Dynamic fluence field modulation in computed tomography using multiple aperture devices

Abstract

A novel beam filter consisting of multiple aperture devices (MADs) has been developed for dynamic fluence field modulation (FFM) in CT. Each MAD achieves spatial modulation of x-ray through fine-scale, highly attenuating tungsten bars of varying widths and spacings. Moiré patterns produced by relative motions between two MADs provide versatile classes of modulation profiles. The dual-MAD filter can be designed to achieve specific classes of target profiles. The designed filter was manufactured through a laser-sintering process and integrated to an experimental imaging system that enables linear actuation of the MADs. Dynamic FFM was achieved through a combination of beam shape modulation (by relative MAD motion) and amplitude modulation (by view-dependent mAs). To correct for gains associated with the MADs, we developed an algorithm to account for possible focal spot changes during/between scans and spectral effects introduced by the MADs. We performed FFM designs for phantoms following two imaging objectives: (1) to achieve minimum mean variance in filtered backprojection (FBP) reconstruction, and (2) to flatten the fluence behind the phantom. Comparisons with conventional FFM strategies involving a static bowtie and pulse width modulation were performed. The dual-MAD filter produced modulation profiles closely matched with the design target, providing varying beam widths not achievable by the static bowtie. The entire range of modulation profiles was achieved by 0.373 mm of MAD displacement. The correction algorithm effectively alleviated ring artifacts as a result of MADs while preserving phantom details such as wires and tissue boundaries. Dynamic FFM enabled by the MADs were effective in achieving the imaging objectives and demonstrated superior FFM capabilities compared to the static bowtie. In an ellipse phantom, the FFM of objective 1 achieved the lowest mean variance in all cases investigated. The FFM of objective 2 produce nearly isotropic local noise power spectrum and homogeneous noise magnitude. The dual-MAD filter provides an effective tool for fluence control in CT to overcome limitations of conventional static bowties and to further enable patient-specific FFM studies for a wide range of dose and image quality objectives.

The z‐sbDBA, a new concept for a dynamic sheet‐based fluence field modulator in x‐ray CT

“…Besides these advantages, the main limitation of the z-sbDBA might be its limitation to symmetrical transmission profiles. Unlike other DBA concepts, 27,28,40,51 the z-sbDBA cannot shift the peak transmission along the fan beam width but is centered on the iso-ray.…”

“…However, most studies reported dose savings in the range of 30% to 50% w.r.t conventional bowtie filters. 29,32,40,46,53,54 Only the inverse CT geometry yields far higher dose savings of up to more than 80%. 37 Comparable improvements in noise homogeneity are e.g.…”

“…In this DBA approach, overlapping structures of highly absorbing bars allow modulating the fluence by using the Moiré effect. [38][39][40] Numerous studies, including the ones mentioned above, have demonstrated that FFM by DBAs has the potential to reduce patient dose and noise anisotropy. Beyond that, FFM is seen as a promising concept for further CT improvements.…”

An optimization algorithm for dose reduction with fluence‐modulated proton CT

“…[44][45][46][47][48] In contrast to the direct filtered backprojection algorithm used in this study, iterative reconstruction employs a regularization method (typically total variation), which reduces noise and whose optimal weight depends on the object and the fluence level. 49 While most fluence modulation studies for x-ray CT have been performed using filtered backprojection, 20,21 a first study 23 investigated a joint optimization of the fluence field and a spatially varying regularization parameter in the iterative reconstruction. For pCT, a comparison of iterative and direct reconstruction 47 showed comparable image quality.…”

“…Fluencemodulated scans 20 can either aim for homogeneous variance across the whole volume, or for region-of-interest (ROI) imaging, where only the relevant part of the image is acquired at low noise and imaging dose is reduced elsewhere. Algorithms [21][22][23][24] and experimental prototypes [25][26][27][28][29] have been developed for fluence modulation in x-ray CT. Recently, fluence-modulated proton computed tomography (FMpCT) has also been proposed 30 and its initial experimental feasibility using pencil beam scanning was investigated.…”