Charge-cloud-based micrometer resolution in deep-silicon photon-counting CT.

Abstract

Purpose: We are developing a monolithic deep-silicon photon-counting sensor targeting spatial resolution on the order of $1\mu m$ . This work investigates how pixel pitch, noise level, threshold number, and threshold placement affect the achievable tangential and wafer-thickness resolution to guide the CMOS electronics and sensor design.

Approach: Allpix Squared simulations were used to evaluate pixel pitches with two noise levels and two threshold-placement schemes over varying threshold numbers. Interaction position was estimated in both dimensions using lookup tables trained on the thresholded pixel output, and performance was quantified using the MTF.

Results: Decreasing pixel pitch strongly improved tangential resolution for Compton interactions, whereas photoelectric interactions showed a weaker dependence on pixel pitch and threshold placement. Equal-counting thresholds approached the achievable resolution with fewer thresholds than equidistant thresholds. For eight equal-counting thresholds, the 10% MTF across the studied pixel sizes ranged from $\text{1284\hspace{0.17em}\hspace{0.17em}}\mathrm{lp}/\mathrm{cm}$ to $\text{2452\hspace{0.17em}\hspace{0.17em}}\mathrm{lp}/\mathrm{cm}$ in the tangential direction and from 15 to $71\mathrm{lp}/\mathrm{cm}$ in the wafer-thickness direction for Compton interactions, compared with 409 to $507\mathrm{lp}/\mathrm{cm}$ and 18 to $79\mathrm{lp}/\mathrm{cm}$ , respectively, for photoelectric interactions. RMSE analysis further showed that some interactions for the $25\mu m$ pitch reached below $2\mu m$ tangentially and a few tens of micrometers in the wafer-thickness direction.

Conclusions: Spatial resolution improved with a threshold number up to about 8 to 10, beyond which additional programmable thresholds gave only modest gain. The achieved tangential resolution also suggests that analyzer-free phase-contrast imaging is feasible for realistic geometries and expected interference patterns.