Evaluation of cone-beam computed tomography over a small field of view in a water bath based on the modulation transfer function with repeating-edge oversampling.

The spatial resolution of cone-beam computed tomography (CBCT) in small fields of view (FOVs) is important for clinical applications. However, it is difficult to measure spatial resolution reliably due to error factors such as noise. The aim of this study was to obtain a modulation transfer function (MTF) more accurately. A CBCT apparatus was used with small FOV. An aluminum pipe slightly tilted at an inclination ratio of 77/3 (25.7) was used as the measurement phantom. The MTF was calculated from the edge image of the phantom. The actual oversampling ratio was determined by regression analysis. The experiment was repeated 16 times and the edge-spread function (ESF) was approximated by the least-square method. Furthermore, a low-pass filter (LPF) was applied to eliminate the component at frequencies above the Nyquist frequency. Finally, the MTF was calculated from the pre-processed ESF. Results showed that pre-processing reduced the noise of the ESF. The MTFs at frequencies of 1.0 and 2.0 LP/mm were 0.59 and 0.18, respectively, in air and 0.52 and 0.16, respectively, in water. The repeating-edge oversampling method combined with ESF pre-processing improved the accuracy of the MTF under clinically relevant conditions with a phantom.