Study of Gd beam filtration for dual-layer flat panel detector based CBCT imaging: proof-of-concept study with 160 mm...

Objective.This study aims at evaluating the gadolinium (Gd) beam filter in generating high quality dual-energy cone beam CT (CBCT) images with a dual-layer flat panel detector (FPD).Approach.First of all, the Cramér-Rao lower bounds (CRLBs) were estimated on dual-energy x-ray projections to determine the optimal settings such as beam filter thickness, CsI:TI scintillator thickness, and copper (Cu) filter thickness between the two detector layers. Afterwards, dual-energy CT imaging were numerically simulated with varied Gd/Cu beam filter thickness and Cu filter between the two detector layers. Finally, physical experiments were validated on our dual-layer FPD based dual-energy CBCT imaging benchtop.Main results.for a specific dual-energy imaging task, in general, optimizations are required in order to achieve satisfactory imaging performance. Compared to the Cu beam filter, the CRLB analyses found that Gd beam filter would generate material basis with lower noise levels, i.e. higher signal-to-noise ratio (SNR). In addition, physical experiments show that the Gd beam filter might be a better choice than the Cu beam filter in ensuring accurate basis decomposition. Specifically, the Gd beam filter yielded higher SNR than the Cu beam filter in all cases: by 41.3% (water) and 52.9% (bone) in numerical simulations; by 31.0% (water) and 0.9% (iodine) for the cylindrical phantom; and by 15.6% (water) and 13.3% (iodine) for the head phantom. However, the virtual mono-chromatic images (VMI) generated with Cu beam filter always have higher contrast-to-noise ratio than with the Gd beam filter. Both numerical simulations and physical experiments show that the inter-layer Cu filter may enhance the basis decomposition performance, but at the expense of reducing the total radiation efficiency.Significance.for 160 mm diameter phantom, this study found that the Gd filter is a promising alternative to the conventional Cu filter in improving the dual-energy basis imaging performance of a dual-layer FPD based CBCT system.