Single scan dual energy cone beam CT using a rotating filter

Abstract

Compared with conventional CT imaging, dual energy CT (DECT) provides more material information using two sets of projection data with different x-ray spectra. Current implementations of DECT typically require two CT scans with different x-ray energies or costly upgrades of x-ray tube or detector. In this work, we propose a single-scan DECT method for conebeam CT (CBCT) imaging by inserting a rotating metal filter between x-ray source and the scanned object. CBCT systems have a relatively slow scanning speed, which offers a possibility of altering the x-ray spectrum via a metal filter for every other projection. We insert a rotating metal filter between the x-ray source and the detector, with the filter rotation synchronized with the x-ray pulse and the detector readout. As such, one single-scan acquires interlacing low- and high-energy CT projections. We then reconstruct high and low CT images using a combined approach of iterative reconstruction and decomposition with similarity matrix and TV based regularization to preserve the image structure and denoising. We have implemented the proposed method on a benchtop CBCT system, and compared the performance with that of a photon-counting detector based system on physical phantoms. On an electron density phantom, our proposed algorithm decreases the noise from a root mean square of error of 2.105 to 0.017(1023 𝑒/𝑐𝑐) compared with a direct decomposition from FBP reconstructed images. On a resolution gauge phantom, our method maintained the resolution of 15 line pairs per mm compared with FBP reconstruction and decomposition results and subdue the image aliasing artifacts under same noise level with that of a TV regularization results without losing spatial resolution. The performance is further validated on an anthropomorphic head phantom. We propose a practical and accurate DECT method on CBCT with insertion of a rotating metal filter. Compared with those of the photon-counting detector based approaches, our method achieves better performances on spatial resolution of decomposed material images and similar performance on electron density measurement, with attractive features of no requirement of scan time increase or costly imaging component upgrades.