A New Image Calibration Technique for Colposcopic Images Wenjing Li * , Marcelo Soto-Thompson, Yizhi Xiong and Holger Lange STI ® Medical Systems, 733 Bishop Street, Honolulu, HI 96813, USA ABSTRACT Colposcopy is a primary diagnostic method used to detect cancer and precancerous lesions of the uterine cervix. During the examination, the metaplastic and abnormal tissues exhibit different degrees of whiteness (acetowhitening effect) after applying a 3%-5% acetic acid solution. Colposcopists evaluate the color and density of the acetowhite tissue to assess the severity of lesions for the purpose of diagnosis, telemedicine, and annotation. However, the color and illumination of the colposcopic images vary with the light sources, the instruments and camera settings, as well as the clinical environments. This makes assessment of the color information very challenging even for an expert. In terms of developing a Computer-Aided Diagnosis (CAD) system for colposcopy, these variations affect the performance of the feature extraction algorithm for the acetowhite color. Non-uniform illumination from the light source is also an obstacle for detecting acetowhite regions, lesion margins and anatomic features. Therefore, in digital colposcopy, it is critical to map the color appearance of the images taken with different colposcopes into one standard color space with normalized illumination. This paper presents a novel image calibration technique for colposcopic images. First, a specially designed calibration unit is mounted on the colposcope to acquire daily calibration data prior to performing patient examinations. The calibration routine is fast, automated, accurate and reliable. We then use our illumination correction algorithm and a color calibration algorithm to calibrate the patient data. In this paper we describe these techniques and demonstrate their applications in clinical studies. Keywords: Cervical Cancer, Calibration, Colposcopy, Computer-Aided Diagnosis 1. INTRODUCTION Uterine cervical cancer is the second most common cancer in women worldwide, with nearly 500,000 new cases and over 270,000 deaths annually. Because invasive disease is preceded by pre-malignant Cervical Intraepithelial Neoplasia (CIN), if detected early and treated adequately, cervical cancer can be universally prevented 1 . Colposcopy is once of the primary diagnostic methods used to detect CIN and cancer, following an abnormal cytological screen (Papanicolaou smear). The purpose of a colposcopic examination is to identify and rank the severity of lesions, so that biopsies representing the highest-grade abnormality can be taken, if necessary. A colposcopic examination involves a systematic visual evaluation of the lower genital tract (cervix, vulva and vagina), with special emphasis on the subjective appearance of metaplastic epithelium comprising the Transformation Zone (TZ) on the cervix. During the exam, a 3-5% acetic acid solution is applied to the cervix, causing abnormal and metaplastic epithelia to turn white. Cervical cancer precursor lesions and invasive cancer exhibit certain distinctly abnormal morphologic features that can be identified by colposcopic examination. Lesion characteristics such as color or opacity, margin shape, blood vessel caliber, intercapillary spacing and distribution, and contour are considered by physicians (colposcopists) to derive a clinical diagnosis. Lugol’s iodine is another contrast solution often used during colposcopy. The color difference of the iodine staining also assists in differentiating the severity of the lesions. However, the color and illumination of the colposcopic images vary with the light sources, the instruments and camera settings, as well as the clinical environment. Typically, the color of the epithelium may look very different (including normal and abnormal findings) in cervical images acquired with different instruments or at different times. This makes the assessment of the color information very challenging, even for an expert. Using an objective image calibration technique (accompanied by corresponding monitor calibration technique) may help the physician/colposcopists to better assess the information in cervical images in terms of diagnosis and severity, as well as facilitate the annotation and the use of telemedicine. * wli@sti-hawaii.com; phone 1 808 540-4768; fax 1 808 540-4850 Medical Imaging 2006: Image Processing, edited by Joseph M. Reinhardt, Josien P. W. Pluim, Proc. of SPIE Vol. 6144, 614466, (2006) · 0277-786X/06/$15 · doi: 10.1117/12.650218 Proc. of SPIE Vol. 6144 614466-1