Implementation of a piecewise-linear dynamic attenuator

A dynamic prepatient attenuator can modulate flux in a computed tomography (CT) system along both fan and view angles for reduced dose, scatter, and required detector dynamic range. Reducing the dynamic range requirement is crucial for photon counting detectors. One approach, the piecewise-linear attenuator (Hsieh and Pelc, Med Phys 2013), has shown promising results both in simulations and an initial prototype. Multiple wedges, each covering a different fan angle range, are moved in the axial direction to change thickness seen in an axial slice. We report on an implementation of a filter with precision components and a control algorithm targeted for a tabletop system. Algorithms for optimizing wedge position and mA modulation and for correcting bowtie-specific beam-hardening are proposed. In experiments, the error between expected and observed bowtie transmission was â1/42 % on average and â1/47 % at maximum for a chest phantom. Within object boundaries, the observed flux dynamic ranges of 42 for a chest and 25 for an abdomen were achieved, corresponding to a reduction factor of 5 and 11 from the object scans without the bowtie. With beam hardening correction, the CT number in soft tissue regions was improved by 79 HU and deviated by 7 HU on average from clinical scanner CT images. The implemented piecewise-linear attenuator is able to dynamically adjust its thickness with high precision to achieve flexible flux control.