Philips Medical Systems DMC GmbH

A first demonstrator system for clinical dark-field chest radiography was recently constructed. Due to its scanning acquisition, conventional exposure regulation systems are not possible. This study presents the adaptation of exposure to each patient individually, using a conventional radiograph as a reference. Additionally, an alternative approach for individual exposure planning based on the patient's body mass index (BMI) is proposed.

Using exposure settings from a conventional radiography system with automatic exposure control (AEC), a patient-specific equivalent attenuator thickness was calculated with polyoxymethylene (POM) as a surrogate material. The tube current at the dark-field system was adapted to reach the target detector dose based on this thickness. This approach was verified with both phantom measurements and patient examinations. The correlation between patient body parameters (weight, BMI, upper bust girth, and under-bust girth) and desired tube current was also evaluated.

A calibration curve was found to transfer individual exposure settings from a conventional device to the dark-field system. Achieved detector doses for both phantom and patient examinations were within permitted ranges. A strong correlation was found between each body parameter and the desired tube current, with BMI showing the strongest correlation (r=0.87,p<10^-29).

An exposure planning approach was successfully implemented at the first clinical dark-field chest radiography system, delivering a detector dose within the required range for different patient sizes. A conventional radiograph with AEC is necessary for this implementation. The strong correlation between BMI and tube current offers an alternative approach for patient-specific exposure control.

X-ray dark-field imaging enables a spatially-resolved visualization of ultra-small-angle X-ray scattering. Using phantom measurements, we demonstrate that a material's effective dark-field signal may be reduced by modification of the visibility spectrum by other dark-field-active objects in the beam. This is the dark-field equivalent of conventional beam-hardening, and is distinct from related, known effects, where the dark-field signal is modified by attenuation or phase shifts. We present a theoretical model for this group of effects and verify it by comparison to the measurements. These findings have significant implications for the interpretation of dark-field signal strength in polychromatic measurements.

Grating-based X-ray phase-contrast and in particular dark-field radiography are promising new imaging modalities for medical applications. Currently, the potential advantage of dark-field imaging in early-stage diagnosis of pulmonary diseases in humans is being investigated. These studies make use of a comparatively large scanning interferometer at short acquisition times, which comes at the expense of a significantly reduced mechanical stability as compared to tabletop laboratory setups. Vibrations create random fluctuations of the grating alignment, causing artifacts in the resulting images. Here, we describe a novel maximum likelihood method for estimating this motion, thereby preventing these artifacts. It is tailored to scanning setups and does not require any sample-free areas. Unlike any previously described method, it accounts for motion in between as well as during exposures.