Repositioning errors in measurement of vertebral attenuation values by computed tomography.

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 three directions of displacement and two directions of rotation were investigated using two sections of cadaver spines each including three 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 units for a single vertebra and 4.9 Hounsfield units for three 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 approximately 15HU for a single vertebra and 8HU for three adjacent vertebrae.