Experimental realization of fluence field modulated CT using digital beam attenuation

Abstract

Purpose Tailoring CT scan acquisition parameters to individual patients is a topic of much research in the CT imaging community. It is now common place to find automatically adjusted tube current options for modern CT scanners. In addition, the use of beam shaping filters, commonly called bowtie filters, is available on most CT systems and allows for different body regions to receive different incident x-ray fluence distributions. However, no method currently exists which allows for the form of the incident x-ray fluence distribution to change as a function of view angle. This study represents the first experimental realization of fluence field modulated CT (FFMCT) for a c-arm geometry CT scan. Methods: X-ray fluence modulation is accomplished using a digital beam attenuator (DBA). The device is composed of 10 iron wedge pairs that modulate the thickness of iron x-rays must traverse before reaching a patient. Using this device, experimental data was taken using a Siemens Zeego c-arm scanner. Scans were performed on a cylindrical polyethylene phantom and on two different sections of an anthropomorphic phantom. The DBA was used to equalize the x-ray fluence striking the detector for each scan. Non DBA, or “flat field” scans were also acquired of the same phantom objects for comparison. In addition, a scan was performed in which the DBA was used to enable volume of interest (VOI) imaging. In VOI, only a small sub-volume within a patient receives full dose and the rest of the patient receives a much lower dose. Data corrections unique to using a piece-wise constant modulator were also developed. Results The feasibility of FFMCT implemented using a DBA device has been demonstrated. Initial results suggest dose reductions of up to 3.6 times relative to “flat field” CT. In addition to dose reduction, the DBA enables a large improvement in image noise uniformity and the ability to provide regionally enhanced signal to noise using VOI imaging techniques. Conclusions The results presented in this paper take the field of FFMCT from the theoretical stage to that of possible clinical implementation. FFMCT, as shown in this paper, can reduce patient dose while maintaining or improving image quality. In addition, the DBA has been experimentally shown to be well suited to implement entirely new imaging methods like photon counting and VOI imaging.

Fluence field modulated CT on a clinical TomoTherapy radiation therapy machine

“…In kilo-voltage CBCT, radiation dose reduction of over 10 times have been demonstrated using FFMCT. 4 This dose reduction can be "traded" for image quality improvement for RT applications. The use of a TomoTherapy system for FFMCT may also be useful for kV CT and CBCT researchers seeking to evaluate FFMCT.…”

Two-dimensional dynamic fluid bowtie attenuators

“…Previous work has simulated dynamic bowtie filters for both types of FFMCT. [1][2][3][4][5][6] As current solid-metal attenuator solutions may be difficult to extend to 2-D modulation, we propose to use liquid-and gas-based attenuators which can be arrayed in a 2-D pattern to implement FFMCT. This idea is not entirely new to x-ray imaging; it was actually proposed in the early 1980s for chest radiography.…”

“…1,3 Extending this solid-based approach to 2-D is possible. One could align two 1-D filter arrays orthogonal to each other.…”

“…Clinically, a real-time optimization 1 or an atlas of positions 14,15 would be used to determine compensator thickness/pressure in place of the more time-consuming procedure outlined in Secs. 2.1 and 2.2.…”

“…These unusable sections of the sinogram were interpolated using redundant rays as described in Ref. 1, and in doing so, artifact creation was mitigated.…”