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Abstract
The influence of silicon wall thickness on the performance of structured CsI(Tl) scintillation screen based on oxidized silicon micropore array template in X-ray imaging was simulated using Geant4 Monte Carlo simulation code in terms of light output (LO), modulation transfer function (MTF) and detective quantum efficiency (DQE). The results show that when the thickness of the silicon wall is less than 0.5 μm, the increase in the bottom light output (BLO) of the screen and the decrease in the spatial resolution of the X-ray imaging system using the screen become more significant as the thickness decreases. At low spatial frequency, the thicker the silicon wall, the lower the DQE. However, the DQE with a thick silicon wall can exceed the DQE with a thin silicon wall at high spatial frequency. All the results provide the quantitative relation between the silicon wall thickness of the structured CsI(Tl) scintillation screen and the quality of the X-ray imaging.
© 2019 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
As a key component of an indirect conversion detector, CsI(Tl) scintillation screen is widely used in X-ray imaging because of its high conversion efficiency from X-ray to scintillation light [1–3]. In order to achieve excellent imaging performance, it is expected that the spatial resolution of the scintillation screen can be improved while maintaining its good X-ray absorption. In general, it is difficult to meet this expectation since better spatial resolution requires a thinner screen and a thinner screen leads to less X-ray absorption [4,5]. To cope with this difficulty, CsI(Tl) scintillation screen with needle-like columnar or microcolumnar structure has been developed by vapor deposition method [3,6,7]. The CsI(Tl) columns parallel to the normal of the screen surface can reduce the lateral spread of scintillation light, thereby improving the spatial resolution of X-ray imaging. However, this approach still has its disadvantages. The crosstalk of the scintillation light between adjacent columns cannot be ignored because the CsI(Tl) columns are close together. To solve this problem, a new type of structured CsI(Tl) scintillation screen has been developed by filling oxidized silicon micropore array template with CsI(Tl) scintillator [8–13]. The silicon wall serves as a barrier for the lateral spread of scintillation light due to its high optical absorption [14]. The SiO2 layer grown on the wall of the silicon pore acts as a total reflective layer to guide the scintillation light with incident angle α smaller than the critical angle αc propagating along the pore channel [15]. In this way, Svenonius et al. [11] have fabricated the structured CsI(Tl) scintillation screens with pore array periods of 40, 30 and 20 μm. The spatial resolutions of X-ray imaging obtained using them are 14, 17 and 22 lp/mm, respectively. More recently, Hormozan et al. [12] and Liu et al. [13] have prepared the structured CsI(Tl) and CsI scintillation screens with 4 μm pore array period, respectively. The spatial resolutions of X-ray imaging obtained using them exceed 100 lp/mm. It is obviously that the scintillation screen with a smaller structure period can result in a higher spatial resolution of X-ray imaging. However, as the structure period decreases, the influence of the thickness of the pore wall on the imaging performance of the scintillation screen tends to be sensitive. If the thickness of the pore wall is kept constant, the screen area occupied by CsI(Tl) scintillator would decrease, thus reducing the X-ray detection efficiency of the scintillation screen. If the screen area occupied by CsI(Tl) scintillator remains same, the thickness of the pore wall must be reduced, which makes it difficult to effectively prevent the crosstalk of scintillation light between adjacent scintillation columns. Therefore, it is necessary to study the influence of the thickness of the silicon wall on the X-ray imaging performance of the structured CsI(Tl) scintillation screen under small period of the pore array.
In this work, the influence of the silicon wall thickness on the performance of the structured CsI(Tl) scintillation screen based on oxidized silicon micropore array template in X-ray imaging was studied. The research was performed by Geant4 Monte Carlo simulation code [16,17], which includes models of scintillation and optical transport processes. The period of the structured CsI(Tl) scintillation screens in the simulation was chosen to be 4 µm, as it is the smallest reported period for X-ray imaging [12,13]. The imaging performance of the structured CsI(Tl) scintillation screen was characterized by light output (LO), modulation transfer function (MTF) and detective quantum efficiency (DQE). The results are of great significance for optimizing the thickness of the silicon wall of the structured scintillation screen to improve the performance of X-ray imaging for different application requirements.
