  A bowtie is a filter used to shape an x-ray beam and equalize its flux
reaching different detector channels. For development of spectral CT with
energy-discriminative photon-counting (EDPC) detectors, here we propose and
evaluate a dynamic bowtie for performance optimization based on a patient model
or a scout scan. Our dynamic bowtie modifies an x-ray beam intensity profile by
mechanical rotation and adaptive adjustment of the x-ray source flux. First, a
mathematical model for dynamic bowtie filtering is established for an
elliptical section in fan-beam geometry, and the contour of the optimal bowtie
is derived. Then, numerical simulation is performed to compare the performance
of the dynamic bowtie in the cases of an ideal phantom and a realistic
cross-section relative to the counterparts without any bowtie and with a fixed
bowtie respectively. Our dynamic bowtie can equalize the expected numbers of
photons in the case of an ideal phantom. In practical cases, our dynamic bowtie
can effectively reduce the dynamic range of detected signals inside the field
of view. Although our design is optimized for an elliptical phantom, the
resultant dynamic bowtie can be applied to a real fan-beam scan if the
underlying cross-section can be approximated as an ellipse. Furthermore, our
design methodology can be applied to specify an optimized dynamic bowtie for
any cross-section of a patient, preferably using rapid prototyping technology.
This fan-beam dynamic bowtie work could be extended to the cone-beam geometry
in a follow-up study.
