Quantitative assessment of noise and spatial resolution properties of ASIR-reconstructed CT images at different percentages of reconstruction

Introduction Computed tomography (CT) is the largest source of population exposure to ionizing radiation in industrialized countries. Recent advances in CT imaging include the development of iterative reconstruction algorithms in order to obtain a dose reduction without compromising the diagnostic quality of images. The purpose of this study was to quantitatively assess noise reduction and spatial resolution in CT imaging with the ASIR (Adaptive Statistical Iterative Reconstruction, GE Healthcare) reconstruction algorithm at different percentages of reconstruction as well as different kVp, mAs and contrasts. Acquisitions of the Catphan-504 phantom were performed on a PET/CT scanner (Discovery-710, GE Healthcare). CT images were reconstructed using both filtered back projection (FBP) and ASIR with different percentages of reconstruction (20%, 40%, 60%, 80%, 100%). The image noise was estimated for different values of scanning parameters (i.e. mAs, kVp, pitch, slice thickness). Then, 3D/2D/1D noise power spectrum was estimated. Also, spatial resolution was assessed by obtaining the modulation transfer function (MTF) for a wide range of scanning parameters values and different contrast objects by the circular Edge Spread Function method and the Point Spread Function method. Image noise decreased (up to 50% as compared to FBP) when increasing the percentage of ASIR reconstruction (behaviour more relevant for higher spatial frequencies). Only for low tube load (<56 mAs) and lower contrast objects (with respect to phantom background - i.e. PMP, Delrin, LDPE and Polystyrene) acquisitions, MTF analysis showed that, increasing the ASIR percentage, the reconstructed images were characterized by an appreciable reduction in spatial resolution, when compared to FBP-reconstructed images. When compared to FBP, ASIR allows a relevant noise reduction without appreciably affecting image quality, except for very low dose and contrast acquisitions.