1. A computing system for creating a synthetic computed-tomography (CT) image volume, comprising: processing circuitry comprising a processor and memory; a rigid registration algorithm, which when implemented by the processing circuitry, is configured to register a reference CT image volume to a first clinical image volume and thereby produce a first registration; a smoothing function, which when implemented by the processing circuitry, is configured to further refine said first registration and thereby produce a smoothed registration; a voxelwise intensity algorithm, which when implemented by the processing circuitry, is configured to further refine said smoothed registration and thereby produce said synthetic CT image volume having characteristics correlated to those of said first clinical image volume; an attenuation coefficient map generating algorithm, which when implemented by the processing circuitry, is configured to derive a map of attenuation coefficients based on said synthetic CT image volume; and an attenuation correction engine, which when implemented by the processing circuitry, is configured to correct attenuation in a second clinical image volume based on said map of attenuation coefficients.