It must be possible to rotate the object to be studied without any part of it escaping the X-ray cone (see below under CT Realisation).   This enables the choice of detector size, as the CT-Mini uses normally a 4 cm detector, so the x, y dimensions of the object cannot normally exceed 4 cms.  

If they do, it will be better to choose the CT-Compact, with a 12 cm detector.   For either of these systems, the small size of the system means that X-rays produced from even low voltage tubes (90kV for the CT-Mini, 130kV for the CT-Compact) will in many cases be sufficient to adequately penetrate the object, producing a good picture.  

Use of the CT-Alpha allows a higher-voltage tube, but the greater distances mean that higher voltage is required to penetrate objects which could have been X-rayed in the smaller systems, so there may not be much advantage.   In addition, the larger detector used in this system has a larger pixel size, so more magnification is needed to achieve the same resolution.

Summarising, choose the smallest system in which it is possible to completely rotate the object in the cone beam.