Quality Management

To rule out unnecessary influences such as motion artifacts (patient) and distortions due to system problems, there are some precautions:

• Proper positioning and fixation of the patient.
  This is especially important with children who do not understand what is being done to them, and, in some cases, require sedation.
• Regular maintenance of the CT scanner.
  A phantom of known dimensions is scanned. Aside from verifying density values, it is checked for correct measurements in the images (scanner) and along the Z axis (table movement); the CT system is adjusted where necessary.
• Regular maintenance of the modeling system.
  A test case of known dimensions is produced and measured; the system is adjusted according to the results.

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Are there still limiting factors? What can be done about those?

Patient positioning and fixation does not entirely exclude patient movement: a patient might twitch during the scan, resulting in minor change of position. If this occurres in a slice with not so important structures, there is no major problem associated with this. The resulting model might show an offset in the corresponding slice position. Otherwise, a scan needs to be repeated.

The choice of scan parameters might also prove to be a limiting factor. In case the patient has been scanned in our institution; there is no problem whatsoever. Occasionally we receive CT data from other institutions, and sometimes there might be problems.

E.g. a FOV (field of view) of 500mm along with a matrix of 256² pixels results in a pixel size of almost 2mm. Taking the influence of partial volume effect (e.g. at the skull base) into consideration, this may result in (worst case assumed) almost 4mm tolerance in contour definition.

Metal artefacts. These contours have to be re-defined manually

Metal artifacts are a major limiting factor. These almost always can be found in maxillofacial scans: inlays of teeth. With their density values overshadowing any bone, the contours in slices affected by metal artifacts have to be defined manually. This requires comparison to previous and following slice positions along with sufficient anatomical knowledge.

Partial volume effect. The right orbit (left side of the image) requires manual correction since due to the effect, some of the bone density values are below the threshold value.

Partial volume effect has always to be dealt with in any structure whose boundaries are not perpendicular to the slicing plane. Within these boundaries, each voxel represents the two neighbouring structures: the density values of e.g. the orbit in the image is an average of the bone and the surrounding soft tissue, and partly lie below the threshold value.
Without correction, the resulting model will have holes in the location of the values in question.

This hip joint has areas of density that are below the threshold value (green contour).

Even with the correct threshold value, in some cases parts of the bone show lower density values (e.g. due to osteoporosis), requiring manual correction.

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