Semi-empirical scattering correction model for MSCT

Abstract

Scattering in CT is a major process that may result in severe image artifacts. In order to suppress this scattering, most CT scanners are equipped with post collimation anti-scatter grid along the fan beam direction. In the longitudinal direction (z-direction) no hardware solution has been implemented since the scattering is negligible in narrow coverage CT scanners. As the coverage becomes wider in recent MSCT scanners the scattering level in the z-direction increases significantly. This scattering increase in z-direction, results in image artifacts appearing as dark shadows along highly attenuating directions.In the present work we measure the scattering level in the z-direction for a wide coverage scanner, using various phantoms. Based on the results, a semi-empirical model for scattering correction in MSCT is presented and validated. The proposed model is based on a subtraction of a low frequency offset. This offset is proportional to the scattering, corresponding to the detector that has the lowest signal at each rotation angle. To validate the model, we first calculate this low frequency offset directly from the scatter measurements and apply it to the data acquired with the wide coverage. We then use a semi-empirical function to estimate the scattering fraction from the raw data, and use it to replace the directly measured scatter values in the correction scheme.Applying the proposed semi-empirical scatter correction model to the data acquired with the wide coverage, the scattering signal is significantly decreased. The images reconstructed from the corrected data exhibit a clear reduction of the artifact level.