Traceable measurement of the instrument transfer function in dXCT

Abstract

In dimensional X-ray CT (dXCT), the definition of a resolution describing parameter (metrological structural resolution) is not standardized conclusively yet. The instrument transfer function (ITF) is currently being discussed in national and international standardization bodies as one candidate for such a parameter. The ITF describes the wavelength-dependent transfer of a surface modulation - which is the main focus of dXCT - in contrast to the modulation transfer function (MTF), a well-known parameter which describes the grey-value amplitude transfer function after volume reconstruction without including a surface determination. In this article, a practical implementation of two differently sized new ITF reference standards is presented. Their idea is to use a height profile - with nearly constant spectral amplitude and bandwidth-limited - which is extended periodically. This profile is diamond-turned on the outer side of a tube and can be segmented to obtain identical pieces. One piece is calibrated pars pro toto with a high-resolution tactile scanner (HRTS) to obtain the reference ITF. Comparisons, CT parameter dependency tests and simulations highlight the benefit of this new type of standards for the determination of the ITF as a measure for resolution.