Image resolution of the Lunar Expert-XL

The Lunar Expert-XL is an example of the latest generation of fan beam dens itometers, with the X-ray source and detector array mounted on a C-arm to e nable supine lateral imaging. Image resolution for anteroposterior (AP) spi ne, femur, hand, forearm and lateral morphometry on the Expert-XL were asse ssed in vitro with the 07-541 Nuclear Associates line pair test pattern. Ea ch scan type was investigated at all available tube currents and scan speed s, and at the maximum, minimum and default bed heights. The effect of soft tissue thickness on resolution was investigated by using varying amounts of Perspex attenuator. The in vitro median lateral (x-axis) resolutions at th e default bed height for the default scan types were 0.9 line pairs (lps)/m m for the 5 mA fast AP spine and femur scans, and 1.0 lps/mm for 1 mA fast hand, forearm and 5 mA fast morphometry scans. This equates to a resolution of about 1 mm. The best resolution achieved was 1.2 lps/mm (0.83 mm), obta inable on all scan modes with the bed at maximum elevation, but only consis tently with the forearm mode. Lower tube current did not affect resolution but did change the range of soft tissue thickness over which an image could be resolved. Turbo scan modes greatly reduced longitudinal (y-axis) resolu tion but had little effect on lateral resolution. This study demonstrates t he importance of including an assessment of resolution when validating new equipment, especially if morphometric investigations are to be conducted.