Purpose: To develop and evaluate the performance an apparatus of fluence modulation and scatter shielding for cone-beam CT, and the corresponding method of image acquisition.
Methods: The apparatus is composed of two assemblies, referred to herein as the dynamic Fluence Modulation (FM) and Scatter Shield (SS), that work in synchrony to form a narrow beam sweeping the entire field of view of the imaging system in each projection. The positioning of the FM with respect to the focal spot leads a change in the flux of x-rays incident on a patient – providing the means to optimize the dose depending on the size, shape and positioning of the organ of interest.Feasibility of this apparatus was verified through a series of Monte Carlo (MC) simulations, followed by incorporating the FM-SS assembly into an existing cone-beam CT system. A series of UHMW cylindrical phantoms were utilized to measure the detected signal level, scatter build-up, and the dose, in presence and absence of the FM-SS apparatus.
Results: Implementation of FM-SS results in near scatter-free projections. The scatter-to-primary ratio dropped from 90% in the absence of the FM-SS to below 15% in its presence. The recorded dose, tallied throughout a phantom with a diameter similar to that of an average-size breast (14 cm), was dropped 32%. The variabilities in the recorded raw signal, resulting from the differences in the attenuation path length through a cylinder, was almost entirely resolved (coefficient of variation dropping from 850% to 20%).
Conclusion: Utilization of the FM-SS apparatus and technique was demonstrated via simulations and physical implementation to result in: significant reductions in dose, reductions in scatter uptake in projections, and a reduction in the required dynamic range of the imager. This novel solution addresses the limitations of bow-tie filters, and high-scatter projections in cone-beam CT geometry.
Funding Support, Disclosures, and Conflict of Interest: We acknowledge partial support from a Small Business Innovation Research grants by National Science Foundation (grant number 2014351) and National Institute of Health (grant number R43CA261381-01).