Scott Sigao Hsieh MS, Presenter: Nothing to Disclose

Norbert J. Pelc ScD, Abstract Co-Author: Research grant, General Electric Company Board of Directors, Real-Time Radiography, Inc Stockholder, General Electric Company Scientific Advisory Board, Samplify Systems Inc Scientific Advisory Board, Radguard, Inc

A dynamic CT bowtie filter capable of modulating its attenuation profile with time may provide superior image quality and reduced patient dose when compared to static bowties. We present a design for such a dynamic bowtie, which may be especially relevant when used with photon-counting detectors, because the reduction in dynamic range enabled by the dynamic bowtie may translate into the usability of detectors with modest count rate capability.

The design goal of the dynamic bowtie is to produce an attenuation profile that is piecewise-linear in fan angle. This goal is achieved with two layers of triangular wedges. Each layer has wedges side by side, and the layers themselves interleave. By scrolling these wedges in the z-direction, triangles of different heights are exposed to the source. These triangles are then used as basis functions for building the desired piecewise-linear function. A triangular compensator makes the attenuation profile constant in z. We evaluated the effectiveness of this design on a clinical thoracic section. A dynamic bowtie using nine wedge elements was compared to a standard GE body bowtie. The triangular wedges were moved with the goal of minimizing the dynamic range on the detector. The actuators driving the wedge elements were assumed to have a finite speed, and the trajectory of the wedges was cast into a convex optimization problem and solved.

A reference system with a static bowtie and ideal tube current modulation was found to have a dynamic range of 59. With the dynamic bowtie, the dynamic range was 11.8, or a fivefold reduction of dynamic range on this object.

A dynamic bowtie capable of achieving a piecewise-linear attenuation profile is possible and can significantly reduce the dynamic range on the detector. Further work is necessary to verify the total performance gains in dose and dynamic range that can be expected in practice.

The dynamic bowtie may enable the adoption of photon-counting detectors with count rates lower than otherwise possible, and may also improve dose efficiency substantially.

Hsieh, S, Pelc, N, Design for a Dynamic Bowtie Achieving a Piecewise-Linear Attenuation Profile.  Radiological Society of North America 2011 Scientific Assembly and Annual Meeting, November 26 - December 2, 2011 ,Chicago IL. http://archive.rsna.org/2011/11009332.html