The use of computed tomography and ultrasonography in the evaluation of orbital masses.

The combined use of computed tomography scans with multiplanar reformation and ultrasonography makes it possible to locate orbital masses with a high degree of accuracy, and sometimes even to determine their histological nature without surgical biopsy. One of the unique features of computed tomography is its ability to distinguish both normal and abnormal structures of various tissue densities; this has contributed to very low false positive and false negative rates. Ultrasonography, while of limited use in the detection of posterior orbital lesions or lesions involving the orbital bones, provides an excellent cost-effective screening test for anterior and midorbit disease. It is also useful for the detection of extraocular muscle enlargement secondary to thyroid disease. In this article, a review of the characteristics and techniques of computed tomography and ultrasonography is followed by discussion of the advantages and limitations of each modality in the evaluation of six common types of orbital tumefaction.