Variation in Attenuation in L1 Trabecular Bone at Different Tube Voltages: Caution Is Warranted When Screening for Osteoporosis With the Use of Opportunistic CT.

OBJECTIVE: The purpose of this study is to investigate the variation in attenuation values (expressed as Hounsfield units) for L1 vertebral body trabecular bone at different tube voltages used in dual-energy CT (DECT) and to remind physicians to consider changes in attenuation values when they approach opportunistic screening for osteoporosis.

MATERIALS AND METHODS: Consecutive patients who underwent DECT examination of the abdomen and pelvis for suspected urolithiasis were included in the study. Attenuation noted on CT of the L1 trabecular bone performed with the use of tube voltages of 80, 100, and 140 kV was recorded. The correlation between the attenuation noted when the tube voltage was 140 kV and the attenuation noted when the tube voltage was either 80 or 100 kV was calculated, and differences in the mean CT attenuation values were compared.

RESULTS: The mean attenuation values from L1 trabecular bone measurement performed for 191 patients were analyzed. As expected, the mean attenuation values decreased as the tube voltage increased. There was a strong correlation between the attenuation values noted when tube voltages of 80 and 140 kV were used (r(2) = 0.97) and those noted when 100 and 140 kV were used (r(2) = 0.96). The mean attenuation value noted at 80 kV was 76.4 HU (65%) higher than that noted at 140 kV (p < 0.001). The mean attenuation value at 100 kV was 45.5 HU (39.9%) higher than that noted at 140 kV (p < 0.001).

CONCLUSION: We confirmed that attenuation values of L1 trabecular bone, unlike attenuation values of fat, fluid, or soft tissue, vary at different CT x-ray tube voltages. Therefore, standard reference attenuation values for trabecular bone seen at 120 kV cannot be applied to other single-energy settings, DECT, or CT examinations where dose modulation software automatically raises or lowers the tube voltage from 120 kV. Knowledge of the specific energy spectra used is essential before performing opportunistic CT evaluation for osteoporosis.