient positioning is very important (see below) Radiation dose management is very important (see below)
Patient Positioning
Patient should be placed supine, head first into the gantry, with the head in the head-holder whenever possible.
Table height should be set such that the external auditory meatus (EAM) is at the center of the gantry.
To reduce or avoid ocular lens exposure, the scan angle should be parallel to a line created by the supraorbital ridge and the inner table of the posterior margin of the foramen magnum. This may be accomplished by either tilting the patient’s chin toward the chest (“tucked” position) or tilting the gantry. While there may be some situations where this is not possible due to scanner or patient positioning limitations, or contraindications to tilting of the head, it is considered good practice to try to perform one or both of these maneuvers whenever possible. Some newer scanners may allow helical acquisitions to be performed while the gantry is tilted.
Immobilization strategies should be used to minimize patient motion – this is essential to acquiring quality images
If lead shielding is used it must be positioned well away from the scan range.
Bismuth shields are easy to use and have been shown to reduce dose to anterior organs in CT scanning. However, there are several disadvantages associated with the use of bismuth shields, especially when used with automatic exposure control or tube current modulation.