Metal Artifact Correction Using KV and Selective MV Imaging
A Keil1*, L Zhu2, J Star-Lack3, R Fahrig1, (1) Stanford University, Stanford, CA, (2) Georgia Institute of Technology, Atlanta, GA, (3) Varian Medical Systems, Mountain View, CASU-E-I-22 Sunday 3:00:00 PM - 6:00:00 PM Room: Exhibit Hall
To improve the image quality of radiotherapy planning CTs for patients with metal implants or fillings by completing the missing kV projection data with selectively acquired MV data that does not suffer from photon starvation. Using both imaging systems that are available on current radiotherapy devices, streaking artifacts are avoided and the soft tissue contrast is restored, even in areas where the kV photons do not contribute any information. This enables a better delineation of structures of interest in planning CT images for patients with metal objects.
An algorithm for combining kV and MV projection data from the two on-board imagers of a radiotherapy device is presented in this work. It only requires selective MV imaging with the high energy X-rays being collimated onto the metal implants, ensuring that the patient dose does not increase significantly. The algorithm can cope with non-identical geometries of the two imagers and is based on stitching together kV and MV sinograms by estimating a ratio between them. A numerical head phantom with two dental fillings and two soft tissue patterns was used to quantitatively evaluate the proposed hybrid reconstruction algorithm. A structural similarity index (SSIM) with respect to the ground truth data was computed for two ROIs. Realistic, polychromatic spectra were used for both imagers with 120 keV(p) and 6 MeV(p). The patient dose was limited to about 6 cGy for both acquisitions combined.
The reconstruction results yield visually as well as objectively better results (SSIM=74.8%) than a simple sinogram interpolation of the kV data (SSIM=69.7%) or a reconstruction from the original data (SSIM=17.9%).
We have successfully implemented a new reconstruction method for hybrid kV-MV cone beam CT reconstruction that enables a better planning of radiotherapy treatments for patients with metal implants without compromising their safety.