Assessment of calibration methods for estimating bone mineral densities in trauma patients with quantitative CT: An anthropomorphic phantom study

Rationale and Objectives. Osteoporosis may contribute to the increased morbidity and mortality of elderly persons involved in motor vehicle accidents. Such patients commonly undergo whole-body computed tomographic (CT) studies that may be analyzed with quantitative CT. Various quantitative CT calibration techniques were investigated for use with patients who have suffered trauma, who are typically scanned on a backboard. Materials and Methods. Lumbar simulator phantoms were used to simulate small and large patients. Vertebral spongiosa inserts with a wide range of bone and fat compositions were placed in the phantoms, and their bone mineral densities (BMDs) were measured by using calibration lines derived from the CT numbers of a calibration standard. Four calibration techniques were tested. In three the lumbar simulator and the calibration standard were scanned simultaneously, with the standard placed beneath the backboard (method 1), on top of the backboard adjacent to the lumbar simulator (method 2), or on top of the abdomen region of the lumbar simulator (method 3). The fourth technique employed a single calibration line derived from a separate scan of the calibration standard beneath the small lumbar simulator without the backboard, with correction for patient body size. Results. The best overall results were obtained with the single calibration line method. The root mean square errors of the BMD values were 2.9-18.4, 2.5-7.5, 2.5-14.9, and 0.3-2.8 mg/cm(3) for methods 1, 2, 3, and 4, respectively (ranges represent variations in the errors of the measured BMDs of the inserts due to changes in scanner table height and lumbar simulator phantom size). Conclusion. The single calibration line method is an accurate means of measuring BMD in trauma patients.