In x-ray imaging, improving spatial resolution is an important goal, but developing detectors with smaller pixels is technically challenging.
We demonstrate a technique for improving the spatial resolution by utilizing the fact that linear attenuation coefficients of all substances within the human body can be expressed, to a good approximation, as a linear combination of two basis functions, or three if there is iodine contrast present in the image. When the x rays pass an interface parallel to the beam direction, the exponential attenuation law makes the linear attenuation coefficient measured by the detector a nonlinear combination of the linear attenuation coefficients on each side of the interface. This so-called nonlinear partial volume effect causes the spectral response to be dependent on the steepness of interfaces in the imaged volume.
In this work, we show how this effect can be used to improve the spatial resolution in spectral projection x-ray imaging and quantify the achievable resolution improvement. We simulate x-ray transmission imaging of sharp and gradual changes in the projected path length of iodine contrast with an ideal energy-resolving photon-counting detector and demonstrate that the slope of the transition can be determined from the registered spectrum. We simulate piecewise-linear transitions and show that the algorithm is able to reproduce the transition profile on a subpixel scale. The FWHM resolution of the method is 5-30 % of the pixel width.
The results show that an energy-resolving detector can be used to improve spatial resolution when imaging interfaces of highly attenuating objects.
Mats Persson and Mats Danielsson, "Resolution improvement in x-ray imaging with an energy-resolving detector," Proc. SPIE 10132, Medical Imaging 2017: Physics of Medical Imaging, 101321D (Presented at SPIE Medical Imaging: February 15, 2017; Published: 9 March 2017); https://doi.org/10.1117/12.2254572.
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