Acquisition of CT images with comparable diagnostic power can potentially be achieved with lower radiation exposure than the current standard of care through the adoption of hardware-based fluence-field modulation (e.g. dynamic bowtie filters). While modern CT scanners employ elements such as static bowtie filters and tube-current modulation, such solutions are limited in the fluence patterns that they can achieve, and thus are limited in their ability to adapt to broad classes of patient morphology. Fluence-field modulation also enables new applications such as region-of-interest imaging, task specific imaging, reducing measurement noise or improving image quality. The work presented in this paper leverages a novel fluence modulation strategy that uses “Multiple Aperture Devices” (MADs) which are, in essence, binary filters, blocking or passing x-rays on a fine scale. Utilizing two MAD devices in series provides the capability of generating a large number of fluence patterns via small relative motions between the MAD filters. We present the first experimental evaluation of fluence-field modulation using a dual-MAD system, and demonstrate the efficacy of this technique with a characterization of achievable fluence patterns and an investigation of experimental projection data.
We introduce a novel strategy for fluence field modulation (FFM) in x-ray CT using multiple aperture devices (MADs).
MAD filters permit FFM by blocking or transmitting the x-ray beam on a fine (0.1-1 mm) scale. The filters have a number
of potential advantages over other beam modulation strategies including the potential for a highly compact design, modest
actuation speed and acceleration requirements, and spectrally neutral filtration due to their essentially binary action. In this
work, we present the underlying MAD filtration concept including a design process to achieve a specific class of FFM
patterns. A set of MAD filters is fabricated using a tungsten laser sintering process and integrated into an x-ray CT test
bench. A characterization of the MAD filters is conducted and compared to traditional attenuating bowtie filters and the
ability to flatten the fluence profile for a 32 cm acrylic phantom is demonstrated. MAD-filtered tomographic data was
acquired on the CT test bench and reconstructed without artifacts associated with the MAD filter. These initial studies
suggest that MAD-based FFM is appropriate for integration in clinical CT system to create patient-specific fluence field
profile and reduce radiation exposures.