Adrian Howansky

Deriving depth‐dependent light escape efficiency and optical Swank factor from measured pulse height spectra of scintillators

Purpose: Pulse height spectroscopy has been used by investigators to deduce the imaging properties of scintillators. Pulse height spectra (PHS) are used to compute the Swank factor, which describes the variation in scintillator light output per x‐ray interaction. The spread in PHS measured below the K‐edge is related to the optical component of the Swank factor, i.e., variations in light escape efficiency from different depths of x‐ray interaction in the scintillator, denoted Symbol. Optimizing scintillators for medical imaging applications requires understanding of these optical properties, as they determine tradeoffs between parameters such as x‐ray absorption, light yield, and spatial resolution. This work develops a model for PHS acquisition such that the effect of measurement uncertainty can be removed. This method allows Symbol to be quantified on an absolute scale and permits more accurate estimation of the optical Swank factor of scintillators. Symbol. No Caption available. Symbol. No Caption available. Methods: The pulse height spectroscopy acquisition chain was modeled as a linear system of stochastic gain stages. Analytical expressions were derived for signal and noise propagation through the PHS chain, accounting for deterministic and stochastic aspects of x‐ray absorption, scintillation, and light detection with a photomultiplier tube. The derived expressions were used to calculate PHS of thallium‐doped cesium iodide (CsI) scintillators using parameters that were measured, calculated, or known from literature. PHS were measured at 25 and 32 keV of CsI samples designed with an optically reflective or absorptive backing, with or without a fiber‐optic faceplate (FOP), and with thicknesses ranging from 150–1000 μm. Measured PHS were compared with calculated PHS, then light escape model parameters were varied until measured and modeled results reached agreement. Resulting estimates of Symbol were used to calculate each scintillators optical Swank factor. Symbol. No Caption available. Results: For scintillators of the same optical design, only minor differences in light escape efficiency were observed between samples with different thickness. As thickness increased, escape efficiency decreased by up to 20% for interactions furthest away from light collection. Optical design (i.e., backing and FOP) predominantly affected the magnitude and relative variation in Symbol. Depending on interaction depth and scintillator thickness, samples with an absorptive backing and FOP were estimated to yield 4.1–13.4 photons/keV. Samples with a reflective backing and FOP yielded 10.4–18.4 keV−1, while those with a reflective backing and no FOP yielded 29.5–52.0 keV−1. Optical Swank factors were approximately 0.9 and near‐unity in samples featuring an absorptive or reflective backing, respectively. Symbol. No Caption available. Conclusions: This work uses a modeling approach to remove the noise introduced by the measurement apparatus from measured PHS. This method allows absolute quantification of Symbol and more accurate estimation of the optical Swank factor of scintillators. The method was applied to CsI scintillators with different thickness and optical design, and determined that optical design more strongly affects Symbol and Swank factor than differences in CsI thickness. Despite large variations in Symbol between optical designs, the Swank factor of all evaluated samples is above 0.9. Information provided by this methodology can help validate Monte Carlo simulations of structured CsI and optimize scintillator design for x‐ray imaging applications. Symbol. No Caption available. Symbol. No Caption available. Symbol. No Caption available.

Direct measurement of Lubberts effect in CsI:Tl scintillators using single x-ray photon imaging

The imaging performance of an indirect flat panel detector (I-FPD) is fundamentally limited by that of its scintillator. The scintillator’s modulation transfer function (MTF) varies as a function of the depth of x-ray interaction in the layer, due to differences in the lateral spread of light before detection by the optical sensor. This variation degrades the spatial frequency-dependent detective quantum efficiency (DQE(f)) of I-FPDs, and is quantified by the Lubberts effect. The depth-dependent MTFs of various scintillators used in I-FPDs have been estimated using Monte Carlo simulations, but have never been measured directly. This work presents the first experimental measurements of the depth-dependent MTF of thallium-doped cesium iodide (CsI) and terbium-doped Gd2O2S (GOS) scintillators with thickness ranging from 200 – 1000 μm. Light bursts from individual x-ray interactions occurring at known, fixed depths within a scintillator are imaged using an ultra-high-sensitivity II-EMCCD (image-intensifier, electron multiplying charge coupled device) camera. X-ray interaction depth in the scintillator is localized using a micro-slit beam of parallel synchrotron radiation (32 keV), and varied by translation in 50 ± 1 µm depth intervals. Fourier analysis of the imaged light bursts is used to deduce the MTF versus x-ray interaction depth z. Measurements of MTF(z,f) are used to calculate presampling MTF(f) with RQA-M3, RQA5 and RQA9 beam qualities and compared with conventional slanted edge measurements. Images of the depth-varying light bursts are used to derive each scintillator’s Lubberts function for a 32 keV beam.

