A New Concept for Fluence Field Modulation in X-ray CT: the z-sbDBA

Abstract

In this work, we present a new concept for dynamic beam attenuation, the z-aligned sheet-based dynamic beam attenuator (z-sbDBA). Like the previously presented sbDBA, it allows dynamically adapting the x-ray intensity over the projection angle and also the fan beam angle by changing the tilt angle of the z-sbDBA. In contrast to the sbDBA, however, the absorbing sheets of a z-sbDBA are parallel to the detector rows. In addition, the height of the absorbing sheets varies over the fan angle – very low near the central beam, increasing toward larger fan beam angles. This facilitates designing the absorption profile of the z-sbDBA, which was not feasible with the sbDBA. Due to the changed orientation of the attenuation sheets, pronounced structures along the fan beam width are avoided, reducing the risk of ring artifacts in the reconstructed image. A prototype of the z-sbDBA, mounted on a drive, has been built and investigated. Using a clinical computed tomography (CT) scanner, we experimentally demonstrate that variable and smooth intensity profiles can be realized by the controlled change of the angular position of the z-sbDBA. Reconstructed images do not reveal substantial artifacts, thus proving the necessary stability of the acquisition technique. We also show that the variance across a reconstructed image can be changed as a function of the tilt angle. Our experimental results demonstrate that the new z-sbDBA concept maintains the main advantage of the sbDBA concept, which is the dynamic fluence field modulation (FFM) of the emitted x-ray beam. In addition, our findings show that due to the improved z-sbDBA structuring several drawbacks of the sbDBA can be overcome by a) avoiding pronounced structures along the fan beam angle, b) requiring only small tilt angles and c) allowing for a flexible design of the transmission profile propagated toward the patient.