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The reconstruction of 2D images from fan-beam transmission measurements is performed with filtered-backprojection (FBP) and an ordered subsets maximum likelihood (OSML) method. A special software phantom, which allows measuring the resolution and noise in a non-ambiguous way, is used to simulate transmission tomography scans with different signal-to-noise ratios. The noise and modulation transfer function is calculated for both FBP and OSML reconstruction at several positions distributed over the field of view (FOV). The OSML method is able to achieve a nearly homogeneous high resolution over the whole field-of-view, which is not possible with FBP, because the OSML reconstruction from the focus-centered projections intrinsically uses the improvement of the resolution due to the quarter detector offset without any extra re-binning. In addition, the reconstruction with OSML has a higher resolution than FBP at a similar level of noise, or equivalent, the OSML method achieves a lower level of noise compared to FBP at the same resolution. The reconstruction with OSML can save a factor of about 2 of dose with respect to FBP.