Exhibit Hall | Forum 6
Purpose: Multi-energy KV-CBCT offers advantages including enhanced soft tissue visualization by reduction of signal from overlapping anatomy such as bone. A common approach to realize dual energy (DE) imaging is to expose the patient to two different KV spectra. However, this leads to increased patient dose, longer image acquisition and susceptibility to motion artefacts. A novel dual-layer on-board imager, enabling simultaneous acquisition of images with two different energy spectra, is being designed. Towards optimization of the design, this study evaluates the application of material removal in projection images.
Methods: A computational model of the imager, comprised of two stacked Cesium Iodide detector panels with active area of 43×43 cm² was implemented in GEANT4 Application for Tomographic Emission (GATE). A 26×30×3 cm³ planar acrylic phantom consisting of 1,5,10,20 mm aluminum, teflon, water-equivalent material (WEM, Plastic Water), acrylic and polyethylene inserts was imaged using a 140 kVp x-ray spectrum. A weighted logarithmic subtraction of the images from the two layers of the imager was used to generate DE images. Weights were iteratively generated to minimize CNR between regions-of-Interest (RoI) within the 1 mm insert and background. The material subtraction performance was assessed using the contrast relative to that of single energy images.
Results: An optimum weight of 0.59, 0.68, 2.26, 1.34 and 0.80 was obtained for aluminum, teflon, WEM, acrylic and polyethylene inserts respectively. Sequential application of these weights on the images results in 100% reduction of the material contrasts relative to the single energy image in all cases.
Conclusion: Dual energy imaging in a single exposure with a dual layer imager shows significant reduction in subject contrast of overlapping anatomy. Subtraction of anatomical noise would enhance CNR leading to better visualization of soft tissue for application in soft tissue tumor tracking and adaptive radiation therapy.