Repositioning errors in measurement of vertebral attenuation values by computed tomography

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Abstract
Difficulty in repositioning makes a major contribution to reproducibility errors in measurements of bone mineral by computed tomography (CT). The magnitude of measurement variations arising from incremental movements in each of 3 directions of displacement and 2 directions of rotation were investigated using 2 sections of cadaver spines each including 3 vertebral bodies. Within the range of variation in position readily attainable by currently available scan plane selection techniques, changes in measured attenuation were small. Estimated variations resulting from compounded errors of different movements averaged 8.3 Hounsfield U [HU] for a single vertebra and 4.9 HU for 3 adjacent vertebrae. The largest change in measured attenuation which could be attributed with 95% confidence to repositioning error within the quoted limits of scan plane selection was estimated to be .apprx. 15 HU for a single vertebra and 8 HU for 3 adjacent vertebrae.