Solid-state flat panel imager with avalanche amorphous selenium

Active matrix flat panel imagers (AMFPI) have become the dominant detector technology for digital radiography and fluoroscopy. For low dose imaging, electronic noise from the amorphous silicon thin film transistor (TFT) array degrades imaging performance. We have fabricated the first prototype solid-state AMFPI using a uniform layer of avalanche amorphous selenium (a-Se) photoconductor to amplify the signal to eliminate the effect of electronic noise. We have previously developed a large area solid-state avalanche a-Se sensor structure referred to as High Gain Avalanche Rushing Photoconductor (HARP) capable of achieving gains of 75. In this work we successfully deposited this HARP structure onto a 24 x 30 cm2 TFT array with a pixel pitch of 85 μm. An electric field (ESe) up to 105 Vμm-1 was applied across the a-Se layer without breakdown. Using the HARP layer as a direct detector, an X-ray avalanche gain of 15 ± 3 was achieved at ESe = 105 Vμm-1. In indirect mode with a 150 μm thick structured CsI scintillator, an optical gain of 76 ± 5 was measured at ESe = 105 Vμm-1. Image quality at low dose increases with the avalanche gain until the electronic noise is overcome at a constant exposure level of 0.76 mR. We demonstrate the success of a solid-state HARP X-ray imager as well as the largest active area HARP sensor to date.

Investigation of the screen optics of thick CsI(Tl) detectors

Flat panel imagers (FPI) are becoming the dominant detector technology for digital x-ray imaging. In indirect FPI, the scintillator that provides the highest image quality is Thallium (Tl) doped Cesium Iodide (CsI) with columnar structure. The maximum CsI thickness used in existing FPI is ~600 microns, due to concerns of loss in spatial resolution and light output with further increase in thickness. The goal of the present work is to investigate the screen-optics for CsI with thicknesses much larger than that used in existing FPI, so that the knowledge can be used to improve imaging performance in dose sensitive and higher energy applications, such as cone-beam CT (CBCT). Columnar CsI(Tl) scintillators up to 1 mm in thickness with different screen-optical design were investigated experimentally. Pulse height spectra (PHS) were measured to determine the Swank factor at x-ray energies between 25 and 75 keV, and to derive depth-dependent light escape efficiency i.e. gain. Detector presampling MTF, NPS and DQE were measured using a high-resolution CMOS optical sensor. Optical Monte Carlo simulation was performed to estimate optical parameters for each screen design and derive depth-dependent gain and MTF, from which overall MTF and DQE were calculated and compared with measured results. The depth-dependent imaging performance parameters were then used in a cascaded linear system model (CLSM) to investigate detector performance under screen- and sensor-side irradiation conditions. The methodology developed for understanding the optics of thick CsI(Tl) will lead to detector optimization in CBCT.

Toward Scintillator High‐Gain Avalanche Rushing Photoconductor Active Matrix Flat Panel Imager (SHARP‐AMFPI): Initial fabrication and characterization

PURPOSE We present the first prototype Scintillator High-Gain Avalanche Rushing Photoconductor Active Matrix Flat Panel Imager (SHARP-AMFPI). This detector includes a layer of avalanche amorphous Selenium (a-Se) (HARP) as the photoconductor in an indirect detector to amplify the signal and reduce the effects of electronic noise to obtain quantum noise-limited images for low-dose applications. It is the first time avalanche a-Se has been used in a solid-state imaging device and poses as a possible solution to eliminate the effects of electronic noise, which is crucial for low-dose imaging performance of AMFPI. METHODS We successfully deposited a solid-state HARP structure onto a 24 × 30 cm2 array of thin-film transistors (TFT array) with a pixel pitch of 85 μm. The HARP layer consists of 16 μm of a-Se with a hole-blocking and electron-blocking layer to prevent charge injection from the high-voltage bias and pixel electrodes, respectively. An electric field (ESe ) up to 105 V μm-1 was applied across the a-Se layer without breakdown. A 150 μm thick-structured CsI:Tl scintillator was used to form SHARP-AMFPI. The x-ray imaging performance is characterized using a 30 kVp Mo/Mo beam. We evaluate the spatial resolution, noise power, and detective quantum efficiency at zero frequency of the system with and without avalanche gain. The results are analyzed using cascaded linear system model (CLSM). RESULTS An avalanche gain of 76 ± 5 was measured at ESe = 105 V μm-1 . We demonstrate that avalanche gain can amplify the signal to overcome electronic noise. As avalanche gain is increased, image quality improves for a constant (0.76 mR) exposure until electronic noise is overcome. Our system is currently limited by poor optical transparency of our high-voltage electrode and long integrating time which results in dark current noise. These two effects cause high-spatial frequency noise to dominate imaging performance. CONCLUSIONS We demonstrate the feasibility of a solid-state HARP x-ray imager and have fabricated the largest active area HARP sensor to date. Procedures to reduce secondary quantum and dark noise are outlined. Future work will improve optical coupling and charge transport which will allow for frequency DQE and temporal metrics to be obtained.

