Quantitative optimization of X-ray image acquisition with respect to object thickness and anode voltage - A comparison using different converter screens

The aim of this study is to present a new measurement scheme for optimizing image quality with respect to object thickness and anode voltage in polychromatic X-ray imaging. For demonstrating the high technological potential of this scheme, we show three results: a. Step-wise increasing the object thickness (aluminum) and adding four sheets of sandpaper for emulating structure, the optimal anode voltage UA is found from maximal amplitudes in the signal-to-noise ratio (SNR) spectra; b. Exchanging the Flatpanel (FP) with a lens-coupled indirect detector (LCID) and using two different converter screens (thick-transparent vs. thin powder) for the latter, the effects of scintillator material and thickness on the optimization are revealed; c. Resolution, i.e. energy-dependent modulation transfer function (MTF), is recorded for the different detectors/screens, for supporting the interpretation of the SNR results. The ground assumption (SNR theory) from which an optimal UA (which equals highest X-ray energy in the polychromatic spectrum) can be determined for any material/thickness is confirmed. For the Flatpanel (FP), the optimal anode voltage UAopt increases from 40kV to 128kV for aluminum slabs of 2mm to 10mm thickness. Both, SNR amplitudes and UAopt, change significantly when the detector and/or converter screen is exchanged. The detector SNR and MTF amplitudes degrades in all cases when the X-ray spectrum is hardened. The degradation of MTF is most pronounced for the thick transparent LuAG screen. Unlike DQE, SNR spectra correctly include object structure and polychromatic superposition of signal and noise. Using SNR amplitudes proves to be a reliable and versatile routine for carrying out complex optimization tasks. These measurements require very little time and effort. Using MTF measurements facilitates additional evaluations, particularly regarding the role of the X-ray converter screen.