2. Materials and methods
2.1 Screen structures
The simulated scintillation screen is formed by filling an oxidized silicon micropore array template with CsI(Tl) scintillator. Its structure is shown in Fig. 1. The micropores are square pores with a depth of 100 µm and are arranged in a square array with a period of 4 µm. The micropore wall is composed of a silicon wall covered with 0.1 µm thick SiO2 layer at each side. Owing to the absorption coefficient of silicon at 550 nm wavelength (i.e. the emission peak position of CsI (Tl)) is 7 × 103 cm−1, a 2.3 µm thick silicon wall can basically prevent the crosstalk of scintillation light between adjacent CsI(Tl) microcolumns [14]. Therefore, the maximum thickness of silicon wall was set to 2.3 μm, and the range of the thickness varied from 0.1 to 2.3 µm. In addition, the area of the structured scintillation screen was set to 600 × 600 μm2. The screen was attached to an ideal photoelectric imaging device.
Fig. 1 Structure of the simulated CsI (Tl) scintillation screen (left) top view, (right) enlarged side view.
2.2 Simulation process
The influence of the silicon wall thickness on the performance of the structured CsI(Tl) scintillation screen in X-ray imaging was characterized by the variations of LO, MTF and DQE. When a beam of X-rays irradiates vertically from above to the structured CsI(Tl) scintillation screen, the scintillation light will be generated through the interaction between the incident X-ray and the scintillator in the screen. The mean number of scintillation photons generated per unit X-ray energy is called total light output (TLO). The generated scintillation light can be emitted in all directions. Due to the microcolumn array structure, the scintillation photons with incident angles α smaller than the critical angle αc will be totally reflected and propagate along the microcolumn channels, while the others will be absorbed by the silicon walls or transmit into the adjacent CsI(Tl) microcolumns. The scintillation photons arriving at the bottom of the structured CsI(Tl) scintillation screen were detected by an ideal photoelectric imaging device. The mean number of scintillation photons detected per unit X-ray energy is called bottom light output (BLO). The optical coupling agent between the structured scintillation screen and the photoelectric imaging device was silicon oil with refractive index n = 1.43.
MTF is used to characterize the ability of an imaging system to reproduce information in spatial frequency domain. In this work, it was simulated according to the standard edge measurement method [18]. An edge spread function (ESF) was acquired by using an edge phantom, which was the response of the X-ray imaging detector to an opaque object with a straight edge. The orientation of the edge line was titled 2.86° with respect to the alignment direction of the scintillation array to allow for spatial oversampling. The MTF was calculated by discrete Fourier transform (DFT) of a line spread function (LSF), which was derived from the differential of the ESF along the direction (assuming the direction of the x-axis) perpendicular to the straight edge [19,20].
The MTF was normalized to its value at zero frequency. The spatial resolution of the X-ray imaging system was defined by the frequency where MTF = 0.1.DQE is the spectral representation in frequency domain of the signal-to-noise characteristics of a given detector configuration. It was simulated using the recently developed Fujita-Lubberts-Swank (FLS) method [20–22], which considers MTF, noise power spectrum (NPS) and Swank factor AS. The expression for the DQE is as follows.
