Cervical surface shape recovery using digital imaging colposcopy

A common application of digital imaging colposcopy in cervical examinations is the measurement of lesion dimensions and areas. Typically this is done by interactively marking the region of interest on a cervix image, calculating the corresponding pixel dimensions and then scaling to the colposcope optics. Until now no one has suggested a solution to the effects of the cervical surface slant on these measurements. Away from the cervical os the surface slant is large and lesion dimensions there will be underestimated, possibly leading to a misinterpretation of the lesion's progression. In this paper we discuss a non-invasive method for determining the surface geometry of the cervix using digital imaging colposcopy. The method is an application of shape-from-shading techniques used to determine the surface slant at all points in the cervix image. From the surface slant we can calculate area corrections to measurements made on the image. In our initial investigations we have applied this method to area measurements of circular regions drawn on spherical test targets. Our results indicate that we can obtain improvements in area measurement errors of factors between 3 and 6, resulting in relative errors of a few percent.