Monte Carlo investigations of the effect of beam divergence on thick, segmented crystalline scintillators for radiotherapy imaging

Abstract

The use of thick, segmented scintillators in electronic portal imagers offers the potential for significant improvement in x-ray detection efficiency compared to conventional phosphor screens. Such improvement substantially increases the detective quantum efficiency (DQE), leading to the possibility of achieving soft-tissue visualization at clinically practical (i.e. low) doses using megavoltage (MV) cone-beam computed tomography. While these DQE increases are greatest at zero spatial frequency, they are diminished at higher frequencies as a result of degradation of spatial resolution due to lateral spreading of secondary radiation within the scintillator--an effect that is more pronounced for thicker scintillators. The extent of this spreading is even more accentuated for radiation impinging the scintillator at oblique angles of incidence due to beam divergence. In this paper, Monte Carlo simulations of radiation transport, performed to investigate and quantify the effects of beam divergence on the imaging performance of MV imagers based on two promising scintillators (BGO and CsI:Tl), are reported. In these studies, 10-40 mm thick scintillators, incorporating low-density polymer, or high-density tungsten septal walls, were examined for incident angles corresponding to that encountered at locations up to approximately 15 cm from the central beam axis (for an imager located 130 cm from a radiotherapy x-ray source). The simulations demonstrate progressively more severe spatial resolution degradation (quantified in terms of the effect on the modulation transfer function) as a function of increasing angle of incidence (as well as of the scintillator thickness). Since the noise power behavior was found to be largely independent of the incident angle, the dependence of the DQE on the incident angle is therefore primarily determined by the spatial resolution. The observed DQE degradation suggests that 10 mm thick scintillators are not strongly affected by beam divergence for detector areas up to approximately 30x30 cm2. For thicker scintillators, the area that is relatively unaffected is significantly reduced, requiring a focused scintillator geometry in order to preserve spatial resolution, and thus DQE.

FIG. 1

Schematic view of the geometry of the detector (consisting of a segmented scintillator and an overlying copper plate), the radiation beams, and the associated coordinate system employed in the MTF simulations. The origin of the coordinate system, indicated by point O in the figure, is located at the center of the copper surface, on the x-ray side. The z-axis is perpendicular to the top surface of the detector and parallel to the detector thickness. The x- and y-axes are oriented so as to coincide with the directions of the grid formed by the septal walls. Planes A, B, C and D represent the direction of beams of parallel X rays incident on the detector at various angles θ with respect to the z-axis and at an angle φ with respect to the y-axis. Note that the dimensions of the drawing are not to scale.

FIG. 2

Pre-sampled MTF for four incident beam angles (0°, 2.24°, 4.47° and 6.69°) using segmented scintillators with polymer septal walls. The results are shown for 10 – 40 mm thick BGO scintillators in (a), (c), (e) and (g) and CsI:T1 scintillators in (b), (d), (f) and (h). In this figure and in figure 3, for each detector configuration, the progressive degradation in MTF shown by the various lines corresponds to increasing incident beam angle. Note that the simulation results are compared to the measured MTF from the conventional EPID.

FIG. 3

Pre-sampled MTF for four incident beam angles (0°, 2.24°, 4.47° and 6.69°) using segmented scintilllators with tungsten septal walls. The results are shown for 10 – 40 mm thick BGO scintillators in (a), (c), (e) and (g) and CsI:T1 scintillators in (b), (d), (f) and (h). The simulation results are compared to the measured MTF from the conventional EPID.

FIG. 4

Simulated NNPS for two incident beam angles (0° and 6.69°). Results are shown for 10 and 40 mm thick, segmented BGO and CsI:T1 detectors with polymer and tungsten septal walls.

FIG. 5

DQE for four incident beam angles (0°, 2.24°, 4.47°, and 6.69°) using 10 – 40 mm thick, segmented BGO and CsI:T1 detectors with polymer septal walls. In this figure and in figure 6, for each detector configuration, the progressive degradation in DQE shown by the various lines corresponds to increasing incident beam angle. Note that the results are compared to DQE measured from the conventional EPID.

FIG. 6

DQE for four incident beam angles (0°, 2.24°, 4.47° and 6.69°) using 10 – 40 mm thick, segmented BGO and CsI:T1 detectors with tungsten septal walls. The results are compared to DQE measured from the conventional EPID.