Investigation of random gain variations in columnar CsI:Tl using single x-ray photon imaging

The x-ray imaging performance of an indirect flat panel detector (I-FPD) is degraded by random variations in its scintillator’s conversion gain. At energies below the K-edge, these variations are caused by depth-dependence in light collection from within the scintillator, and intrinsic fluctuations in the number of optical photons (Nph) emitted per absorbed x-ray. At fixed energy, the former effect can be quantified by the average depth-dependent gain Nph (𝑧). The latter effect can be evaluated using a Fano factor FN, defined as the variance in Nph divided by its mean at fixed interaction depth. Neither phenomenon has been directly measured in non-transparent scintillators used in medical I-FPDs, namely columnar CsI:Tl. This work presents experimental measurements of Nph(𝑧) and FN in a columnar CsI:Tl scintillator with 1000 μm thickness. X-ray interactions were localized to fixed depths (±10 μm, 100 μm intervals) in the scintillator using a microslit beam of parallel synchrotron radiation (32 keV). Light bursts from single interactions at each depth were imaged using an II-EMCCD optical camera, and their magnitude was characterized by 2D summation of their image pixel values. The II-EMCCD camera was calibrated to convert summed pixel values to numbers of optical photons detected per event. The number distributions of photons collected per event were represented in histograms as “depth-localized pulse height spectra” (DLPHS), from which𝑁̅ph (𝑧) and FN were derived. The II-EMCCD’s noise contribution to these measurements was estimated and removed from FN. Depth-dependent and intrinsic variations in the gain of columnar CsI:Tl are compared.

Dual screen sandwich configurations for digital radiography

Motivated by recent advances in TFT array technology for display, this study develops a theoretical treatment of dual granular scintillating screens sandwiched around a light detector and applies this to investigate possible improvements in imaging performance of indirect active-matrix flat-panel imagers (AMFPI’s) for x-ray applications, when dual intensifying screen configurations are used. Theoretical methods, based on previous studies of granular intensifying screens, are developed and applied to calculate modulation transfer function (MTF), normalized noise power spectrum (NNPS), Swank factor (As), Lubberts function L(f), and spatial frequency-dependent detective quantum efficiency (DQE(f)) for a variety of detector configurations in which a pair of screens are sandwiched around a light sensing array. Single-screen front illuminated (FI) and back illuminated (BI) configurations are also included in the analysis. DQE(f) is used as a performance metric to optimize and compare the performance of the various configurations. Large improvements in performance in MTF and DQE(f) are found possible, when the substrate layer between the light sensing array and the intensifying screen is optically thin. The ratio of the thicknesses of the two screens which optimizes DQE performance is generally asymmetric with the thinner screen facing the incident flux, and the ratio depends on the x-ray attenuation length in the phosphor material.

Evaluation of novel multilayer x-ray detector designs using rapid Monte Carlo simulation

Direct and indirect active matrix flat-panel imagers (AMFPI) have become the dominant technology in digital radiography and fluoroscopy, and further improvements in imaging performance are being sought through novel detector designs. Two novel multilayer x-ray detectors are proposed to improve the DQEs of existing AMFPI in R/F and CBCT applications that require high DQE and wide dynamic range. Both detectors utilize a back-irradiation (BI) geometry, and incorporate both a-Se and scintillators in their designs. The first design, the Hybrid-AMFPI is a composite direct/indirect detector that aims to improve the quantum efficiency of a-Se (with a maximum thickness of 1 mm due to carrier trapping) by adding a scintillator. The second design, the BI-SHARP-AMFPI (Back-Irradiated Scintillator HARPAMFPI), uses a High Gain Avalanche Rushing Photoconductor (HARP) a-Se layer to detect and amplify optical photons from an x-ray scintillator. This work uses the Fujita-Lubberts-Swank (FLS) Monte Carlo (MC) framework proposed by Star-Lack et al. to investigate the potential improvements in imaging performance of these detectors and the optimal detector configuration. Simulations were carried out at RQA5 and RQA9 standard beam qualities. Both front-irradiation (FI) and BI geometries were evaluated to demonstrate the advantage of BI. Our simulations confirm that the DQE of the Hybrid AMFPI is substantially improved at low spatial frequencies compared to an otherwise identical direct AMFPI. Additionally, the role of gain matching of direct and indirect signal (a consideration unique to multilayer AMFPI) is investigated in the imaging performance of both the Hybrid and BI-SHARP-AMFPI.

