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PURPOSE Scintillators and photoconductors used in energy integrating detectors (EIDs) have inherent variations in their imaging response to single-detected X-rays due to variations in X-ray energy deposition and secondary quanta generation and transport, which degrades DQE(f). The imaging response of X-ray scintillators to single X-rays may be recorded and studied using single X-ray imaging (SXI) experiments; however, no method currently exists for relating SXI experimental results to EID DQE(f). This work proposes a general analytical framework for computing and analyzing the DQE(f) performance of EIDs from single X-ray image ensembles using a spatial frequency-dependent pulse-height spectrum. METHODS A spatial frequency (f)-dependent gain, g∼(f) , is defined as the Fourier transform of the imaging response of an EID to a single-detected X-ray. A f-dependent pulse-height spectrum, Pr[g∼(f)] , is defined as the 2D probability density function of g∼(f) over the complex plane. Pr[g∼(f)] is used to define a f-dependent Swank factor, AS (f), which fully characterizes the DQE(f) degradation due to single X-ray noise. AS (f) is analyzed in terms of its degradation due to Swank noise, variations in the frequency-dependent attenuation of |g∼(f)| , and noise in argg∼(f) which occurs due to variations in the asymmetry in each single X-ray's imaging response. Three example imaging systems are simulated to demonstrate the impact of depth-dependent variation in g∼(f) , remote energy deposition, and a finite number of secondary quanta, on Pr[g∼(f)] , AS (f), MTF(f), and NPS(f)/NPS(0), which are computed from ensembles of single X-ray images. The same is also demonstrated by simulating a realistic imaging system; that is, a Gd2 O2 S-based EID. Using the latter imaging system, the convergence of AS (f) estimates is investigated as a function of the number of detected X-rays per ensemble. RESULTS Depth-dependent g∼(f) variation resulted in AS (f) degradation exclusively due to depth-dependent optical Swank noise and the Lubberts effect. Conversely, the majority of AS (f) degradation caused by remote energy deposition and finite secondary quanta occurred due to variations in argg∼(f) . When using input X-ray energies below the K-edge of Gd, variations in the frequency-dependent attenuation of |g∼(f)| accounted for the majority of AS (f) degradation in the GOS-based EID, and very little Swank noise and variations in argg∼(f) were observed. Above the K-edge, however, AS (f) degradation due to Swank noise and variations in argg∼(f) greatly increased. The convergence of AS (f) was limited by variation in argg∼(f) ; imaging systems with more variation in argg∼(f) required more detected X-rays per ensemble. CONCLUSIONS An analytical framework is proposed that generalizes the pulse-height spectrum and Swank factor to arbitrary f. The impact of single X-ray noise sources, such as the Lubberts effect, remote energy deposition, and finite secondary quanta on detector performance, may be represented using Pr[g∼(f)] , and quantified using AS (f). The approach may be used to compute MTF(f), NPS(f), and DQE(f) from ensembles of single X-ray images and provides an additional tool to analyze proposed EID designs.

Identifying the correct Line-of-Response (LOR) in positron emission tomography (PET) requires accurate localization of the first interaction between incident gamma ray and detector. Improving the accuracy of this localization typically entails offsetting losses in detector efficiency, complexity, or cost. In this paper, we propose a solution that can localize scattered gammas without losses in detector efficiency, utilizing Prism-PET - a single-sided detector module with pixelated light guide. Using Monte Carlo simulations of gamma-detector interactions, we train a Convolutional Neural Network to predict the first interaction position of gammas incident on a detector block.

The x-ray imaging performance of an indirect flat panel detector (I-FPD) is intrinsically limited by its scintillator. Random fluctuations in the conversion gain and spatial blur of scintillators (per detected x-ray) degrade the detective quantum efficiency (DQE) of I-FPDs. These variations are often attributed to depth-dependence in light escape efficiency and spatial spread before detection. Past investigations have used theoretical models to explore how scintillator depth effects degrade DQE(f), however such models have not been validated by direct measurements. Recently, experimental methods have been developed to localize the depth of x-ray interactions in a scintillator, and image the light burst from each interaction using an ultra-high-sensitivity optical camera. This approach, referred to as depth-localized single x-ray imaging (SXI), has enabled direct measurements of both depth-dependent and fixed-depth variations in scintillator gain and spatial resolution. SXI has been used recently to measure depth-dependence in the average gain and modulation transfer function (MTF) of columnar CsI:Tl, which is the scintillator-of-choice for medical I-FPDs. When used in a depth-dependent cascaded linear system model, these SXI measurements accurately predict the presampling MTF(f) of CsI:Tl-based I-FPDs as measured using the slanted-edge method. However, such calculations underestimate the CsI:Tl noise power spectrum (NPS), and thereby overestimate its DQE when compared to conventional measurements. We hypothesize that some of this discrepancy is caused by fixed-depth variations in CsI:Tl spatial resolution, which are not considered in current models. This work characterizes these variations directly using depth-localized SXI and examines their impact on scintillator DQE(f).

PURPOSE: Turbid (powder or columnar-structured) scintillators are widely used in indirect flat panel detectors (I-FPDs) for scientific, industrial and medical radiography. Light diffusion and absorption within these scintillators is expected to cause depth-dependent variations in their x-ray conversion gain and spatial blur. These variations degrade the detective quantum efficiency of I-FPDs at all spatial frequencies. Despite their importance, there are currently no established methods for directly measuring scintillator depth effects. This work develops the instrumentation and methods to achieve this capability. METHODS: An ultra-high-sensitivity camera was assembled for imaging single x-ray interactions in two commercial Gd(2)O(2)S:Tb (GOS) screens (Lanex Regular and Fast Back, Eastman Kodak Company). X-ray interactions were localized to known depths in the screens using a slit beam of parallel synchrotron radiation (32 keV), with beam width (~20 µm) much narrower than the screen thickness. Depth-localized x-ray interaction images were acquired in 30 µm depth-intervals, and analyzed to measure each scintillator’s depth-dependent average gain [Formula: see text] and modulation transfer function MTF(z,f). These measurements were used to calculate each screen’s expected MTF(f) in an energy-integrating detector (e.g. I-FPD). Calculations were compared to presampling MTF measurements made by coupling each screen to a high-resolution CMOS image sensor (48 μm pixel) and using the slanted-edge method. RESULTS: Both [Formula: see text] and MTF(z,f) continuously increased as interactions occurred closer to each screen’s sensor-coupled surface. The Regular yielded 1351 ± 66 and 2117 ± 54 photons per absorbed x-ray (42-66 keV(−1)) in interactions occurring furthest from and nearest to the image sensor, while the Fast Back yielded 833 ± 22 and 1910 ± 39 photons (26-60 keV(−1)). At f = 1 mm(−1), MTF(z,f) varied between 0.63-0.78 in the Regular and 0.30-0.76 in the Fast Back. Calculations of presampling MTF(f) using [Formula: see text] and MTF(z,f) showed excellent agreement with slanted-edge measurements. CONCLUSIONS: The developed instrument and method enable direct measurements of the depth-dependent gain and spatial resolution of turbid scintillators. This knowledge can be used to predict, understand, and potentially improve I-FPD imaging performance.

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 which𝑁ph (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.

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.

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.

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.

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 detector's 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.

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.