Open AccessA pitfall of using the circular‐edge technique with image averaging for spatial resolution measurement in iteratively reconstructed CT images
Author(s) -
Narita Akihiro,
Ohkubo Masaki
Publication year - 2020
Publication title -
journal of applied clinical medical physics
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.83
H-Index - 48
ISSN - 1526-9914
DOI - 10.1002/acm2.12821
Subject(s) - optical transfer function , iterative reconstruction , imaging phantom , convolution (computer science) , noise (video) , image resolution , optics , mathematics , artificial intelligence , fourier transform , computer vision , physics , computer science , image (mathematics) , mathematical analysis , artificial neural network
The circular‐edge technique using a low‐contrast cylindrical object is commonly used to measure the modulation transfer functions (MTFs) in computed tomography (CT) images reconstructed with iterative reconstruction (IR) algorithms. This method generally entails averaging multiple images of the cylinder to reduce the image noise. We suspected that the cylinder edge shape depicted in the IR images might exhibit slight deformation with respect to the true shape because of the intrinsic nonlinearity of IR algorithms. Image averaging can reduce the image noise, but does not effectively improve the deformation of the edge shape; thereby causing errors in the MTF measurements. We address this issue and propose a method to correct the MTF. We scanned a phantom including cylindrical objects with a CT scanner (Ingenuity Elite, Philips Healthcare). We obtained cylinder images with iterative model reconstruction (IMR) algorithms. The images suggested that the depicted edge shape deforms and fluctuates depending on slice positions. Because of this deformation, image averaging can potentially cause additional blurring. We define the deformation function D that describes the additional blurring, and obtain D by analyzing multiple images. The MTF measured by the circular‐edge method (referred to as MTF') can be thought of as the multiplication of the true MTF by the Fourier transformation (FT) of D . We thus obtain the corrected MTF (MTF corrected ) by dividing MTF' by the FT of D . We validate our correction method by comparing the calculated images based on the convolution theorem using MTF' and MTF corrected with the actual images obtained with the scanner. The calculated image using MTF corrected is more similar to the actual image compared with the image calculated using MTF', particularly in edge regions. We describe a pitfall in MTF measurement using the circular‐edge technique with image averaging, and suggest a method to correct it.