Monte Carlo simulation of inverse geometry x-ray fluoroscopy using a modified MC-GPU framework

Abstract

Scanning-Beam Digital X-ray (SBDX) is a technology for low-dose fluoroscopy that employs inverse geometry x-ray beam scanning. To assist with rapid modeling of inverse geometry x-ray systems, we have developed a Monte Carlo (MC) simulation tool based on the MC-GPU framework. MC-GPU version 1.3 was modified to implement a 2D array of focal spot positions on a plane, with individually adjustable x-ray outputs, each producing a narrow x-ray beam directed toward a stationary photon-counting detector array. Geometric accuracy and blurring behavior in tomosynthesis reconstructions were evaluated from simulated images of a 3D arrangement of spheres. The artifact spread function from simulation agreed with experiment to within 1.6% (rRMSD). Detected x-ray scatter fraction was simulated for two SBDX detector geometries and compared to experiments. For the current SBDX prototype (10.6 cm wide by 5.3 cm tall detector), x-ray scatter fraction measured 2.8-6.4% (18.6-31.5 cm acrylic, 100 kV), versus 2.1-4.5% in MC simulation. Experimental trends in scatter versus detector size and phantom thickness were observed in simulation. For dose evaluation, an anthropomorphic phantom was imaged using regular and regional adaptive exposure (RAE) scanning. The reduction in kerma-area-product resulting from RAE scanning was 45% in radiochromic film measurements, versus 46% in simulation. The integral kerma calculated from TLD measurement points within the phantom was 57% lower when using RAE, versus 61% lower in simulation. This MC tool may be used to estimate tomographic blur, detected scatter, and dose distributions when developing inverse geometry x-ray systems.

Keywords: MC-GPU; Monte Carlo; X-ray fluoroscopy; inverse geometry; scanning-beam digital x-ray.

Figure 1

Inverse geometry x-ray system. An electron beam is raster scanned over a large area target, dwelling at fixed positions behind a multi-hole collimator. The collimator defines a sequence of narrow overlapping x-ray beams (A) directed at a small photon-counting detector. Tomosynthesis images are reconstructed at multiple planes (B) for each scan frame. Part (C) shows the geometry and input variables used in the modified MC-GPU code.

Figure 2

Scan maps for regular and regional-adaptive-exposure (RAE) scanning (A and B), and an example tomosynthesis reconstruction from an RAE scan (C). Each pixel in a scan map represents the relative output from a collimator hole in a 71 × 71 scan. The scale ranges from 0 to 8 to indicate the number of hole illuminations per scan frame. In MC-GPU simulation, relative x-ray outputs are specified in the range [0 1]. An RAE scan reduces the number of illuminations in higher transmission regions, e.g. the lung fields lateral to the spine in (C).

Figure 3

Tomosynthesis reconstructions produced by experiment with the current SBDX prototype (A) and from SBDX simulations using the current rectangular detector (B) and previous square detector (C). In the simulations, the sphere position increases in 0.5 cm increments toward the detector, in row major order (top left to bottom right). The in-plane sphere is indicated with a yellow box.

Figure 4

Comparison of artifact spread functions along the horizontal image direction (A), for the current SBDX prototype. The ASF was measured from a 2.38 mm diameter ball bearing. Parts (B) and (C) show the profile data that was used to calculate the ASFs.

Figure 5

(A) Measured and simulated kerma at the phantom entrance plane, for regular and regional adaptive (RAE) scanning. Imaging was performed for an anthropomorphic chest phantom. (B, top row) shows SBDX images reconstructed from simulated detector images, for each scanning mode. (B, bottom row) shows the change in axial kerma distribution resulting from the regional adaptive exposure method.

Figure 6

Simulated and experimental x-ray scatter fraction for current (A) and previous (B) SBDX systems imaging isocentered acrylic phantoms (100 kV). Data for the previous SBDX system was taken from studies reported in Ref..

Figure 7

Simulated and experimental x-ray scatter fraction as a function of air gap, for the current SBDX prototype imaging a 23.3 cm acrylic phantom (100 kV).

Figure 8

(A) Composition of detected scatter for previous and current SBDX prototypes, for an isocentered 23.3 cm acrylic phantoms The scatter composition for an example conventional cone-beam geometry (5 cm air gap, no grid) is shown for comparison. (B) Composition of scatter versus acrylic phantom thickness, for previous and current generations of SBDX.

Figure 9

Simulated x-ray scatter fraction as a function of beam area, expressed as a multiple of the detector area, for the current SBDX geometry and a 23.3 cm thick acrylic phantom (100 kV). The horizontal line represents the experimental value under the same conditions.