CT head (technique)

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CT head technique describes how a CT head is performed.

Technique

The technique for performing a CT of the head depends on the scanner available and fall into two broad camps:

  • step-and-shoot (sequential)
  • volumetric acquisition (helical) 

Helical is the most common technique.

Step-and-shoot

Step-and-shoot scanning was the first described technique but as largely been superseded in more modern scanners in favor of helical scanning and volumetric datasets (see below)

In step-and-shoot scanning, the CT tube/detector perform a complete resolution around a stationary patient (shoot) generating a single axial image. The table/gantry then advances a specific distance (step) and the process is repeated to acquire the next axial image.  Traditionally, these were 10 mm slices through the cerebrum and 5 mm slices through the base-of-skull and posterior fossa.

Initially, scanners were fixed such that the scanning plane was at right angle to the floor. The initial axial plane described for CT brain was the orbitomeatal line. Relatively soon, however, the ability to tilt the scan plane became possible and the standard plane was then shifted to one parallel to the orbital roof. This had the advantage of avoiding the lens (at least in some patients) and reduce the artefact from dental fillings which would be projected below the posterior fossa.

Volumetric acquisition

More recently, CT scanners have become able to obtain a complete volume of scan data by continuously scanning as the gantry is moved. This generates a helical scan path through the patient (thus helical scanning). The advantage of this technique is that it generates a complete 3D volume of data which in turn allows the creation of multi-planar reconstruction with thick or thin slices using different algorithms.

The axial plane can then be chosen to match any desired plane, regardless of the position the patient's head was in when scanned. Increasingly, the standard axial plane is being set to match that of MRI scans often parallel to the tuberculum sellae-occipital protuberance line (which is close to parallel to the AC-PC line). Coronal and sagittal reconstructions are then usually at right angles to this.

The ability to create MPRs quickly and easily does result in a significant increase in the number of images to be reviewed and the amount of space required to save them to disc.

It is not inconceivable to see a CT brain resulting in 3-plane 4 mm soft and 3-plane 1 mm bone reconstructions being sent to PACS with a 3D reformat and even the 0.6 mm overlapping data (to allow reimport into the volume rendering system for future use). We have moved from the traditional 30 images for a CT brain to a mammoth set of data with 1000+ images.