Rainer Grimmer, Abstract Co-Author: Nothing to Disclose

Yiannis Kyriakou PhD, Presenter: Nothing to Disclose

Marc Kachelrieß PhD, Abstract Co-Author: Nothing to Disclose

To reduce scatter and cupping artifacts in X-ray CT scanners with tube voltage modulation and shaped prefiltration.

Scatter and beam hardening are prominent artifacts in X-ray CT. Currently, there is no precorrection method that inherently accounts for tube voltage modulation and shaped prefiltration. We generalized a method based on binary tomography of homogeneous objects [Proc. IEEE ICME, 891-895, 2007] to account not only for beam hardening but also for scatter and to allow for detector pixel-specific precorrections. This implies that our calibration technique handles varying tube voltage and shaped prefiltration. An initial reconstruction of a homogeneous calibration object (water phantom) is segmented to obtain a binary image. This image is forward projected. The projection data obtained are fitted to the original projection data using non-linear physical models. Beam hardening is modeled using a rational function (7 parameters per projection value) while scatter is modeled using the pep-model (3 parameters per projection value). A smoothness constraint is applied to the parameter space to regularize the underdetermined system of non-linear equations. The parameters determined are then used to precorrect CT scans. Our algorithm was evaluated using simulated data of a flat panel scanner with tube voltage variation and bow tie prefiltration and real data of a flat detector cone-beam CT scanner (OBI, Varian, Switzerland). The results were compared to standard reconstructions with a conventional empirical precorrection function applied to all detector channels.

In simulation studies our correction model proved to be nearly perfect and the algorithm showed its abilities by correcting the beam-hardening and scatter effects. The relative error between the estimated parameters and those used for simulation was below 2.5%. Reconstructions of patient data showed significantly less artifacts than the standard approach.

The new approach efficiently reduces scatter and beam hardening artifacts and can correctly address scans with tube voltage modulation. Thereby it appears superior to and more versatile than conventional methods. Since no assumptions on the calibration phantom, apart from being homogeneous, are necessary it is a practical way to calibrate CT scanners.

Diagnosis directly benefits from the improved reduction of scatter and beam hardening artifacts.

Grimmer, R, Kyriakou, Y, Kachelrieß, M, Scatter and Cupping Precorrection for Flat Detector CT based on Binary Tomography (BT).  Radiological Society of North America 2009 Scientific Assembly and Annual Meeting, November 29 - December 4, 2009 ,Chicago IL. http://archive.rsna.org/2009/8015263.html