Precise determination of paraspinous musculature by quantitative CT.

The ability to quantify muscular size has important implications in monitoring the effects of muscle disorders, in assessing the efficacy of interventions aimed at abbreviating muscle atrophy, and in examining the association between muscle and bone as a correlate of osteoporosis. We have developed an autocontouring technique for the precise determination of paraspinous musculature that can be implemented as an adjunct to our current CT method of spinal trabecular densitometry without additional scanning time or radiation exposure. Using two distinct patient groups, we evaluated the validity of this technique by assessing intra- and interobserver variability. Using the average coefficient of variation (CV) as an estimate of precision, we found intraobserver variability to be essentially equivalent whether evaluated in young, healthy men (0.69%) or in older, mildly osteoporotic women (0.74%). When the muscle evaluations of two observers were compared, the variability was somewhat higher, 1.81 and 1.83% in older and younger subjects, respectively. We scanned an additional 10 subjects twice, with intermediate repositioning, and found the reproducibility (CV) of determining paraspinous muscle area to be 0.97%. Given this estimated high level of precision, we derived the approximate magnitude of change in muscle size that could be observed using two-point measurements in time. Small average changes, on the order of 1-2%, could be detected using small groups of subjects (10-20/group). Moreover, we found that, even in the individual patient, relatively small changes (3-6%) could be detected given our low imprecision estimate. We believe this quantitative approach holds promise for monitoring muscle changes in vivo as well as for rigorously exploring the relationship between muscle and bone.