where DQE(0) is the value of DQE at zero frequency calculated by the following equation.where is the quantum detection efficiency of a screen, which can be evaluated as the ratio of the mean number of X-rays that deposit their energy in the screen to the mean number of X-rays incident on the screen. AS is the Swank Factor arised from the fluctuations in the number of scintillation photons emitted from the screen per absorbed X-ray photon. It is given bywhere Mn is the nth moment of the scintillation light pulse height spectrum (PHS). The NPS is the output noise power spectrum of an X-ray imaging detector; it quantitatively describes the magnitude of the noise in an imaging detector as a function of frequency. Each normal incident X-ray photon that interacts with a scintillation screen produces a 2D point spread function (PSF). The NPS produced by each detected X-ray photon along the x-axis direction was computed by summing the 2D PSF in the y-axis direction and taking the square of the magnitude of its discrete Fourier transform was obtained by averaging NPS(f) over all detected eventsFinally, it was normalized by its value at zero frequency, i.e., = 1.2.3 Simulation parameters
The simulation was carried out using version 10.0 of Geant4 Monte Carlo simulation code, which had been widely used for performing scintillation process and scintillation light propagation in X-ray imaging detector [16,17]. A number of optical parameters such as emission spectrum, light yield, optical attenuation coefficient, refractive index etc. needed to be used in the simulation. The emission spectrum and scintillation light yield of CsI(Tl) were cited from [23] and [24], respectively. The optical attenuation coefficient was determined by the transmission spectrum of CsI(Tl) crystal [25]. The refractive index of CsI(Tl) in the wavelength range from 300 to 900 nm was cited from [26]. The refractive indices, optical attenuation coefficients of Si and SiO2 were cited from [27–30]. The energy of the incident X-ray was set to 20 keV. The number of X-rays launched per flood image is 2 × 107, which is high enough to avoid biasing the DQE estimate [20].
3. Results and discussion
3.1 Light output
The variations of TLO and BLO of the structured CsI(Tl) scintillation screen as functions of the silicon wall thickness are given in Fig. 2. It can be seen that the TLO and BLO of the screen decrease from 30878 ph/MeV to 4362 ph/MeV and from 6916 ph/MeV to 760 ph/MeV, respectively, with the thickness of the silicon wall increasing from 0.1 µm to 2.3 µm. The decrease of the TLO with the increase of the thickness of the silicon wall is due to the decrease of X-ray absorption caused by the reduction of the screen area occupied by CsI(Tl) scintillator. When the thickness of the silicon wall increases from 0.1 µm to 2.3 µm, the proportion of the screen area occupied by CsI(Tl) scintillator to the entire screen area decreases from 85.6% to 14.1%. The BLO, like the TLO, decreases with the increase of the thickness of the silicon wall. However, when the thickness is less than 0.5 μm, the change ofthe BLO with the thickness becomes more rapid. This is because, in addition to the variation of the screen area occupied by CsI(Tl) scintillator, the influence of the variation of the silicon wall on the prevention of the cross talk of scintillation light between adjacent CsI(Tl) microcolumns can't be ignored. The distributions of the scintillation light that reaches to the bottom of the structured CsI (Tl) scintillation screens with different silicon wall thicknessesare shown in Fig. 3. It can be seen that the silicon wall with 0.5 μm thickness of the structured CsI(Tl) scintillation screen can basically suppress the crosstalk of scintillation light between adjacent microcolumns. As the thickness decreases further, the suppression becomes more and more difficult.
Fig. 2 Total light output (left) and bottom light output (right) of the structured CsI(Tl) scintillation screen as functions of silicon wall thickness.
Fig. 3 Distributions of the scintillation light that reaches to the bottoms of the structured CsI(Tl) scintillation screens with different silicon wall thicknesses d.
3.2 Modulation transfer function
The MTFs of the X-ray imaging systems using the structured CsI(Tl) scintillation screens with different silicon wall thicknesses are displayed in Fig. 4. The thinner the silicon wall, the worse the MTF. The spatial resolution of the X-ray imaging systems using the structured CsI(Tl) scintillation decreases from 124.2 lp/mm to 66.8 lp/mm as the silicon wall thickness reduces from 2.3 μm to 0.1 μm. There is no other reason for such a decrease except that reducing the thickness will reduce the ability of the silicon wall to prevent the crosstalk of the scintillation light between adjacent CsI(Tl) microcolumns. When the thickness is less than 0.5 μm, the resolution decreases more rapidly with the decrease of the thickness, which is compatible with the variation of the BLO with the thickness.
Fig. 4 MTFs of the X-ray imaging systems using the structured CsI(Tl) scintillation screens with different silicon wall thicknesses.
