Helical and conventional CT in the imaging of metallic foreign bodies in the orbit.

Abstract

To compare helical CT to conventional CT imaging in the assessment of orbital metallic foreign bodies with regard to image quality, scanning time, and radiation exposure. Twenty-four standardized metallic foreign bodies, placed into the orbit (anterior, epibulbar, posterior) of cadaver heads were studied. Helical CT scanning in the axial plane with multiplanar reconstruction of coronal and sagittal images was performed as well as conventional CT imaging with direct scanning in the axial and the coronal planes (Tomoscan SR 7000, Philips, The Netherlands). Two masked observers consensually graded the studies using 5 predetermined criteria. Radiation dose delivered to the lens and scanning time were measured for the helical and the conventional CT imaging workup. Helical CT imaging scored statistically significantly better with regard to overall accuracy of foreign body localization and presence of beam-hardening streak artifacts from dental fillings. Conventional CT scored significantly better with regard to stair-step artifacts. The radiation dose delivered to the lens was 35.4 mGy for helical CT imaging and 73.9 mGy for conventional CT workup (axial and coronal scanning). Total scanning time was 18 seconds for helical CT axial scanning and 104 seconds for conventional CT axial and coronal scanning. Helical CT is superior to conventional CT imaging, because it can provide adequate information about orbital metallic foreign bodies with a single acquisition, thus reducing both the number of examinations and the radiation exposure for the patient.