An Implementation of Dual Energy CT Scanning

Abstract

A prereconstruction method for dual energy (PREDECT) analysis of CT [computed tomography] scans is described. In theory, this method can eliminate beam hardening and produce an accuracy comparable with monoenergetic scans and provide the effective atomic number and electron density of any voxel scanned. This implementation proves these statements and eliminates some of the objectionable noise. A phantom with a cylindrical sleeve-like compartment containing known amounts of high atomic number material simulating a removable skull was constructed. Conventional scans, with and without this beam hardener, were done of a water bath containing tubes of high electron and high atomic number material. Dual energy scans were then done for PREDECT. To increase the effective separation of the low and high energy beams by using more appropriate tube filtration, a beam filter changer containing Er, W, Al and steel. Er, W and steel were used at high energy and Al, steel and Er were used at low energy for data acquisition. The reconstructions were compared visually and numerically for noise levels with the original steel only filtration. A decrease in noise was found down to .apprx. 1/2 the prior level when Er/Al or W/Al replaced the steel/steel filter. Er and W were equally effective. Steel/Er and steel/Al also significantly reduced image noise. The noise in the photoelectric (P) and Compton (C) images is negatively correlated. At any pixel, if the noise is positive in the P image, it is most probably negative in the C. Using this fact, the noise was reduced by postreconstruction processing.