A static multi-slit collimator system for scatter reduction in cone-beam CT

Abstract

A multiple-slit collimator (MSC) design was introduced for scatter reduction in cone-beam computed tomography (CBCT). Unlike most other collimators, the open and closed septa of the proposed MSC are placed in an equi-angular interval on a circular track of the central sagittal plane. Therefore, one gantry rotation provides only the half of necessary dataset and two gantry rotations are needed to obtain full information. During the first gantry rotation, the MSC position relative to the source is fixed. For the second rotation, the MSC is rotated by the equi-angle interval. We assume signals under the closed septa are totally attributed to scatter radiation. Then, scatter contributions under open septa are determined by interpolating them.Monte Carlo (MC) simulations for two virtual phantoms (one with a simple geometry and the other with two heterogeneities simulating the bone and the lung) were performed to evaluate the effectiveness of the system. Using the method developed, we could obtain images with significant scatter reduction. Contrast ratio (CR) improvement factors were 1.165 in a 2D projection view, and 1.210 and 1.223 at the central and peripheral slice of the reconstructed CBCT image of the simple geometry phantom.This preliminary study demonstrated that the proposed MSC, together with the imaging process technique, had a great potential to reduce scatter contribution in CBCT. Further studies will be performed to investigate the effect of various factors, such as reducing the detector size, increasing the number of history of MC simulation, and including many structures with different densities.

Figure 1

A simple diagram showing the multi‐slit collimator (MSC) and imaging geometry: (a) a sectional view on the central sagittal plane; (b) a sectional view on an axial plane corresponding to an open septum. As illustrated, the MSC consists of open and closed septa that are placed in equi‐angular interval on a circular track of the central sagittal plane.

Figure 2

Scatter reduction scheme: (a) a raw projection image during the first gantry rotation; (b) a raw projection image during second rotation; (c) and (d) the projection images of the first and second rotation after subtraction of the scatter signals that are approximated by interpolating (and/or extrapolating) scatter signals under the closed septa; (e) the sum of images (c) and (d).

Figure 3

Anterior‐to‐posterior projection images and their profiles: (a) projection image of the simple phantom and its x‐axis profile (b) and its z‐axis profile (c); (d) projection image of the lung‐n‐bone phantom and its x‐axis profile (e) and its z‐axis profile (f). [‘$\mathrm{P}+\mathrm{S}$’, ‘P’, ‘1 cm’, ‘2 cm’, and ‘3 cm’ indicate primary plus scatter, primary only, 1 cm width MSC, 2 cm width MSC, and 3 cm width MSC, respectively.]

Figure 4

Relative errors of the obtained profiles in with respect to the primary radiation only profiles following the z‐axis: (a) the simple phantom, and (b) the lung‐n‐bone phantom. [‘1 cm’, ‘2 cm’, and ‘3 cm’ indicate 1 cm width MSC, 2 cm width MSC, and 3 cm width MSC, respectively.]

Figure 5

CR improvement factor of a 2D projection image for the simple phantom with the dotted regions representing the CR calculation regions (black: PMMA, white: paraffin).

Figure 6

Central‐line intensity profile at the center slice of the reconstructed CBCT image of the simple phantom [‘wo’, ‘1 cm’, ‘2 cm’, and ‘3 cm’ indicate no MSC, 1 cm width MSC, 2 cm width MSC, and 3 cm width MSC, respectively.]

Figure 7

CR improvement factor of the CBCT image for the simple phantom; the two dotted boxes indicate the CR calculation regions.

Figure 8

Energy profile (in z‐direction) that the detector received in the case of 1 cm width MSC: [‘$\mathrm{P}+\mathrm{S}$’ indicates primary plus scatter, ‘1 cm’ and ‘$1\mathrm{c}\mathrm{m}\mathrm{\_}\mathrm{r}$’ indicate two projections of a pair obtained in a given angle (i.e. one in the first MSC position and the other with the MSC rotated by one equi‐angle interval), and ‘$1\mathrm{c}\mathrm{m}+1\mathrm{c}\mathrm{m}\mathrm{\_}\mathrm{r}$’ represents the summation of them without any image processing.]