Reliability of computed tomography measurements in assessment of thigh muscle cross-sectional area and attenuation

31 patients from an ongoing study of rehabilitation and muscle atrophy after ACL reconstruction were included in the study. Axial CT images with slice thickness of 10 mm at the level of 150 mm above the knee joint were analyzed by two investigators independently at two times with a minimum of 3 weeks between the two readings using NIH ImageJ. CSA and the mean attenuation of individual thigh muscles were analyzed for both legs.Mean CSA and mean attenuation values were in good agreement both when comparing the two observers and the two replicates. The inter- and intraclass correlation (ICC) was generally very high with values from 0.98 to 1.00 for all comparisons except for the area of semimembranosus. All the ICC values were significant (p < 0,001). Pearson correlation coefficients were also generally very high with values from 0.98 to 1.00 for all comparisons except for the area of semimembranosus (0.95 for intraobserver and 0.92 for interobserver).This study has presented ImageJ as a method to monitor and evaluate CSA and attenuation of different muscles in the thigh using CT-imaging. The method shows an overall excellent reliability with respect to both observer and replicate.Injury of the anterior cruciate ligament (ACL) is a common injury especially in younger people involved in different kinds of sport activity [1,2]. It is well known that atrophy of the quadriceps femoris muscle is linked to disuse both pre- as well as postsurgical in patients with ACL and/or meniscal damage. Several strategies for treatment, both conservative and surgical, have been used [3]. Different surgical strategies have been applied and there is still an ongoing development of the surgical techniques [2,4]. For evaluation of outcome and follow-up there is a need for a reliable method for measuring muscle size. There is also a need for evaluating of atrophy of the different muscles of the thigh especially as some of the techniques involve the use of grafts either from gracilis and/or se