Radiologic Measurement of Extraocular Muscle Volumes in Patients with Graves' Orbitopathy: A Review and Guideline

Objective: Toevaluate and compare techniques for extraocular muscle (EOM) volume measurement and to provide guidelines for future measurements. Design: Systematic review. Results: Existing techniques used to measure extraocular muscle volumes on radiologic scans can be divided into manual outlining, computer assisted and automated segmentation. Both computed tomography (CT) and magnetic resonance (MR) image datasets can be used. On CT scans, one best measures muscle volume using region grow segmentation, accepting an overestimation of true volume by inevitable inclusion of nonmuscular tissue. On high resolution MRI scans, single muscles can be outlined manually, but measurements include only part of the muscle due to poor tissue contrast at the orbital apex. Measurement errors can be reduced 3.5% by exact horizontal repositioning. A measured volume change of at least 6-17% is required to demonstrate a significant difference. Conclusion: Currently the best choice for EOM volume measurements on CT images is computer assisted grey value segmentation and on MRI images is manual outlining of individual muscles. Because of the time required and the complexity of the measurements, present EOMvolume measurement is as yet only suitable for research purposes.