TH-C-110-04: Dual Energy CT Imaging for Density Measurements

Abstract

Purpose: Diagnostic CT scanners use polychromatic x-ray beams, which introduces errors from beam hardening. If the object is composed of a single material corrections for beam hardening can be exact but if the object is heterogenous some errors would remain. Dual energy CT can avoid these errors and can also provide atomic number information. Methods: If transmission measurements at two or more energies are obtained, even with polychromatic beams, more specific information about the object can be obtained. In the diagnostic energy range and if there are no materials with k-edges in the spectrum, the x-ray attenuation can be described by a linear combination of two processes (photoelectric absorption and Compton scattering) or, equivalently, two basis materials. The process of calculating the amount of the two basis functions implicitly corrects for beam hardening and therefore can lead to improvements in quantitative accuracy. Implementations of dual energy include dual-source, rapidly switching kVp sources, dual layer detectors and energy discriminating detectors. Results: Dual energy CT can correct for beam hardening. It can quantify effective atomic number and electron density in each voxel. It can also quantify the amounts of specific materials even in heterogenous voxels. The precision of the dual energy information is limited by statistics and the accuracy can be compromised by scatter. Conclusion: Dual energy CT can provide additional information for quantitative measurements with CT but its limitations need to be appreciated as well. Research sponsored by GE Healthcare. The learning objectives are to familiarize the audience with 1. basic principles of dual energy CT 2. implementations of dual energy CT 3. basis material decomposition 4. dual energy CT can characterize materials and correct for nonlinearity (e.g., beam hardening) 5. limitations of dual energy CT