Optimization of the geometry and speed of a moving blocker system for cone-beam computed tomography scatter correction

Abstract

Purpose: X-ray scatter is a significant barrier to image quality improvements in cone-beam computed tomography (CBCT). A moving blocker-based strategy was previously proposed to simultaneously estimate scatter and reconstruct the complete volume within the field of view (FOV) from a single CBCT scan. A blocker consisting of lead stripes is inserted between the X-ray source and the imaging object, and moves back and forth along the rotation axis during gantry rotation. While promising results were obtained in our previous studies, the geometric design and moving speed of the blocker were set empirically. The goal of this work is to optimize the geometry and speed of the moving block system.

Methods: Performance of the blocker was examined through Monte Carlo (MC) simulation and experimental studies with various geometry designs and moving speeds. All hypothetical designs employed an anthropomorphic pelvic phantom. The scatter estimation accuracy was quantified by using lead stripes ranging from 5 to 100 pixels on the detector plane. An iterative reconstruction based on total variation minimization was used to reconstruct CBCT images from unblocked projection data after scatter correction. The reconstructed image was evaluated under various combinations of lead strip width and interspace (ranging from 10 to 60 pixels) and different moving speed (ranging from 1 to 30 pixels per projection).

Results: MC simulation showed that the scatter estimation error varied from 0.8% to 5.8%. Phantom experiment showed that CT number error in the reconstructed CBCT images varied from 13 to 35. Highest reconstruction accuracy was achieved when the strip width was 20 pixels and interspace was 60 pixels and the moving speed was 15 pixels per projection.

Conclusions: Scatter estimation can be achieved in a large range of lead strip width and interspace combinations. The moving speed does not have a very strong effect on reconstruction result if it is above 5 pixels per projection. Geometry design of the blocker affected image reconstruction accuracy more. The optimal geometry of the blocker has a strip width of 20 pixels and an interspace three times the strip width, which means 25% detector is covered by the blocker, while the optimal moving speed is 15 pixels per projection.

Keywords: cone-beam CT; imaging artifacts; moving blocker; optimization; scatter correction.

Figure 1

Illustration of the blocker and its location in an on‐board imaging system. (a) Anterior view of the blocker. (b) Cross section view of the blocker. (c) A blocker is inserted between the X‐ray source and the imaging object and moves back and forth along the gantry rotation axis z during CBCT acquisition. The source‐to‐blocker distance is 310 mm, the source‐to‐axis distance is 1000 mm while the source‐to‐detector distance is 1536 mm.

Figure 2

Part of the detector plane to illustrate of the excluded pixels for eliminating penumbra. The blocker has a strip width of 1.6 mm and an interspace of 3.2 mm, corresponding to a 10‐pixel‐wide blocked region and a 20‐pixel‐wide unblocked region at detector plane. The blocker moving speed is 15 pixels per projection (13.2 mm s⁻¹), 6‐ pixel‐wide region around the strip edge were excluded.

Figure 3

(a) A customized moving blocker system mounted on an Elekta Synergy XVI system. Components of the moving blocker system are labeled as (a) a lead‐strip blocker (3.2 mm lead strip width and 3.2 mm interspace); (b) a linear motion guide actuator (KR20, THK); (c) a bipolar stepper motor with controller(PD1141, TRINAMIC). (b) Three customized blockers, lead strips were 3.2 mm in thickness and 20 pixels (3.2 mm) in width. Interspace was 20 pixels (3.2 mm) for A1, 40 pixels (6.4 mm) for A2 and 60 pixels (9.6 mm) for A3.

Figure 4

Scatter estimation error in the unblocked regions of detector panel, as both strip width and interspace varied from 5 to 100 pixels (as projected at the detector plane). (a) 0° projection (frontal); (b) 45° projection; (c) 90° projection (lateral).

Figure 5

Axial (top row) and coronal slices (bottom row) of the reconstruction from simulated data. (a) and (f): Benchmark, simulated non blocker image; (b) and (g): B10G50, speed 10 pixels per projection ( RMSE  = 7.5); (c) and (h): B20G20, speed 20 pixels per projection ( RMSE = 19.8); (d) and (i): B20G20, speed 1 pixels per projection ( RMSE = 45.8); (e) and (j): B10G10, moving speed 20 pixels per projection ( RMSE = 202). Display window [−800, 1000] HU.

Figure 6

Difference images between the reconstructed images from partially unblocked projections and reconstructed images from full projections. (a), (b) and (c): difference images between Figs. 5(b)–5(d) and Fig. 5(a); (d), (e) and (f): difference images between Figs. 5(g)–5(i) and Fig. 5(f); Display window [−350, 350] HU.

Figure 7

CBCT image reconstruction error when different moving blockers were applied; no scatter was presented. The blocker lead strip width was 10 and 20 pixels, the gap width was 10, 20, 30, 40, 50 and 60 pixels, and the blocker moving speed was from 1 pixel per projection to 30 pixels per projection. (We did not show the scenarios where RMSE is above 60).

Figure 8

Axial (top row) and coronal slices (bottom row) of the reconstruction from simulated data. (a) and (f): Benchmark, simulated non blocker and scatter free image; (b) and (g): B10G50, speed 10 pixels per projection( RMSE  = 28.9); (c) and (h): B20G20, speed 20 pixels per projection( RMSE  = 30.7); (d) and (i): B20G20, speed 1 pixels per projection( RMSE  = 54.5); (e) and (j): B20G60, moving speed 20 pixels per projection( RMSE  = 28.2). Display window [−800, 1000] HU.

Figure 9

CBCT image reconstruction error upon application of different moving blockers. Simulations of the blocker lead strip width was 10 and 20 pixels, the gap width was from 10 to 60 pixels, and the blocker moving speed was from 1 to 30 pixels per projection. (We did not show the scenarios where RMSE was above 70).

Figure 10

Axial (top row) and coronal slices (bottom row) of the anthropomorphic pelvis phantom. (a) and (f): Benchmark, simulated non blocker and scatter free image; (b) and (g): reconstruction from scatter‐contaminated projection data; (c) and (h): B20G20, speed 8 pixels per projection; (d) and (i): B20G60, speed 15 pixels per projection. (e) and (j): reconstruction provided by Varian OBI system, scatter corrected by asymmetric kernel approach provided in the software. The inferior‐superior length is different in the experiments of Elekta and Varian systems. Display window [−800, 1000] HU.

Figure 11

Comparison of the horizontal profile, as indicated by a dashed line in Fig. 10(b), for simulated non blocker and scatter free image, scatter contaminated CBCT, and scatter corrected CBCT images with two different moving blockers and with Varian software in Figs. 10(a)–10(e), respectively.