We studied the intra-and inter-equipment variations in the mean CT numbers of the central part of a vertebral body for six X-ray CT equipment. We used the lower torso phantom and the CaCO; calibration phantom. These phantoms were scanned by abdominal routine examination parameters, and the mean CT numbers of a vertebral body included in the lower torso phantom were measured. The intra-equipment variation of 1.9%(coefficient of variation: CV) for the mean CT numbers was obtained in the X-ray CT equipment. Using CaCO reference lines, we obtained the mineral contents of the vertebral body. The intra-equipment variation for the mineral contents was 2.2% (CV). The variations of the mean CT numbers and mineral contents were almost the same. The inter-equipment variation of the mean CT numbers was 4.7% (CV) and of the mineral contents was 2.8% in the six X-ray CT equipment. Therefore, we believe that the CT number obtained for a vertebral body can be used as an estimation of the bone mineral content in the X-ray CT equipment. And, if we allow interequipment variation of about 5%, we propose that the CT numbers obtained in several X-ray CT equipment can be compared.
