Iterative reduction of artifacts in computed tomography images using forward projection and an edge-preserving blur filter

Abstract

We present an iterative method for reducing artifacts in computed tomography (CT) images. First, a filter is applied to the experimental projection data that adaptively expands the detector element size in regions with low photon counts, until the desired number of photons are detected. The initial image is then calculated using an existing reconstruction technique. In each iteration, artifacts and noise in the current image are reduced by using an edge-preserving blur filter. Metal pixels (determined from the initial image) are replaced with smaller values. The resulting image is forward projected. Rays that go through metal are replaced with the forward projected values. Rays that do not pass near metal are kept at the experimental values. Filtered backprojection is then performed on the new projection data to determine the updated image. Finally, after the last iteration, metal pixels are copied from the initial image.

Description

REFERENCES CITEDU.S. Patent Documents[0001]5,907,594May 1999Lai378 / 45,933,471August 1999Kalvin378 / 47,023,951 B2April 2006Man378 / 87,636,461 B2December 2009Spies382 / 1282008 / 0273651November 2008Boas378 / 4OTHER PUBLICATIONS[0002]G. Glover, et al, “An algorithm for the reduction of metal clip artifacts in CT reconstructions,”Medical Physics, vol. 8, pp. 799-807, November-December 1981.[0003]W. Kalender, et al, “Reduction of CT artifacts caused by metallic implants,”Radiology, vol. 164, pp. 576-77, August 1987.[0004]A. C. Kak and M. Slaney, Principles of computerized tomographic imaging, IEEE Press, New York, 1988.BACKGROUND OF THE INVENTION[0005]A computed tomography (CT) scanner uses X-rays to determine the three-dimensional structure of an object. X-ray beams (“rays”) are passed through the object from different angles, and “detector elements” (also known as “detectors”) on the other side measure the intensity of each attenuated ray. “Ray” can also refer to the path traversed by X-rays ...

AI Technical Summary

Benefits of technology

"The present invention is a method for reducing metal artifacts in computed tomography (CT) images. It uses forward projection to replace detector measurements that involve metal. An adaptive filter is applied to the experimental projection data to expand the size of each detector element until at least 30 photons are detected. The initial image is constructed using a combination of metal pixels and non-metal pixels. Metal pixels are then erased, and filtered backprojection is iterated four times. On each iteration, rays that pass through metal are replaced with forward projected values from the previous iteration. The end result is a self-consistent set of projection data with the metal removed. The invention also addresses other sources of artifacts in CT images, such as beam hardening, edge effects, and patient motion. It is the first metal artifact reduction technique that has been shown to produce statistically significantly better image quality and change the imaging diagnosis."

Problems solved by technology

Second, the forward projected values don't match the original projection data exactly, due to beam hardening, density outside the reconstructed region, and other factors.

Furthermore, a dark streak through soft tissue may cause it to be incorrectly classified as air (which would introduce new artifacts).

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