Compton scatter effects in CT reconstructions

Compton scatter of x-ray quanta is the dominant attenuation mechanism in medical diagnostic imaging. In this paper, it is shown that the scatter-to-primary ratio determines the nature and intensity of scatter artifacts in computed tomography (CT) reconstructions and that this ratio, while significantly lower than in radiographic and fluoroscopic examinations, can still be significant in CT. It is found that high spatial-frequency artifacts can arise even though the detected scatter intensity has little or no high-frequency modulation. Reconstructions from x-ray data are presented which show "cupping" as well as dark streaks connecting high-attenuation regions. Correction of the data from measurements of scatter eliminates these artifacts. It is, moreover, observed that the intensity of the artifacts is often diminished when a beam-shaping attenuator is employed. Calculations of scatter intensity are developed from a model which includes single-event and two-event scatter. This analysis is in good agreement with measurements on round water phantoms. Extension to other detector geometries shows, not unexpectedly, that detectors with poorer collimation yield larger scatter artifacts.