Spectral analysis of scattered radiation in CT

In the framework of Spectral Computed Tomography (Spectral CT), scattered X-ray radiation is examined for its spectral composition and spatial distribution by means of Monte Carlo simulations. A reliable material (e.g. bone / contrast agent) separation and quantification requires a precise knowledge of the transmitted X-ray spectrum especially for low energy photons. Unfortunately, for lower energies the primary intensity is increasingly covered by scattered radiation. The detected scattered radiation can be classified into two main categories with respect to their scattering history. The first category contains purely Rayleigh or one-time Compton scattered photons which typically have small scattering angles and an energy spectrum similar to that of the transmitted primary radiation. The second category comprises multiple Compton scattered photons with a spectral composition which is typically softer than that of the transmitted primary photons. In regions of strong beam attenuation (i.e. in the X-ray shadow of a scanned object), the scattered radiation is mainly composed of multiple Compton scattered photons. As a consequence, the spectrally resolved scatter-to-primary ratios strongly increase at low energies. High-quality anti-scatter grids can be used to reduce especially the detection of multiple Compton-scattered photons. A quantitative evaluation of measured photon energies below a certain limit between 30 keV and 50 keV (depending on the phantom geometry and the applied anti-scatter grid) is challenging, since primary photons are superposed by a significantly higher amount of scattered photons.