Correction Method of Nonlinearity Due to Logarithm Operation for X-Ray CT Projection Data with Noise in Photon-Starved State

Shin-ichiro IWAMOTO
Akira SHIOZAKI

Publication
IEICE TRANSACTIONS on Information and Systems   Vol.E90-D    No.10    pp.1697-1705
Publication Date: 2007/10/01
Online ISSN: 1745-1361
DOI: 10.1093/ietisy/e90-d.10.1697
Print ISSN: 0916-8532
Type of Manuscript: PAPER
Category: Biological Engineering
Keyword: 
computed tomography,  correction of nonlinearity,  CT number,  noise distribution,  streak artifacts,  

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Summary: 
In the acquisition of projection data of X-ray CT, logarithm operation is indispensable. But noise distribution is nonlinearly projected by the logarithm operation, and this deteriorates the precision of CT number. This influence becomes particularly remarkable when only a few photons are caught with a detector. It generates a strong streak artifact (SA) in a reconstructed image. Previously we have clarified the influence of the nonlinearity by statistical analysis and proposed a correction method for such nonlinearity. However, there is a problem that the compensation for clamp processing cannot be performed and that the suppression of SA is not enough in photon shortage state. In this paper, we propose a new technique for correcting the nonlinearity due to logarithm operation for noisy data by combining the previously presented method and an adaptive filtering method. The technique performs an adaptive filtering only when the number of captured photons is very few. Moreover we quantitatively evaluate the influence of noise on the reconstructed image in the proposed method by the experiment using numerical phantoms. The experimental results show that there is less influence on spatial resolution despite suppressing SA effectively and that CT number are hardly dependent on the number of the incident photons.

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