Determining size and characteristics of metal intraocular foreign bodies using helical CT scan

Purpose. To carry out a pilot study comparing the size of various types of metallic intraocular foreign bodies as imaged on computed tomographic (CT) scan to their actual size, and to determine the features of different types of metals on CT scan. Methods. Metallic foreign bodies of predetermined standardized sizes were prepared from iron, silver, copper, aluminium, and lead. The metallic pieces/shards were each inserted into the vitreous of a cow's eye and helical CT scan (MX twin, Picker) was performed to image the intraocular foreign bodies, using soft tissue and bone window settings. Slice section thickness was 1.3 mm, with intervals of 0.6 mm. Milliampere per second was 265 and pitch 0.7. Results. The size of all types of metals as measured on CT was larger than the actual size. Iron was enlarged by a factor of 2.29, silver 1.77, copper 1.26, and aluminum 1.17. All metals had features including central core, ring density, and artifacts which varied for each type of metal, giving each one a characteristic appearance. Conclusions. Helical CT scan could be useful in estimating the type and size of an intraocular metal foreign body made from iron, silver, copper, aluminium, or lead.