BEAM HARDENING ERRORS IN POST-PROCESSING DUAL-ENERGY QUANTITATIVE COMPUTED-TOMOGRAPHY

A computer simulation study was performed to assess the errors due to x-ray beam hardening in the fat and bone estimates of a post-processing dual-energy quantitative computed tomography technique. The ''central'' calibration method was employed in which calibration standards are inserted within a torso phantom of a size similar to that of the ''patient.'' Although beam hardening errors are reduced with this method, they still occur as a result of mismatches between the torso phantom and patient body sizes. Two mismatch situations were investigated. In one, a single torso phantom was used for all subject sizes (i.e., one-size-fits-all). In the other, closest matches were made from a set of three different sized torso phantoms (small, medium, and large): In all cases, the compositions of the calibration standards that were inserted into the torso phantoms consisted of bone, fat (glycerol trioleate), and an average fat-free red marrow. Fifteen patient sizes were simulated ranging from 20 to 34 cm in diameter. There were 21 patients of each size. The vertebrae in these subjects contained known amounts of bone mixed in marrows of composition determined from chemical analyses of cadaver marrow samples. Vertebrae consisting of mixtures of the calibration standard materials were also studied. The computed effective x-ray beam energies at the vertebra location for the various subject sizes ranged from 54.3 to 56.4 keV at 80 kVp and from 74.4 to 78.8 keV at 140 kVp. Maximum root mean square errors due to beam hardening were 1.2 mg/ml bone and 0.007 mass fraction fat for one-size fits-all central calibration, and 0.31 mg/ml bone and 0.002 mass fraction fat for closest match central calibration. These errors are two to four times smaller than the maximum beam hardening errors obtained when the calibration standards and vertebrae are made of identical materials. This study indicates that for a realistic situation in which the compositions of the calibration standards and patient's marrow differ to some extent, the contribution of beam hardening to the overall error in fat and bone estimates is small. Furthermore, these errors can be minimized through the use of a central calibration technique in which a closest match to the patient's body size is made from a set of three different sized torso phantoms.