Purpose: Iodine contrast agent is widely used in liver cancer at CT simulation to enhance detectability of tumor. However, its application in cone beam CT (CBCT) for image guidance is still limited because of poor image quality and excessive dose of contrast agent during multiple treatment fractions. In this study, we develop a multi-energy element-resolved (MEER) CBCT framework to enhance detectability of low-concentration iodine contrast agent.
Methods: We acquired a kVp swithching scan on a conventional CBCT platform with 600 acquired projections in a full gantry rotation using 80,100, and 120kVps. We developed an MEER-CBCT reconstruction algorithm that simultaneously reconstructed x-ray attenuation images at each kVp, a density image, and elemental composition images (H, O, Ca, and I). The composition of each voxel was subject to a constraint of a sparse representation of materials in a dictionary containing typical human tissues and iodine. To evaluate our method, we performed simulation studies and experimental validations using a Gammex phantom with inserts containing water and different concentrations of iodine solution (175 mgI/mL). We analyzed iodine detectability as quantified by contrast-to-noise ratio (CNR) and compared results with those of single energy CBCT images reconstructed by the standard filter back projection (FBP) method with 600 projections.
Results: In both the simulation and experiment, MEER-CBCT achieved similar contrast enhancement as FBP method but much higher CNR. An average CNR of MEER-CBCT was ~6 times higher than that of the FBP method. At 4% iodine solution concentration, FBP method achieved ~250 HU enhancement and CNR of ~2.5, considered as the standard CNR for successful tumor visualization. MEER-CBCT achieved the same CNR but at ~8 times lower iodine concentration of 0.5%.
Conclusion: We developed a novel MEER-CBCT framework and demonstrated its potential to visualize iodine contrast at ~8 times reduction of iodine concentration.