Technical Note: Sheet-based dynamic beam attenuator - A novel concept for dynamic fluence field modulation in x-ray CT.

PURPOSE

It has been a long-standing wish in computed tomography (CT) to compensate the emitted x-ray beam intensity for the patient's changing attenuation during the rotation of a CT data acquisition. The patient attenuation changes both spatially, along the fan beam angle, and temporally, between different projections. By modifying the pre-patient x-ray intensity profile according to the attenuation properties of the given object, image noise can be homogenized and dose can be delivered where it is really needed. Current state-of-the-art bowtie filters are not capable of changing attenuation profiles during the CT data acquisition. In our work, we present the sheet-based dynamic beam attenuator (sbDBA), a novel technical concept enabling dynamic shaping of the transmission profile.

METHODS

The sbDBA consists of an array of closely spaced, highly attenuating metal sheets, focused toward the focal spot. Intensity modulation can be achieved by controlled defocusing of the array such that the attenuation of the x-ray fan beam depends on the fan angle. The sbDBA concept was evaluated in Monte-Carlo (MC) simulations regarding its spectral and scattering properties. A prototype of the sbDBA was installed in a clinical CT scanner and measurements evaluating the feasibility and the performance of the sbDBA concept were carried out.

RESULTS

Experimental measurements on a CT scanner demonstrate the ability of the sbDBA to produce an attenuation profile that can be changed in width and location. Furthermore, the sbDBA shows constant transmission properties at various tube voltages. A small effect of the flying focal spot (FFS) position on the transmission profile can be observed. MC simulations confirm the essential properties of the sbDBA: In contrast to conventional bowtie filters, the sbDBA has almost no impact on the energy spectrum of the beam and there is negligible scatter emission toward the patient.

CONCLUSIONS

A new concept for dynamic beam attenuation has been presented and its ability to dynamically shape the transmission profile has successfully been demonstrated. Advantages compared to regular bowtie filters including the lack of filter-induced beam hardening and scatter have been confirmed. The novel concept of a DBA paves the way toward region of interest (ROI) imaging and further reductions in patient dose.