2022 年 40 巻 5 号 p. 261-272
The mechanism of edge artifacts in point spread function (PSF) reconstruction was investigated by a simulation study. A two-dimensional (2D) numerical phantom comprises 184 mm diameter disk background and 4.6-30 mm diameter disk signal with contrast 9 (expressed as signal/background −1). Parallel beam projection data were blurred by detector response of 1D Gaussian function with 5 mm full width at half maximum (FWHM). Projection matrix only considers detector response function, ensuring that the edge artifacts are not masked or affected by other components. Image reconstruction was performed using ordered subset expectation maximization (OSEM) without regularization and with regularization by generalized Gaussian function. Without regularization, the object-specific modulation transfer function (OMTF), defined as the ratio of power spectrum of original and reconstructed images, was ~1 up to 1.6 cycles/cm, lower than the kernel frequency support of the detector response function (1/FWHM: 2.0 cycles/cm) and then decayed rapidly. This frequency response is similar to the original image and was smoothed using a Butterworth filter of order 4 and a cutoff frequency of 2.94 cycles/cm, producing the edge artifacts in image space. In PSF reconstruction, the edge artifacts are generated because the frequency response has the shape of low-pass Butterworth filter, lacking high-frequency components.
