Meetings » 1st CTUG Meeting » Principles of Multi-Slice CT Scanners

Principles of Multi-Slice CT Scanners

Maria Lewis

ImPACT, St.George's, Hospital, London


The last big impact of technology on CT scanning occurred in 1989 with the introduction of helical CT into clinical practice. This was followed in 1991, by the launch of the CT Twin, the first 3rd generation dual detector bank scanner which at the time failed to make a great impression on the CT market. In 1998, four of the major CT manufacturers built on the experiences with this early multi-slice model and launched scanners capable of imaging four slices simultaneously. It appears that these models will herald big changes in CT practice in the near future.

The four new multi-slice models feature different designs of detector banks, based either on an adaptive array or matrix array principle. They also have varying specifications in terms of minimum scan time, range of available slice widths and maximum length imaged in a single rotation.

The concept of helical pitch must be reconsidered for multi-slice systems as two different definitions of this quantity have been adopted by the manufacturers. One definition utilises the nominal imaged slice width, whereas the other is based on the irradiated dose profile.

To make most efficient use of the data collected by the multi-array detector banks, interpolation algorithms have been extended beyond the 3600 and 1800 linear interpolations used commonly in single detector array models. Some manufacturers also recommend the use of non-integer pitch values to further improve image quality.

The effect of different pitches on image quality and dose needs to be evaluated, as with multi-slice systems there is no longer a straightforward relationship between pitch and image quality. Choice of pitch and imaged slice width will be dependent on the clinical application.

In clinical use, multi-slice systems can be utilised to advantage in the scanning of large volumes in a short time. Alternatively, certain applications may benefit from scanning with narrow slice widths in the same time as with wide slices on conventional systems, and without encountering tube cooling delays. The faster rotation times available on these systems also open up possibilities in the field of cardiology. Finally, the capability of using narrow slice widths routinely results in improved image quality for applications involving 3-D reconstructions.

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