Abstract
Cone beam CT (CBCT) is widely used as an imaging guidance tool in radiation therapy. Yet tumor delineation on CBCT is sometimes challenging, such as in liver cancer, leading to tumor positioning uncertainty. Iodine contrast agent is typically used in liver cancer at CT simulation to enhance detectability. Its application in CBCT is still limited due to poor image quality and excessive contrast agent use in multiple treatment fractions. In this study, we develop a multi-energy element-resolved (MEER) CBCT framework achieved on a standard CBCT platform to enhance detectability of low-concentration Iodine contrast agent. We employ a kVp-switching technique to acquire a multi-energy CBCT scan on standard CBCT platform. We develop an element-resolved CBCT reconstruction algorithm that simultaneously reconstructs x-ray attenuation images at each kVp, a density image, and elemental composition images (H, O, Ca and I). The algorithm solves an optimization problem under the constraint that elemental composition of each voxel is a sparse representation of a dictionary containing compositions of typical human tissues and iodine. To demonstrate feasibility and effectiveness of our method, we perform Monte Carlo simulation studies and experimental validations using a Gammex phantom with inserts containing water and iodine solution (175 mgI/mL) of different concentrations. We consider a kVp switching scan with 600 projections acquired in a full gantry rotation under 80, 100, and 120 kVps. Experiment is realized on the on-board imaging system of a linear accelerator available in our clinic. We analyze iodine detectability as quantified by contrast-to-noise ratio (CNR). We compare results with those of single energy CBCT images reconstructed by the standard filter back projection (FBP) method with 600 projections. Results of simulation and experimental study agree with each other. MEER-CBCT and FBP method achieve similar contrast enhancement. At 4% iodine solution concentration, FBP method achieve ∼250 HU enhancement and CNR of ∼2.5, considered to be the standard CNR for successful tumor visualization. In contrast, MEER-CBCT achieve the same CNR but at the 8 times lower iodine concentration of 0.5%. We have developed a novel MEER-CBCT framework realized on a standard CBCT platform to improve detectability of low-concentration iodine. Simulation and experimental studies demonstrated potential of MEER-CBCT to visualize iodine contrast at a 8 times reduction of iodine concentration.Abstract 208; Table 1Iodine Solution ConcentrationSimulationExperimentMEER-CBCTFBPMEER-CBCTFBPEnhancement (HU)CNREnhancement (HU)CNREnhancement (HU)CNREnhancement (HU)CNR0.1%28.03.430.40.421.91.823.40.30.5%48.95.951.60.755.02.557.90.51%74.48.375.91.079.87.076.80.92%127.313.8129.31.7144.112.4140.31.63%179.016.4179.72.4206.412.3202.62.15%280.923.8281.23.7308.925.4305.63.3 Open table in a new tab
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