MO-AB-BRA-07: Low Dose Imaging with Avalanche Amorphous Selenium Flat Panel Imager

PURPOSE We present the first active matrix flat panel imager (AMFPI) capable of producing x-ray quantum noise limited images at low doses by overcoming the electronic noise through signal amplification by photoconductive avalanche gain (gav). The indirect detector fabricated uses an optical sensing layer of amorphous selenium (a-Se) known as High-Gain Avalanche Rushing Photoconductor (HARP). The detector design is called Scintillator HARP (SHARP)-AMFPI. This is the first image sensor to utilize solid-state HARP technology. METHODS The detectors electronic readout is a 24 × 30 cm2 array of thin film transistors (TFT) with a pixel pitch of 85 µm. The HARP structure consists of a 15 µm layer of a-Se isolated from the high voltage (HV) and signal electrode by a 2 µm thick hole blocking layer and electron blocking layer, respectively, to reduce dark current. A 150 µm thick structured CsI scintillator with reflective backing and a fiber optic faceplate (FOP) was coupled to the semi-transparent HV bias electrode of the HARP structure. Images were acquired using a 30 kVp Mo/Mo spectrum typically used in mammography. RESULTS Optical sensitivity measurements demonstrate that gav = 76 ± 5 can be achieved over the entire active area of the detector. At a constant dose to the detector of 6.67 µGy, image quality increases with gav until the effective electronic noise is negligible. Quantum noise limited images can be obtained with doses as low as 0.18 µGy. CONCLUSION We demonstrate the feasibility of utilizing avalanche gain to overcome electronic noise. The indirect detector fabricated is the first solid-state imaging sensor to use HARP, and the largest active area HARP sensor to date. Our future work is to improve charge transport within the HARP structure and utilize a transparent HV electrode.

TH-CD-207B-06: Swank Factor of Segmented Scintillators in Multi-Slice CT Detectors: Pulse Height Spectra and Light Escape

PURPOSE Pulse height spectra (PHS) have been used to determine the Swank factor of a scintillator by measuring fluctuations in its light output per x-ray interaction. The Swank factor and x-ray quantum efficiency of a scintillator define the upper limit to its imaging performance, i.e. DQE(0). The Swank factor below the K-edge is dominated by optical properties, i.e. variations in light escape efficiency from different depths of interaction, denoted e(z). These variations can be optimized to improve tradeoffs in x-ray absorption, light yield, and spatial resolution. This work develops a quantitative model for interpreting measured PHS, and estimating e(z) on an absolute scale. The method is used to investigate segmented ceramic GOS scintillators used in multi-slice CT detectors. METHODS PHS of a ceramic GOS plate (1 mm thickness) and segmented GOS array (1.4 mm thick) were measured at 46 keV. Signal and noise propagation through x-ray conversion gain, light escape, detection by a photomultiplier tube and dynode amplification were modeled using a cascade of stochastic gain stages. PHS were calculated with these expressions and compared to measurements. Light escape parameters were varied until modeled PHS agreed with measurements. The resulting estimates of e(z) were used to calculate PHS without measurement noise to determine the inherent Swank factor. RESULTS The variation in e(z) was 67.2-89.7% in the plate and 40.2-70.8% in the segmented sample, corresponding to conversion gains of 28.6-38.1 keV-1 and 17.1-30.1 keV-1 , respectively. The inherent Swank factors of the plate and segmented sample were 0.99 and 0.95, respectively. CONCLUSION The high light escape efficiency in the ceramic GOS samples yields high Swank factors and DQE(0) in CT applications. The PHS model allows the intrinsic optical properties of scintillators to be deduced from PHS measurements, thus it provides new insights for evaluating the imaging performance of segmented ceramic GOS scintillators.