Practical alignment method for X-ray spectral measurement in micro-CT system based on 3D printing technology.

This study presents a practical alignment method for X-ray spectral measurement in a rotating gantry based micro-computed tomography (micro-CT) system using three-dimensional (3D) printing technology. In order to facilitate the spectrometer placement inside the gantry, supporting structures including a cover and a stand were dedicatedly designed and printed using a 3D printer. According to the relative position between the spectrometer and the stand, the upright projection of the spectrometer collimator onto the stand was determined and then marked by a tungsten pinhole. Thus, a visible alignment indicator of the X-ray central beam and the spectrometer collimator represented by the pinhole was established in the micro-CT live mode. Then, a rough alignment could be achieved through repeatedly adjusting and imaging the stand until the pinhole was located at the center of the acquired projection image. With the spectrometer being positioned back onto the stand, the precise alignment was completed by slightly translating the spectrometer-stand assembly around the rough location, until finding a "sweet spot" with the highest photon rate and proper distribution of the X-ray photons in the resultant spectrum. The spectra were acquired under precise alignment and misalignment of approximately 0.2, 0.5, and 1.0mm away from the precise alignment position, and then were compared in qualitative and quantitative analyses. Qualitative analysis results show that, with slight misalignment, the photon rate is reduced from 1302 to 1098, 1031, and 416 photons/second (p/s), respectively, and the characteristic peaks in the acquired spectra are gradually deteriorated. Quantitative analysis indicates that the energy resolutions for characteristic peak of Kα1 were calculated as 1.56% for precise alignment, while were 1.84% and 2.40% for slight misalignment of 0.2mm and 0.5mm. The mean energies were reduced from 43.93keV under precise alignment condition to 40.97, 39.63 and 37.78keV when misaligned. Accurate spectral measurements in micro-CT systems are significantly influenced by the alignment precision. This practical alignment method using 3D printing technology could be readily applied to other rotating gantry based micro-CT systems with modified design of the supporting structures and careful considerations of the spectrometer and gantry dimensions.