A method for determining the modulation transfer function from thick microwire profiles measured with x-ray microcomputed tomography

This study describes a model-dependent method to determine the modulation transfer function (MTF) in the transversal plane, obtained by a microcomputed tomography (micro-CT) system from profiles of a thick wire phantom instead of a thin wire phantom, and the study evaluates the feasibility of the proposed method in the MTF determination of micro-CT systems.

The MTF is generally calculated as the absolute value of the normalized Fourier transform from the point spread function obtained by scanning a thin wire phantom. Since the wire is not a point source, the raw MTF is corrected for the finite size of the wire phantom; a wire with too large a diameter introduces inaccuracies in the MTF values. Therefore, we solved the MTF determination from profiles of a thick wire phantom via MTF modeling on the basis of the symmetric Lévy function that generalizes Gaussian and Lorentzian functions. We then applied the method to profiles of wire phantoms (1 mm, 2 mm, and 3 mm in diameter) measured by a clinical CT system to evaluate the applicable diameter range of the thick wire phantom. Two types of reconstruction kernels (standard and sharp) were used in the clinical CT. The performance of the method was evaluated using microwire phantoms (10 and 30 μm in diameter) measured by a synchrotron radiation micro-CT (SRμCT) system, in which the Shepp-Logan filter and Ramachandran-Lakshminarayanan filter were used as the reconstruction kernel. The MTFs obtained using thin wire phantoms of 0.1 mm and 3 μm in diameter were regarded as the gold standard MTFs for the clinical CT and SRμCT, respectively. The root-mean-square error (RMSE) and relative error (RE) of the 10% value of the MTF were used to measure the difference between the MTF determined by the method and the gold standard.

The mean RMSEs for two types of reconstruction kernels of three wire phantoms (1, 2, and 3 mm in diameter) were 0.0085, 0.012, and 0.021, respectively. The mean REs for the 1-, 2-, and 3-mm wire phantoms gave the same values of 2.0%, 3.5%, and 3.5%, respectively, for two types of reconstruction kernel. The MTFs determined from thick wire phantoms reveal the spatial resolution for the two kernels. The mean RMSEs for two types of reconstruction kernels of the microwire phantoms of 10 and 30 μm in diameter were 0.0045 and 0.0035, respectively. The mean REs of the two wire phantoms of 10 and 30 μm diameter had 4.0% and 3.1%, respectively, for two types of reconstruction kernel.

Experimental data presented in this paper support the effectiveness of the model-dependent method based on the symmetric Lévy function. We conclude that the method is a useful approach for measuring the spatial resolution in the x∕y-scan plane (transversal orientation) of micro-CT systems by substituting a thick wire phantom for a thin wire phantom.