Computational modelling of pixelated CdZnTe detectors for x- and γ- ray imaging applications

Abstract

Cadmium Zinc Telluride (CdZnTe) detectors are currently used in medical imaging systems employing γ-ray photons. As new imaging techniques such as photon-counting and energy-weighted x-ray imaging are gaining research interest, CdZnTe is seen under a new light for potential use in computed tomography, tomosynthesis and other x-ray imaging applications. However, being relatively expensive, CdZnTe could be favoured by advanced computational modelling to assist in detector and imaging system optimisation. In this work, pixelated CdZnTe detectors are computationally modelled using an integrated framework that combines the Finite Element and Monte Carlo numerical methods to obtain realistic detector models.Various detector thickness and pixel sizes are designed and their performance is investigated in terms of charge induction efficiency, detection efficiency and energy resolution. Detection efficiency and energy resolution are assessed for monoenegergetic photon beams within the energy range used in medical x-ray imaging applications such as mammography and computed tomography. Some of the capabilities of the framework are demonstrated. Small pixel sizes, below 100µm are prone to charge transport effects such as diffusion, especially in larger thickness (> 0.5 mm) and may have limited use in pixelated geometries. Detection efficiency is affected by fluorescence and photon escape as thickness and pixel size decrease. Energy resolution is affected by beam geometry and can vary from ∼3% to 11% depending on the beam width. The framework provides a generic platform and a powerful tool that can be used in the design and optimisation of semiconductor detectors made from any semiconductor material, imaging systems and signal correction techniques. KEYWORDS: Solid state detectors; X-ray detectors; Detector modelling and simulations II (electric fields, charge transport, multiplication and induction, pulse formation, electron emission, etc); Detector modelling and simulations I (interaction of radiation with matter, interaction of photons with matter, interaction of hadrons with matter, etc)

The Effects of Spectral X-Ray Photon Counting Detector Parameters on Detector Performance: Thickness and Pitch

“…The finite element model was used to simulate the charge induction efficiency (CIE) of the various pixel pitch-thickness combinations, by implementing Prettyman's adjoint continuity equations [27], as described in our previous works [28] [29] [22]. CIE is defined as = / (2) where Q is the free charge produced in the detector by the ionising interaction of an incident photon and q is the charge induced at the collecting electrode due to the movement of these free charges.…”

Effect of collimator design and anode dimensions on gamma-cameras based on pixelated CdZnTe

“…[6] and [7]. The framework is based on the combination of GATE Monte Carlo toolkit with results obtained from the COMSOL Multiphysics TM FEM software.…”

“…A computational method that combines two modeling layers of detector and system modeling (i.e. Monte Carlo and Finite Element Method (FEM)) [6,7] was employed that accounts for signal induction and charge transport effects which were not explicitly considered in previous collimator optimization studies [5].…”

CdTe Based Energy Resolving, X-ray Photon Counting Detector Performance Assessment: The Effects of Charge Sharing Correction Algorithm Choice

“…The adjoint continuity equations of Prettyman [29] were used to calculate the CIE values of a range of regularly space points within CdTe pixels of the various thickness and pitch combinations considered in this work, as described in previous publications [24,30,31]. Note that due to the large number of geometries simulated in this work the CIE maps for centre pixels will be applied to all pixels, neglecting the electric field deviations for corner and edge pixels considered in the previous publications [25].…”