3.3 Detective quantum efficiency
The DQEs of the X-ray imaging systems using the structured CsI(Tl) scintillation screens with different silicon wall thicknesses are shown in Fig. 5. An X-ray image recorded by a scintillation detector can be described as a multistage cascade process: X-ray absorption by the screen, generation and propagation of scintillation light in the screen, and collection of the scintillation light from the bottom of the screen by a photoelectric imaging device. The effect of each stage i on DQE of an X-ray imaging detector can be expressed as [31]
where , and are the average gain/efficiency and Poisson excess in gain for the scintillation detector at stage i, respectively. In this work, the DQE can be expressed asThe parameters obtained using Geant 4 simulation code are shown in Table 1. At low spatial frequency, the thicker the silicon wall, the lower the DQE. The DQE at zero spatial frequency of the imaging system decreases from 0.565 to 0.097 as the silicon wall thickness increases from 0.1 μm to 2.3 μm. This is mainly due to the decrease of quantum detection efficiency of primary X-rays caused by the reduction of the screen area occupied by CsI (Tl) scintillator. Because of the columnar structure of the scintillation screen, the distribution of the conversion gain from the number of interacting X-rays to the number of scintillation photons exiting the bottom of the screens significantly deviates from the Poisson distribution. Therefore, the DQE at zero spatial frequency is also related to the Poisson excess, which is different from an unstructured scintillation screen. As the spatial frequency increases, the influence of MTF on DQE becomes so important that it gradually turns to a major factor, as shown in the fourth term of the denominator of Eq. (9). All the DQEs with different siliconwall thicknesses decrease with the increase of the spatial frequency because they gradually become proportional to the square of the MTFs. The thicker the silicon wall, the better the MTF, so the DQE with a thick silicon wall can exceed the DQE with a thin silicon wall at high spatial frequency. That is, if one wants to retain more low-frequency information at the expense of high-frequency information in X-ray imaging, he can choose a screen with a thin silicon wall; if one wants to retain more high-frequency information at the expense of low-frequency information in X-ray imaging, he can choose a screen with a thick silicon wall.Fig. 5 DQEs of the X-ray imaging systems using the structured CsI(Tl) scintillation screens with different silicon wall thicknesses.
Table 1. Description of the gains, Poisson excesses in the gains, and MTFs for each stage i of the structured CsI(Tl) scintillation detectors with different silicon wall thicknesses.
4. Conclusion
The influence of silicon wall thickness on the performance of the structured CsI(Tl) scintillation screen based on oxidized silicon micropore array template in X-ray imaging was simulated using Geant4 Monte Carlo simulation code. The performance was characterized by LO, MTF and DQE. The micropores of the templates are square pores with a depth of 100 µm and are arranged in a square array with a period of 4 µm. The range of the silicon wall thickness varied from 0.1 to 2.3 µm. When the thickness of the silicon wall is less than 0.5 μm, the BLO of the scintillation screen increases more rapid with the decrease of the thickness. This is because, in addition to the increase of the screen area occupied by CsI (Tl) scintillator, the reduction in the ability of the silicon wall to prevent the cross talk of scintillation light between adjacent CsI(Tl) microcolumns can’t be ignored. This phenomenon can be further demonstrated by the distributions of the scintillation light that reaches to the bottoms of the structured CsI(Tl) scintillation screens with different silicon wall thicknesses. The thinner the silicon wall, the worse the MTF. When the thickness is less than 0.5 μm, the spatial resolution of the X-ray imaging system using the structured CsI(Tl) scintillation screen decreases more rapidly with the decrease of the silicon wall thickness, which is compatible with the variation of the BLO with the thickness. At low spatial frequency, the thicker the silicon wall, the lower the DQE. However, the DQE with a thick silicon wall can exceed the DQE with a thin silicon wall at high spatial frequency. All the results provide the quantitative relation between the silicon wall thickness of the structured CsI(Tl) scintillation screen and the quality of the X-ray imaging. They are very useful for optimizing the thickness of the silicon wall of the structured scintillation screen to improve the performance of X-ray imaging for different application requirements.
Funding
National Natural Science Foundation of China (NSFC) (11675121, 11475128 and 11775160).
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